Cnidarians

There are approximately 10,000 different species of cnidarians, and the majority of these animals are found in the marine environment. Cnidarians are classified either as polyps or medusae. The polyps include the corals (Figure 3-1), hydrae, and anemones (Figure 3-2). The medusae include the true jellyfish and the box jellyfish. Cnidarians can be found as individuals or colonies, depending on the group. Most are carnivorous, subduing and killing their prey with specialized cells called cnidocytes (see Anatomy and Physiology). Jellyfish are one species with cnidocytes, which also are venomous to humans.

Figure 3-1 Maze brain coral (Platygyra sp.). These stony corals are popular in the aquarium hobby.

(Photo by Trevor Zachariah.)

Figure 3-2 Bubble tip anemone (Entacmaea quadricolor). As indicated by this photo, these anemones are popular with clown fish.

(Photo by Trevor Zachariah.)

Gastropods

There are approximately 60,000 different species of gastropods, and these animals are primarily aquatic, although terrestrial forms exist. Among the aquatic gastropods, the major-ity of animals are found in benthic habitats. Gastropods are the most numerous and diverse class within the phylum Mollusca and are the only group to have members that have evolved a terrestrial lifestyle (the Pulmonata). The gastro-pods are primarily represented by the snails (Figure 3-3) and slugs.

Figure 3-3 Turbo snail (Turbo sp.). These marine gastropods are commonly recommended for marine aquaria. In this image, note the dextral twist of the shell, the extended antennae, the oral opening between them, and the large foot.

(Photo by Trevor Zachariah.)

Arachnids

There are approximately 70,000 different species of arachnids, and more than 80% of the animals in this group are spiders and mites. The majority of arachnids (Figure 3-4) are terrestrial and carnivorous. The arachnids, like the cnidarians, may possess specialized tools (e.g., venom) to capture and kill prey. In some species (e.g., spiders and scorpions), the venom can also be harmful to humans. Certain species of arachnids, such as the mites and ticks, have evolved to live as parasites on vertebrate hosts.

Figure 3-4 Chilean rose spider (Grammostola rosea). Because of its hardy and docile nature, this species of giant spider is commonly found in captivity.

(Photo by Trevor Zachariah.)

Myriapods

There are approximately 13,000 different species of myriapods, all of which are terrestrial. Centipedes (≈3000 species) are from the order Chilopoda, and most of these animals are nocturnal predators. These invertebrates possess fangs, which they use to envenomate their prey or potential predators. Millipedes (≈10,000 species) are from the order Diplopoda, and all of these animals are nocturnal detritivores (Figure 3-5). Diplopods possess more legs than any other animal. Unlike the chilopods, diplopods are secretive and prefer to hide from predators rather than challenge them. Diplopods do not pose any real danger to humans.

Figure 3-5 A fire millipede (Aphistogoniulus sp.). Millipedes of a variety of colors and sizes are available to hobbyists.

(Photo by Trevor Zachariah.)

Insects

The insects represent the largest group of invertebrates, with over 900,000 described species. It is estimated that more than 75% of the animal species on Earth are insects. The greatest diversity among any group of living animals is seen with the insects. Most are terrestrial, but some have also developed the ability to fly. Some species of insects have developed parasitic life cycles, many of which include human involvement. Some species of insects can pose a danger to animals and humans through their defensive mechanisms (e.g., bees). Numerous species are of economic importance to humans as pests.

Crustaceans

There are approximately 42,000 different species of crustaceans, most of which are aquatic. The taxon is a diverse group, with many of the inconspicuous species playing a central role in ecologic webs. Some of the larger species are important to aquaculture.

Echinoderms

There are approximately 6,000 different species of echinoderms. Many of these animals are common to the commercial aquarium trade, including the sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers, and sea lilies (Figure 3-6). All of these species are marine and benthic. The echinoderms share a five-part radial body plan, also known as pentamerous symmetry, and have the ability to voluntarily move connective tissue, known as catch connective tissue (see Anatomy and Physiology).

Figure 3-6 A common sea star found in the marine trade. These echinoderms remain quite popular with hobbyists.

(Photo by Trevor Zachariah.)

Cnidarians

Whether in the form of a polyp or medusa, all cnidarians have a basic body plan that is radially symmetric. There are two tissue layers: the epidermis, which lines the outside of the animal, and the gastrodermis, which lines the inside of the animal. These layers are separated by a nonliving layer of elastic, gelatinous material known as the mesoglea, which provides structure and buoyancy without metabolic cost. Tentacles ring the mouth, the single opening to the digestive system (Figure 3-7). All surface tissues perform direct gas exchange.

Figure 3-7 Sebae anemone (Heteractis sp.). In the center of the image is the slit mouth of the anemone, surrounded by a number of tentacles.

(Photo by Trevor Zachariah.)

In the tentacles, and sometimes in the living tissue layers of the body, cnidocytes can be found. Cnidocytes contain a specialized, secreted organelle called a cnida. When stimulated by chemical or tactile cues, the cnidae use osmotic pressure to propel a hollow tube with great force out of the cell toward prey or predators. The tube can be used to physically subdue or inject venom into prey. The venom used in cnidae can be quite potent and may also be used for defense. Each cnida can be used only once, after which the cnidocyte must secrete a new one.

The nervous system is comprised of a web of neurons, and attached to it are sensory systems that belie the simplicity of this web. Statocysts assist with balance, ocelli sense light, and sensory lappets register tactile stimuli. Box jellies take this one step further and have complex eyes that may be capable of forming images.

The muscles of cnidarians are comprised of epitheliomuscular cells. Cnidarian muscles serve two purposes: (1) contractile functions to assist with locomotion, and (2) intracellular food digestion. The muscles are arranged in longitudinal and circular arrangements, allowing for a variety of types of movement. The most common locomotive movement for jellyfish is jet propulsion, and that is accomplished by contracting and releasing the body against the mesoglea, which acts as a spring.

