11.2.1 External Gross Anatomy

Anatomy of the adult form has been described (Bell and Lightner 1988; Felgenhauer 1992a; Johnson 1980; Phillips et al. 1980). The body is covered by a nonliving cuticle also termed a carapace (Figure 11.1). The cuticle is formed from two types of tissues. First is the hard carapace that makes up the majority of the cuticle. The second is the soft rubber‐like carapace, termed arthrodial membrane, which connects the hard carapace pieces and, together with muscle, etc., provides for flexing of the hard carapace, thus forming joints. Externally the lobster’s body is elongated and divided into a cephalothorax and an abdomen. The hard carapace that makes up most of the cephalothorax is further divided into head and thoracic regions by various indentations on the carapace. The dorsum of the head forms a hard arrow‐like protuberance termed a rostrum that covers the eyes and other parts of the head. Crabs and some other decapods lack a rostrum. Both articulated large and small spines and nonmovable spines, as well as other protective and sensory projections of the carapace, are noted on various species of decapods. Embryologically, appendages of decopod crustaceans originate as a protopodite that attaches to the body. Projecting from the protopodite are both a endopodite and an exopodite. If the appendage contains both of these branches it is termed biramus. If either the endopodite or exopodite regresses during development, the appendage is termed uniramus. The protopodite itself is further divided into two segments, the lateral coxopodite (coxa) and the medial basiopodite (basis). As the decapod crustacean larva develops, each of these portions of the appendage further differentiates into one or more specific structures (i.e., antennae, gill, gill bailer, etc.) (Pearse et al. 1987).

Decapod crustaceans have two pairs of antennae (Figure 11.2). Each pair is composed of both a small and large antenna. The smaller of the two is located just below the eyes and is characterized by two flagella of about equal length extending from the protopod. The larger antennae are lateral to the smaller and the protopod of each contains the prominent nephridiopore.

The bilateral mandibular appendage forms the ventrum of the mouth and has surfaces that both grind and cut food. A palp extends anteriorly from the mandibles in American lobsters (Figure 11.2). The bilateral first and second maxilla form the ventral parts of the mouth. The scaphognathite (gill bailer) originates from the endopod of the second maxillae and is used to ventilate the gills.

The thorax is formed by the fusion of eight embryological segments (somites). The first three segments are termed maxillipeds (Figure 11.2) and are biramous. From the coxa, in maxillipeds two and three, extends a gill and an epipodite. Laterally from the basis of maxilliped three extends a prominent leg‐like segmented structure that is used in breaking down food and in grooming (Figure 11.2) (Felgenhauer 1992a; Phillips et al. 1980; Ruppert et al. 2004).

Five pairs of legs, termed pereiopods, extend ventrally from the posterior five cephalothorastic segments (somites). They are considered uniramous since they do not have an exopod originating on the protopodite. Each leg can be divided (proximally to distally) into the following segments: coxa (coxopodite), basis (basipodite), ischium (ischiopodite), merus (merepodite), carpus (carpopodite), propodus (propopodite), and dactylus (dactylopodite) (Figure 11.3). In some swimming crabs (such as the blue crab, Callinectes sapidus), the propodus and dactylus are flat and broad and provide oar‐like structures.

A cheliped (claw) is formed by a rigid extension (finger) of the propodus that interacts with the dactylus (movable finger). In many lobsters and crabs, the first three pair of legs are modified and form chelipeds. In the American lobster, the first pair of chelipeds are greatly enlarged and are differentiated into a crusher and a cutter claw. Some species show a subchelate claw in which the dactyl folds against the propodus to form a claw‐like structure.

In males, bilaterally, at the base of the fifth leg on the medial side of the coxa are the gonadopores (Figure 11.4). The female gonadopores are located on the medial side of the coxa of the third walking legs (Figure 11.5). A seminal receptacle, composed of two lateral chitinized walls with a deep anterior to posterior linear pocket that extends dorsally into the body of the animal, is located on the midline between the fourth and fifth pereiopod in female lobsters and other similar decapods (Figures 11.5 and 11.6). It is used for the storage of spermatophores. In some species, such as the ghost crab (Inachus phalangium), spermatophores are placed into a seminal receptacle located within the end of the gonadopore. Seminal receptacles are storage chambers that retain sperm till spawning occurs (Krol et al. 1992).