The mouth opens into the coelenteron, a cavity that is lined with the gastrodermis. The coelenteron may be partitioned into pouches or canals to increase the surface area of the gastrodermis and distribute nutrients throughout the animal. Many species also harbor mutualistic algae, known as zooxanthellae, which provide another source of nutrition.

Cnidarians have an amazing ability to heal and regenerate. This is also reflected in the fact that asexual reproduction by budding or fission is common to cnidarians. Sexual reproduction also occurs, and most cnidarians are gonochoric. Fertilization is external and is followed by the development of a ciliated, planktonic, nonfeeding larva, known as the planula.

Gastropods

Many gastropod species are characterized by the presence of a shell. The shell of most species is coiled in a dextral manner (e.g., right-handed), whereas a few species possess sinistral shells (e.g., left-handed). The central axis of the shell is the columella, and the opening at the base of the shell is the aperture. The first revolution of the shell is known as the body whorl, and this is where most of the visceral mass of the animal resides. The remaining whorls are collectively known as the spire, which culminates at the peak, or apex, of the shell. There are also many species with reduced or nonexistent shells.

Gastropods are connected to their shells by a columellar muscle, which extends into the foot and to the operculum, if present. The foot is a mass of muscles and connective tissues, and it functions as the locomotory organ of the animal. Some species possess an operculum on the foot, which is a rigid disc that acts as a door to the aperture when the animal is retracted into its shell.

The internal anatomy of gastropods is determined by their development, which includes a 180-degree torsion of the body. This leaves the majority of the mass of the body (and the shell, in those species which possess one) atop the head and foot. The process of torsion also causes a reduction or absence of some of the organs—ctenidia, osphradium, kidney, heart auricle—on the side corresponding to the direction of the torsion (e.g., left side for sinistral and right side for dextral).

The space within the body whorl created by the torsion process is known as the mantle cavity. As the name implies, the mantle cavity is lined with the tissue known as the mantle, which secretes the shell. In the terrestrial species, the mantle cavity is modified into a sac-like structure with increased vasculature and an opening called the pneumostome. Thus equipped, the mantle cavity functions as a primitive lung. In the aquatic species, the mantle cavity contains the ctenidia, or gills, adjacent to which is the osphradium, an organ specialized for chemoreception. The heart lies within the pericardial sac and is located within the mantle cavity. The heart is a single ventricle and supplies hemolymph to an open circulatory system. Some species possess an elongated siphon for intake of water into the mantle cavity.

Gastropods have a tubular digestive system that begins with a mouth and ends with an anus. The basic organs of the digestive tract include a buccal cavity, esophagus, stomach, intestine, and rectum. There are many variations on this theme, including a crop as part of the esophagus, a crystalline style, a gizzard, and a cecum. The crystalline style is found in the stomach of herbivorous species and provides mechanical and enzymatic assistance with digestion. The nudibranchs, or sea slugs, have a highly branched digestive tract. They prey upon cnidarians, and the digestive tract delivers undigested cnidocytes to the many tentacles that sprout from their bodies.

Some gastropod species possess a proboscis within which the buccal cavity resides. Within the buccal cavity is a specialized feeding organ known as the radula. The radula is a layer of chitinous teeth that lie over a mass of cartilage and muscle known as the odontophore. The odontophore supports and moves the radula, which has evolved into many different forms. In some species, the radula serves as a harpoon, whereas in others, it serves as a rasp. The function of the radula is based on the feeding mode of the gastropod. The teeth of the radula vary in number and are continually worn and replaced.

Gastropods sense the world through a variety of organs. Most gastropods have two eyes, which can be found on eyestalks or at the base of the cephalic tentacles. In a few species, the eyes may be capable of forming images; however, the majority of gastropods are able only to sense light through their eyes. Tentacles primarily serve chemoreceptive and mechanoreceptive purposes. The osphradium serves a chemoreceptive purpose. A pair of statocysts can be found in the foot and provide a sense of orientation. Also, a few species of gastropod appear to have magnetoreceptors.

Almost all gastropods reproduce sexually. Primitive gastropods are gonochoric and perform external fertilization. More evolved species, including those that are terrestrial, are hermaphroditic and rely on internal fertilization. The planktonic larval stage of gastropods is called a veliger, and it is characterized by a large, ciliated organ known as the velum. The velum is used for locomotion, food collection, and gas exchange.

Arachnids

The body plan of the arachnids has two major components: the prosoma and the opisthosoma (Figure 3-8). The prosoma is comprised of a fused head and thorax that is covered dorsally by a carapace and ventrally by a sternal plate. The pleurae join the two and are flexible, which allows them to move relative to each other. The opisthosoma, or abdomen, contains the majority of the internal organs. In spiders, the prosoma and opisthosoma are joined by a narrow bridge called the pedicel. In scorpions, the two body segments are fused, and the opisthosoma is segmented and divided into two parts: the anterior mesosoma and the posterior metasoma, or tail. The metasoma is comprised of five to seven segments and, at its terminus, has a telson with a stinger.

Figure 3-8 Mexican redknee spider (Brachypelma smithi). Line A delineates the region of the prosoma, and line B delineates the region of the opisthosoma. The pedicel is the narrowing that connects the two regions.

(Photo by Trevor Zachariah.)

Most of the appendages originate on the prosoma. The most cranial pair of appendages are called the chelicerae. The chelicerae help to grasp and tear prey and bear the fangs in those species that have them (Figure 3-9). After the chelicerae, the next pair of appendages are called the pedipalps. The pedipalps come in a variety of forms and can serve different functions. In spiders, the pedipalps are similar to the legs but lack the metatarsal segment, whereas in scorpions, they terminate with pincers. In both groups of arachnids, the pedipalps are used to grasp prey and assist with copulation. The remaining prosomal appendages are the walking legs, of which there are four pairs. Each leg has seven segments, including (proximal to distal) the coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. The distal end of each leg bears a tarsal claw and, in many species, scopulae, dense tufts of hair that allow the arachnid to climb. The last set of spider appendages originate on the posterior end of the opisthosoma and include the three pairs of spinnerets. The spinnerets are variously modified to meet the needs of each species. Any of the appendages of arachnids may be autotomized, or voluntarily detached, and regenerated after several molts.

Figure 3-9 Mexican redknee spider (Brachypelma smithi). Note the large chelicerae and the attached fangs. The opening of the mouth is found between the bases of the chelicerae.

(Photo by Trevor Zachariah.)

The external openings to the arachnid respiratory organs are found on the abdomen. In spiders, the small openings, or spiracles, allow air to enter the multilayered, internal book lung, so called because hemolymph and air spaces interdigitate and look like the pages of a book in cross-section. Scorpions have four pairs of book lungs, one for each anterior opisthosomal segment. Primitive spiders, like the giant spiders, or tarantulas, have two pairs of book lungs. More evolved spiders have branching, tubular tracheae, which perform gas exchange directly with the tissues. The book lungs are the site of gas exchange. Hemolymph is circulated throughout the body of arachnids by an open circulatory system. The heart is located on the dorsal midline of the opisthosoma, surrounded by a pericardium, and connected to an open-ended arterial system. The movement of hemolymph throughout the body is achieved by a combination of pressure and suction from the heart and its sac. Hemolymph is light blue in color due to the oxygen-carrying molecule hemocyanin, which contains two copper atoms rather than iron as in the hemoglobin of mammals.

The opisthosoma also contains the majority of the digestive system. This includes the extensive diverticula of the midgut where most of digestion takes place. Also in the opisthosoma are the Malpighian tubules and stercoral pocket, which are involved in excretory processes. The gonads and silk glands are also present in the abdomen.

The prosoma contains the anterior portion of the digestive tract, including the diverticula of the midgut, which extend through the pedicel; the sucking stomach; and the mouth, which is just posterior and ventral to the chelicerae. A pair of venom glands are also found in the prosoma of spiders. These glands are under voluntary control and are connected directly to the fangs. The prosoma is highly muscular and contains pseudoskeletal, cartilage-like structures called endosternites, which serve to anchor the muscles. The prosomal muscles maintain hemolymph pressure by contracting and relaxing the carapace and sternal plate, which in turn allows for extension of the appendages.

Sensory perception in arachnids is achieved through a number of specialized organs. Spiders are covered in different types of hairs that allow for sensing of tactile, seismic, and chemical stimuli. Scorpions possess paired pectines, which are paired comb-like organs used to detect chemical and seismic stimuli. The pectines are located caudal to the last pair of legs.¹ Both spiders and scorpions possess eyes, although the number (up to 12) varies. The visual acuity of arachnids can also vary among species. It is generally believed that giant spiders have poor vision, whereas jumping spiders (family Salticidae) can make well-developed images.

In many of the New World giant spiders, hairs are used as a defense mechanism. Urticating hairs, located on the opisthosoma, are small, barbed structures that can be discharged to ward off a threat. Giant spiders use their caudal pair of legs to rapidly kick these hairs into the air. When the urticating hairs settle on the body surfaces of a potential predator, they cause severe irritation. Individual spiders may have more than 1 million urticating hairs on their abdomen, at a density of approximately 10,000 hairs per square millimeter.² The urticating hairs are replaced after each successive molt.

Arachnids reproduce by internal fertilization; however, because of their predatory nature, copulation can be somewhat dangerous. Female spiders produce egg sacs, whereas female scorpions gestate their eggs internally and are ovoviviparous. Maternal care is rare among spiders, whereas scorpions invest significant energy into caring for their young. Female scorpions carry their newly delivered young on their dorsum to reduce predation.

The life span of some scorpion species may reach 25 years.³ The most common species kept in captivity, the African emperor scorpion (Pandinus imperator), has a life span of 3 to 8 years.¹ Male giant spiders live only a short time after reaching sexual maturity and have a life span of 6 to 18 months, on average.⁴ Female giant spiders, however, live significantly longer, with anecdotal reports of individuals surpassing 30 years of age. With proper captive care, it would not be uncommon for female giant spiders to live in excess of 20 years.

Myriapods

The myriapod body plan is elongated and composed of numerous segments (Figure 3-10). Each segment, except for the head and anal segments, bears either one (centipedes) or two (millipedes) pairs of legs, although the first few segments in millipedes bear only one pair of legs. Despite their names, centipedes and millipedes do not have 100 and 1000 legs, respectively. In actuality, these animals generally have 40-60 and 150-200 legs, respectively. Most of the body mass of the myriapod consists of the trunk. Millipedes are cylindrical in shape, with a hard, calcified exoskeleton. Centipedes are dorsoventrally flattened, with no waxy outer cuticle layer. Centipedes are built for speed when catching prey, whereas millipedes are slow and built for powerful digging.

Figure 3-10 Giant African black millipede (Archispirostreptus gigas). Note the large size and long, tubular body form.

(Photo by Trevor Zachariah.)

Besides the legs, myriapods have other appendages that are important for their survival. A pair of antennae are located on the head and the jaws, providing sensory input. Centipedes also possess a pair of forcipules on the first body segment, which are essentially venomous fangs used for acquiring prey. On the anal segment of centipedes is a pair of anal legs. These structures can have various functions, including tactile, defense, and aggression, depending on the species.³

The digestive system in all myriapods is long and tubular. Centipedes have a pharynx and esophagus that represent the majority of the gut length, whereas the millipede gut consists primarily of midgut. Millipedes have salivary glands associated with the oral cavity, whereas centipedes have a variety of glands associated with the pharynx and esophagus.³ The paired Malpighian tubules serve as the primary excretory organs. Millipedes have a layer of tissue that surrounds the midgut that has both energy storage (e.g., glycogen) and detoxification properties.³

The heart is a tubular organ that lies dorsally along the length of the trunk. Ostia act to move blood in and out of the heart. The dorsal pericardial sinus is formed by a horizontal membrane, as is the ventral perineural sinus. The perivisceral sinus is located in the middle of these other sinuses.³ Myriapods have an open circulatory system.

In myriapods, gas exchange occurs via tubular trachea that delivers oxygen directly to the tissues. The trachae open to the environment through spiracles in the exoskeleton. In most species, the spiracles cannot be closed, which greatly hampers water conservation and results in the necessity of maintaining a humid, moist environment. The spiracles are located ventrally in millipedes and laterally in centipedes.

Sensory structures in myriapods consist mainly of the eyespots and antennae. The eyespots consist of a varying number of ommatidia (individual sensory units), depending on the species. In most myriapod taxa, the ommatidia are not clustered in densities high enough to form a true compound eye, such as is found in insects.³ Myriapods are not believed to be capable of forming images. Instead, it has been suggested that these animals are limited to sensing light and movement.³ The antennae are able to sense both tactile and chemical stimuli.

Myriapods are gonochoric and practice internal fertilization. In general, female centipedes are protective of their egg masses until the young hatch and disperse.³ Centipedes generally have a life span of 4 to 6 years; millipedes live for 1 to 10 years.³

Insects

The diversity of insect morphological forms is astonishing. However, all the forms are derived from modifications of a basic plan. For purposes of brevity, those important to captive insects will be presented.

The insect body is divided into three body sections: the head, thorax, and abdomen. The head bears a single pair of dorsal antennae, a variable number of ocelli, a single pair of compound eyes, and the ventral mouthparts. The mouthparts are modified to reflect the feeding strategy of the taxa. For example, sucking mouthparts are found on moths and butterflies (order Lepidoptera); piercing and sucking mouthparts on aphids, cicadas, and assassin bugs (order Hemiptera); cutting and sponging mouthparts on flies (order Diptera); and chewing and sucking mouthparts on bees and wasps (order Hymenoptera).³

The insect thorax bears three pairs of legs and one or two pairs of wings. The legs each have six segments, including the coxae, trochanter, femur, tibia, tarsus, and pretarsus. Each leg ends in a pair of tarsal claws. The wings of an insect can be modified, even into different structures. Two extreme examples of this include the beetles (order Coleoptera), in which the cranial pair are hardened elytra (e.g., wing covers), and the flies (order Diptera), in which the caudal pair are reduced to gyroscopic halteres to aid in flight.

The abdomen of insects is segmented and relatively devoid of appendages. A terminal pair of cerci are usually present as are, in some species, external genitalia.³ The abdomen is often the largest of the three basic body segments and houses the majority of the viscera.

The digestive system of insects is divided into three regions: the foregut, midgut, and hindgut. The foregut consists of the mouth, pharynx, esophagus, crop, and proventriculus. The midgut is the primary site for food digestion. Attached to the anterior end of the midgut are two to six ceca.³ The hindgut is comprised of the intestine, rectum, and anus. A variable number of Malpighian tubules (e.g., 2 to 250, depending on the taxon) are attached to the anterior end of the hindgut.³ The hindgut serves as the major excretory center in the insect body and resorbs most of the water from the digestive system.

The circulatory system of insects is open, and the tubular, ostiate heart is found in the dorsal abdomen. The aorta extends cranially from the heart and is the only hemolymph vessel. An accessory heart is present at the base of most appendages, and facilitates the delivery of hemolymph to the tissues.

Oxygen is delivered directly to the tissues by tubular trachae, in a fashion that is similar to that of myriapods and some arachnids. Spiracles are found on the thorax and abdomen, but not on the head.³ Unlike the spiracles of myriapods, those of most insects can be closed to prevent loss of moisture or water.

In insects, the perception of multiple types of sensory stimuli is performed by sensilla, or hair-like receptors. These structures are found all over the body, although the majority are found on the appendages.³ Chemical, tactile, temperature, and humidity receptors are found on the antennae and tarsi. The abdominal cerci contain tactile and seismic receptors. The ocelli are used to detect changes in light intensity and aid in orientation.³ Visual stimuli are detected by the compound eye, and each of the ommatidia has its own lens. Thus, contrary to popular misconception, the insect brain, similar to the brain of vertebrates, integrates the information from each ommatidium to form a mosaic image.³ Many insects also have tympanic organs for the detection of sound.

Insects are gonochoric and practice internal fertilization. There are three types of development among the insects. Hemimetabolous development involves juveniles called nymphs, which are dissimilar from adults, are aquatic, and grow and molt until reaching a final molt into the adult form. Paurometabolous development involves juveniles that are also called nymphs; however, these are similar to adults and grow and molt into the adult form. Holometabolous development (e.g., metamorphosis) involves juveniles called larvae, which grow and molt until forming a pupa, from which the adult form emerges. Holometabolous development is a successful life strategy, as approximately 80% of insects (e.g., approximately 740,000 species) utilize it.³

Crustaceans

The crustaceans, like the insects, represent a diverse group of animals, with variable body forms. One of the most commonly recognized forms, the order Decapoda (e.g., crabs, lobsters, crayfish, and shrimps), will be described here (Figure 3-11).

Figure 3-11 Louisiana red swamp crawfish (Procambarus clarkii). This species is a typical decapod crustacean. P. clarkii are farmed extensively for human consumption.

(Photo by Trevor Zachariah.)

The crustacean body is comprised of two sections: the head and trunk. The head is small relative to the trunk and, along with the anterior of the trunk, is covered by a carapace. Five pairs of appendages adorn the head: two pairs of antennae, a pair of mandibles, and two pairs of maxillae. The maxillae assist with feeding. The head also bears one pair of compound eyes that are located on movable stalks.

The anterior section of the trunk bears eight pairs of appendages. The anterior three appendages, or maxillipeds, assist with feeding, while the posterior five, pereopods, are used for walking. There are seven segments of the pereopods, including the coxae, basis, ischium, merus, carpus, propodus, and dactyl. In many species (e.g., crabs, lobsters, and crayfish), the first pereopod is modified into an enlarged cheliped or pincer that is used for defense, food acquisition, and courtship. Limb autotomy is a common occurrence in decapods and most often occurs as a result of combat with conspecifics or in defense against predators. The limbs regenerate with later molts.

The posterior of the trunk is comprised of a variable number of segments, depending on the taxon. The terminal end of the trunk bears a telson and paired uropods, and together, these form a tail fan. The tail fan helps to create the backward thrust of shrimp, crayfish, and lobsters. The five pairs of appendages arising from the posterior trunk are the pleopods. The pleopods are biramous and may be modified to serve a variety of functions, including swimming, burrowing, creating ventilating or feeding currents, brooding eggs, gas exchange, and copulating.³ In crabs, the posterior abdomen and pleopods are reduced and found ventral to the carapace.

The digestive system begins with the cranial mouth and leads into a two-chambered stomach. In the stomach, both mechanical and chemical digestion of food occurs. Absorption of food occurs in the ceca, which are connected near the junction of the stomach and intestine. The intestine follows the length of the abdomen and terminates at an anus in the telson.

Decapods have a compact, ostiate heart. This organ is located dorsally under the carapace. The heart is connected to a system of arteries (seven main arteries leave the heart), capillaries, and venous sinuses.³ Hemolymph is transferred through the circulatory system to the gills for oxygenation. The gills, of which they may have up to 24 pairs, are also responsible for excreting nitrogenous wastes.³ Ion balance in crustaceans is maintained primarily by the antennal glands (e.g., green glands), which are located in the cranial aspect of the head. Urine is created and stored in a bladder that opens near the base of the ventral pair of antennae.

Sensory perception in crustaceans is accomplished by the eyes, antennae, and appendages. The stalked eyes are some-what mobile, and some crustaceans may be able to detect color.³ Setae (hair-like receptors) are capable of detecting chemical stimuli and are primarily found on the antennae and appendages. Aesthetascs, or collections of setae, are located on the dorsal pair of antennae. Statocysts are found at the base of the dorsal antennae and assist with orientation. In some crustaceans, statocysts may also be found on other appendages.³

Crustaceans are primarily gonochoric, with a few (hermaphroditic) exceptions. Internal or external fertilization is possible, depending on the taxon. In most decapod species, females carry the eggs until they hatch. The egg masses are held on the ventrum by the pleopods. Decapods go through an indirect development, and there can be a number of larval stages. Other crustacean taxa may have direct or indirect development.

Echinoderms

The body plan of echinoderms follows pentamerous symmetry. Though a type of radial symmetry, the echinoderms are not closely related to the cnidarians. Pentamerous symmetry is based on a central axis around which five body regions aggregate. The body surfaces are described as oral and aboral (Figure 3-12). Sea cucumbers (class Holothuroidea) maintain pentamerous symmetry in an elongated body form, with the ends of the animals being described as oral and aboral.

Figure 3-12 Bubble tip brittle star (Ophiarachna sp.). This is a view of the oral side of the central disc of a brittle star. In the center of the image is the opening of the mouth.

(Photo by Trevor Zachariah.)

Besides their recognizable body forms, another unique feature of echinoderms is the water-vascular system (WVS). The basic anatomy of this system starts with a madreporite, an eccentric, porous opening that is found on the aboral surface of sea stars and urchins and on the oral surface of brittle stars. The madreporite opens into a stone canal, which then leads to a circumoral ring canal. Leading perpendicularly from the ring canal are the radial canals and blind sacs known as polian vesicles. The radial canals reach into the arms of sea and brittle stars and along the inside to the aboral surface in sea urchins and cucumbers.⁵ Multiple lateral canals direct water from the radial canals to ampullae, each of which is attached to a tube foot. The ampullae are used in a manner that is similar to the bulb of a turkey baster, increasing pressure to extend the tube feet and decreasing pressure to contract them. The function of the polian vesicles has not been fully determined, but it may serve in aiding maintenance of fluid pressures within the WVS.

The madreporite and stone canal function to maintain fluid pressure within the WVS. The stone canal is supported by calcareous ossicles and is lined with cilia, which beat to create water flow. The fluid within the WVS is essentially seawater, with increased cellular, protein, and potassium concentrations.^3,5

The body wall of echinoderms is comprised of regions called ambulacral areas and interambulacral areas. Ambulacral areas represent those regions that bear the tube feet. These two types of areas generally alternate around the oral-aboral axis of sea urchins and cucumbers, whereas only ambulacral areas are found on the oral side of the arms of sea and brittle stars. The exoskeleton of echinoderms contains numerous ossicles, comprised of calcite microcrystals, embedded in the dermis. Ossicle structure can vary at the species level and is often a trait used to identify echinoderms. Some ossicles are modified for specific purposes, such as the paxillae of burrowing sea stars. In these animals, the ossicles form a protective shield for the aboral surface. Pedicellariae are specialized ossicles that occur on many different echinoderms. Pedicellariae are stalked or sessile ossicles that have a set of jaws and are used for defensive purposes. Sometimes these pedicellariae are equipped with poison glands (see Human Health Hazards).

Another unique feature of echinoderm anatomy is catch connective tissue. This tissue is mutable, which allows these animals to vary the rigidity of their bodies at will. The stiffness or softness of the dermis is due to the extracellular matrix, in which nerves have been found to terminate.³ Research has found that calcium ion concentrations vary proportionally to the rigidity of the tissues.³

Cnidarians

As with most aquatic species, water quality is the most important factor associated with the successful management of cnidarians in captivity. Stoskopf⁷ recommends the following guidelines: ammonia levels less than 0.1 ppm, nitrite levels less than 1 ppm, nitrate levels less than 10 ppm, dissolved organic matter levels between 0.5 and 3.0 ppm, undetectable phosphate levels, calcium levels between 400 and 450 ppm for corals, pH between 8.2 and 8.4, alkalinity between 3.2 and 4.5 mEq/L, and a specific gravity (as a surrogate measure for salinity) around 1.025 to 1.027. Trace elements are another important consideration, and these animals depend on the water to provide these essential nutrients. Unfortunately, little is known regarding the specific needs of these animals, so attempts should be made to mimic natural levels of trace elements based on the natural body of water from which these animals are derived.

Water motion is particularly important for cnidarian species (Figure 3-13). Cnidarians are generally classified as being either mobile or sessile. Water motion is essential for both groups because it facilitates nutrient gathering and oxygenation. For mobile species, water motion is important also for transporting the organisms.⁷ Because sessile species cannot move away from their wastes or accumulated organics, water motion serves to disperse potential toxicants. Cnidarians exposed to excessive water motion can be injured. The force of the water movement can push cnidarians into the walls of the aquarium or other fixed objects within the aquarium.⁷ Even sessile cnidarians can be injured by excessive water movement, as the fixed organisms are battered by the substrate. Water motion can be provided by way of a power head, airstone, or wave maker. Water motion is generally measured in gallons of water moved per hour. Determining the most appropriate flow rate depends on aquarium size and volume. The authors generally look at the movement of the cnidarians in the aquarium to deter-mine what is best. If the cnidarians appear to be moving too quickly or are being battered against the aquarium or substrate, then the flow rate should be reduced.

Figure 3-13 Cnidarians are dependent on constant circulating water flow. If these animals are placed into an inappropriate system they can be damaged.

(Photo by Mark A. Mitchell)

Due to their diversity, the cnidarians as a group are well adapted to a variety of temperature ranges; however, individual species may have a relatively narrow tolerance for temperature changes.⁷ As a general guideline, tropical anemones and corals should be kept between 20° and 31° C (68°-87.8° F), with an optimal temperature being around 24° C (75.2° F).⁷ For temperate species, a temperature range of 20° to 24° C (68°-75.2° F) is considered more appropriate.⁷ Thermostatically controlled aquarium heaters are the best method to provide an appropriate temperature range within an enclosure. In larger aquaria, multiple heaters may be required to establish an appropriate temperature range. Thermometers should be placed in different areas of the aquarium to monitor temperature.

Many cnidarians derive a significant amount of their nutrients, sometimes up to 90%, from symbiotic algae (e.g., zoochlorellae or zooxanthellae) that are embedded in their tissues.³ The loss of these organisms, due to improper lighting or water quality conditions, can be devastating for a cnidarian. Full spectral lighting that mimics the sun is considered ideal. The light should provide ultraviolet and visible light. Stoskopf⁷ recommends photosynthetically active radiation (e.g., 400-700nm) with more flux density between 400 and 550nm than between 650 and 700nm. The lighting should be maintained close to the water surface (<6 cm) to maximize its value.

In some cases, high-intensity lighting can overheat the water, and the lighting positioning may need to be altered. Certain components of full-spectrum lighting, such as ultraviolet radiation, are lost within the upper surface of the water. The photoperiod for cnidarians should be set on a 12-hour cycle. If animals are to be reproduced, the day may need to be lengthened (13-14 hours) for some animals. The intensity and diversity of light provided should be based on the cnidarians’ natural position in an aquatic system. For example, animals found in deep-water benthic systems need less intense lighting than those found in shallow coral reef systems. Individuals working with potent full-spectrum lights should wear appropriate protective eyewear and minimize direct skin exposure to reduce the likelihood of developing secondary health problems.

The diet of cnidarians varies with the species of interest. In general, cnidarians are fed on a weekly basis.

Gastropods

Gastropods represent another diverse group of organisms. To simplify the captive care of these animals, it is best to separate them into one of two categories: aquatic or terrestrial. It is important to identify into which group a particular species fits to ensure that they are provided the most appropriate captive conditions.

Aquatic species of gastropods exhibit a great diversity in size, shape, and habits, but most can be kept by following the same basic husbandry practices used for other aquatic animals. The most important aspect of husbandry for aquatic gastropods is the water environment in which they live. The water parameters important to these animals are the same as those for other invertebrates and fish—that is, ammonia, nitrite, nitrate, pH, hardness, alkalinity, and temperature. Gastropods should be maintained in systems that have less than 0.1 ppm ammonia, less than 0.1 ppm nitrite, less than 10 ppm nitrate, a neutral pH, and moderate alkalinity and hardness. Hardness can be especially important to developing gastropods, as this is an important source of calcium and magnesium. The water temperature of an aquatic gastropod aquarium should be based on the animal’s natural climate. Temperate species are more tolerant of temperature fluctuation (e.g., seasonal) and may require a reduction in temperature for aestivation. Most gastropods can derive sufficient oxygen from a nonaerated aquarium; however, aeration may be needed in systems with mixed species (e.g., gastropods and fish) when animal densities are high. Live plants may serve as an important substrate for some aquatic gastropods, especially for those species that lay their eggs on plant leaves.

Aquatic gastropods are generally opportunistic feeders. Most species will consume both animal and plant material. A number of species are detritivore specialists, which endears them to aquarists, whereas others are plant specialists and are not well liked. Invertebrate zoology or gastropod biology texts can provide specific foodstuff recommendations for particular species. Vegetable material, such as romaine lettuce, squash, and zucchini, or animal material (e.g., frozen-thawed silversides) are appropriate offerings.

Gastropods spend the majority of their time in contact with a fixed surface. Because of this, any substrate (surface) in an enclosure should be kept clean. Flat surfaces, both horizontal and vertical, are preferred for gastropod vivaria because they do not impede the movement of the animals. These surfaces are also easier to clean.

Terrestrial species are easily kept in terraria of different sizes; however, vertical enclosures are preferable for arboreal species and horizontal cages for terrestrial species. A moist substrate, such as damp sphagnum moss, should be used to provide moisture and will help to maintain a relatively high humidity level in the enclosure. The substrate should be changed periodically to reduce the likelihood of opportunistic pathogens. Gastropods left on “dirty” substrate are more prone to dermatitis and shell lesions. The frequency of substrate changes will depend on the number of animals, size of the vivarium, and frequency and types of food offered. Cork bark can be placed in the enclosure to provide hiding places for the animals. Because of gastropods’ ability to climb perpendicular surfaces, a tight-fitting, solid lid is recommended (e.g., glass top). This will also help to maintain the high humidity level. Although humidity is important, it is just as important that the air in the enclosure does not become stagnant. Stagnant air can lead to the overgrowth of certain pathogens that can affect gastropods. Gastropods are most comfortable and active with low-intensity lighting. Temperature requirements will vary depending on species; for some species, temperatures exceeding 23° C (73.4° F) should be avoided.⁸ Again, knowledge of where an animal originates can help a veterinarian determine the most appropriate temperature range.

The nutritional requirements for most terrestrial gastropods can be met by offering fresh fruits and dark green, leafy vegetables. These items can be fed to gastropods even if they have slightly passed their expiration point for human consumption; however, these food sources can be contaminated with opportunistic pathogens and should be offered only if the food is properly washed. Terrestrial gastropod diets can be supplemented with limited amounts of dry dog food, as well. Captive animals should be offered food ad lib, as these animals can consume large amounts of food on a regular basis.

Arachnids

The size of an enclosure for a giant spider does not need to be expansive, as most species are not large in size or extremely active. Also, they should not be kept communally, because cannibalism is possible. A 35.6 cm × 25.4 cm × 25.4 cm (14″ × 10″ × 10″) glass container can be used as a basic enclosure and provides ample space for all but the adults of the largest species of giant spiders (e.g., Theraphosa blondi, Lasiodora parahybana, Pseudotheraphosa apophysis). In addition to glass tanks, plastic storage containers, large plastic or glass bottles, and glass or plastic fish bowls can be used. A distinction can be made between arboreal and terrestrial species of giant spiders. With terrestrial species, care must be taken to keep the height of the enclosure to a minimum. All spiders are capable of climbing vertical glass and plastic surfaces and can suffer life-threatening injuries from short falls (see Common Disease Presentations). Arboreal species, however, should be provided an enclosure with a high vertical-to-horizontal ratio (e.g., 3 : 1) (Figure 3-14).

Figure 3-14 Basic arboreal spider habitat. Note the large vertical-to-horizontal ratio.

(Photo by Trevor Zachariah.)

In many cases, the size of the enclosure should be commensurate with the size of the animal. This is especially true for young animals (e.g., breeders often sell spiderlings of approximately 1 cm in length), which could easily become lost in a large terrarium. If an enclosure is capable of accommodating substrate and the appropriate accessories, then it should be adequate in size.

Whatever type of enclosure is used, a secure lid is essential to prevent escape. A missing spider of considerable size can prove to be an uncomfortable situation! Many of the glass tanks and plastic containers made specifically for pets have appropriate lids and locking mechanisms. Other types of enclosures need to have their lids weighted down or specifically constructed for them. The lid should be safe for the animal contained within the enclosure (e.g., giant spiders can injure themselves by catching their claws in the wire junctions of woven screen mesh).

Temperature and humidity are important factors to consider in caring for giant spiders. Again, some knowledge of the animals’ natural habitat is important. Enclosures should be monitored with a thermometer and hygrometer on a regular basis. Optimal temperature ranges vary; however, most fall between 21.1° and 32.2° C (70°–90° F). Therefore, most species can be maintained at ambient room temperature, with few exceptions. Most direct heating methods (e.g., under-tank heaters and heat lamps) are not needed and can reduce humidity levels to life-threatening levels.

For giant spiders, humidity must be closely regulated, even more so than temperature. Excessive humidity can lead to potentially harmful fungal and pest infestations within an enclosure. Insufficient humidity can lead to desiccation and dehydration. There are several ways to regulate humidity: change (1) the moisture in the substrate (e.g., higher water-to-substrate ratio), (2) the size of the water dish, or (3) the amount of ventilation of the enclosure. To increase humidity, the substrate moisture and the water dish size should be increased, while the amount of ventilation is decreased. To reduce the humidity within an enclosure, the opposite should be done. Lightly misting the enclosure with lukewarm water (24.4°-26.7° C, 76°–80° F) can also be done to increase humidity. For large vivaria, live plants can be used to increase humidity. For giant spiders, a general rule is that tropical species require 70% to 100% humidity, desert species 40% to 60% humidity, and temperate species 50% to 70% humidity.^9,10

Giant spiders can vary in their daily periods of activity, and no broad generalizations can be made for this group. For most species of giant spider it is not known whether the animal is diurnal, nocturnal, or crepuscular; however, it is known that all species appear to be extremely averse to bright light.¹⁰ For this reason, giant spiders should not be kept under a direct light source. Also, as stated earlier, direct light sources (e.g., incandescent bulbs) can desiccate an enclosure and its resident. Direct sunlight has the same effect and should be avoided. Low or ambient lighting works well for most species, and a 10- to 12-hour photoperiod is recommended.

There are a variety of substrates available, either commercially or naturally, for use in giant spider enclosures. Organic materials are usually preferred. Examples include topsoil, potting soil, peat moss, sphagnum mass, bark or mulch (except pine or cedar), ground coconut hull, and leaf litter. All of these substrates are good at enhancing humidity levels in an enclosure, but they are also prone to supporting fungal and pest infestations.^9–11 To reduce the likelihood of opportunistic infestations, the material should be dry or only slightly damp.

Nonorganic materials are also popular substrates for giant spiders. Examples include vermiculite, aquarium gravel, artificial turf, and sand. Compared to organic substrates, these materials are not as efficient at maintaining humidity. Sand and aquarium gravel are not recommended, as they can abrade the exoskeleton of a giant spider and provide unstable footing. Many of these nonorganic substrates can be mixed in various ratios to provide an appropriate environment. For burrowing species, the substrate should be packed well and be coarse enough that the risk of burrow collapse is minimized. For giant spiders, Marshall⁹ recommends a substrate depth of 8 to 20 cm for terrestrial species and 2.5 cm for arboreal species. Substrates that are strongly contraindicated include cat litter (too dusty), carpet (potential injury from snagging), newspaper (pest and fungal problems), and wood chips (pest and fungal problems, pine and cedar toxicity).^10,11

Accessories added to a giant spider enclosure can be essential for mimicking the animal’s natural habitat and thus behavior, or can be used for decorative purposes. An essential addition for most species is a shelter of some kind. Types of materials that make good shelters include a flower pot on its side, a flat rock, a sturdy piece of driftwood, cork bark, a real or plastic log, a paper towel tube, an egg crate, or a plastic or live plant. Arboreal species need materials upon which they can climb. Terrestrial species will often dig their own shelter or adopt and modify materials to suit their needs.

Live plants are not essential components to making a vivarium. Although they add a decorative touch, they are not needed to properly maintain most species. Plastic plants can be used and are easier than live plants to disinfect. Live plants used in a giant spider vivarium must be relatively small in size and able to tolerate low light levels. Cacti, or plants with spines or sharp edges, should be avoided because they can be injurious to the spider.⁹ Marshall¹¹ recommends snake plants (Sansevieria spp.), bromeliads (Cryptanthus spp.), peperomias (Peperomia spp.), and climbing plants (e.g., philodendrons (Philodendron spp.) or pothos (Epipremnum spp.) for giant spider enclosures.

Giant spider enclosures need to be cleaned and disinfected on a regular basis. The frequency of cleaning will depend on the amount of waste produced by the animal, the quantity of leftover prey, and the type of substrate used. Remaining food items should be removed after each feeding. A thorough cleaning is recommended if there is a visual buildup of waste material or whenever an animal molts. Dilute dishwashing detergent or bleach (0.2%-0.5%) can be used to disinfect an enclosure. The enclosure and all accessories should be thoroughly cleaned and rinsed before replacing the spider.

Crickets are the most common prey item offered to giant spiders.¹¹ Although crickets do not provide a balanced diet for vertebrates (e.g., inadequate calcium), they appear adequate for invertebrates. However, a varied diet comprised of different invertebrate and vertebrate prey species is still considered more appropriate.⁹ Cockroaches, grasshoppers, mealworms, superworms, kingworms, wax moth larvae, earthworms, and neonatal mice may be used to vary the diet.^8,12 Small lizards, crayfish, goldfish, and raw meat have also been given to giant spiders but are less common and generally not recommended.¹⁰ Captive-reared prey is preferred over wild-caught prey. Wild-caught prey can serve as a source of toxin exposure or infectious disease.¹³ All prey items should be offered a high-quality diet before being offered to the spider. If the prey items are not promptly fed to the spider, they will be of limited nutritional value.

Many giant spider species are stimulated by movement of their prey, so it is usually best to feed live prey items when appropriate. Exceptions to this include vertebrate prey that may injure the spiders. When live prey items are offered, they should be monitored carefully to ensure that they do not harm the spider. This is especially true during the molting process, when the exoskeleton is more fragile; a giant spider should never be fed during a molt. Prey of appropriate size should be offered. A good rule of thumb to follow for spiders is that the prey should be no larger that the spider’s opisthosoma,¹⁰ or one third to one fourth the length of its body.¹² If any prey is left uneaten after 24 hours, it should be removed. The quantity of food offered to a giant spider should be based on the dietary habits of the particular species, its life stage, and the environmental conditions. For example, adult giant spiders should be fed on a weekly^9,12 or, at a minimum, monthly basis,¹⁰ whereas spiderlings should be fed every 2 days.¹²

Water should always be made available to giant spiders, as they will actively drink water. Open water dishes should be provided, though the depth should be limited to prevent drowning. Some sources recommend placing a soaked sponge or cotton in the water dish; however, this is not considered hygienic and can serve as a source of opportunistic bacterial infections for the spider.^10,13 For giant spiders, the water dish must be positioned so that the animal can tilt its mouthparts down to the surface (Figure 3-15).^8,10 The water dish should be cleaned and refilled regularly, or as often as it is soiled.

Figure 3-15 Haitian brown spider (Phormictopus cancerides). This spider is drinking water. Note the position of the dish, the top of which is level with the surface of the substrate. The spider is able to tilt its mouthparts down to the water.

(Photo by Trevor Zachariah.)

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