Dorsally and medially, the carapace covers internal organs. The dorsal surface of the carapace shows indentations which demarcate underlying organ locations. These indentations are especially prominent on crabs. Laterally, the thoracic carapace extends from the body wall to cover the gills. This extension is termed the branchiostegite, and forms the outer wall of the large bilateral branchial chambers. The inner wall of the chamber (the lateral wall of the cephalothorax) is formed by a portion of the carapace termed the epimeron. Branchia (gills) extend from the base of each of the bilateral thoracic appendages and adjacent ventrolateral carapace (arthrodial membrane of the epimeron) and extend dorsally within the branchial chamber. In some crabs, the branchiostegite is expanded laterally and the thinly chitinized membrane lining the inner surface of the expanded branchiostegite forms respiratory lobules that project into the seawater‐bathed space around the gills, thus forming “lungs” lateral to the gills.

Epipodites originate from each coxa adjacent to the branchia of the second and third maxilliped and all five thorasic segments. Epipodites in each segment partially separate and maintain the branchia (gills) (Figure 11.7). The branchia (1–4 gills per segment/leg) are named according to their origin as a podobranch which originates from the coxa of each segment, arthrobranchia which originate from the arthrodial membrane between the coxa and the body wall (up to two may be present), and a pleurobranch which originates from the body wall (epimeron). Rarely are all four possible gills present in each segment.

Three different basic types of gills are noted in decopod crustaceans (Fox 2001; Ruppert et al. 2004; Taylor and Taylor 1992). Trichobranchiate gills are noted in lobsters, crayfish, and several crab species (Figure 11.8a,b). In these species, the gills taper to a point as they reach the dorsum of the branchial chamber. Phyllobranchiates, which include many types of crabs and shrimp, have gills composed of leaf‐like lamellae that extend from the central core (axis) of the gill (Figure 11.8c,d). Dendrobranchiate gills found in penaeid shrimp are composed of a central gill axis from which bilateral secondary lamellae and tertiary filaments extend (Figure 11.8e,f). The secondary lamellae tend to curve toward each other distally from the gill axis.

Oxygen‐carrying water enters the branchial chamber ventrally at the base of the cheplid (modified first legs) and between the remaining legs, and flows over the gills and out via the branchial chamber through exhalant pathways. Exhalant spaces are located dorsally to an area lateral to the mouth. This arrangement can produce one‐way water flow that may increase oxygenation. The scaphognathite of the secondary maxilla helps to create a current that pumps the water in and out of the branchial chamber.

The abdomen is divided into six segments (somites). Dorsally, in lobsters, each segment is covered by a hard carapace. The segments are connected to each other by arthrodial membranes that allow for overlapping of the segments and flexion of the abdomen. Ventrally, the abdominal carapace is composed of bands of hard carapace separated by wide bands of the arthrodial membrane (Figures 11.4 and 11.5). Bilaterally and ventrally are the pleopods (swimmerets). The second to fifth pair are biramous with flat rami (endopod and exopod) edged with a setal fringe (setae are described later). The pleopods help with swimming in the water column. In the Pleocyetmata, fertilized eggs are attached to the pleopods until they hatch. The first pair of pleopods are modified for reproduction. In male lobsters and most other decapods, the first pair of pleopods are highly modified to form gonopods (Figure 11.4), which are chitinized thin structures with a central depressed canal used for transferring spermatophores from the male gonadopore to the female. The canal size and shape vary by species from being a central depression in the lobster to an almost totally enclosed canal in some decapods. The first pleopods in females are smaller than pleopods 2–5, and similar in shape to the male gonopods, but are not highly chitinized as in the male (Figure 11.5).

The tail fan is composed of a central telson and bilateral uropods. The anus empties ventrally on the telson. The bilateral uropods (highly modified pleopods) extend from the base of the telson. Each uropod is composed of a protopod that attaches to the telson base and two large flat leaflets (highly modified endopod and exopod) (see Figure 11.1).

Unlike most other decapods, the cephalothorax of crabs is dorsoventrally flattened and laterally widened (Johnson 1980). In crabs, the abdomen is greatly reduced in size and is retroflexed back against the ventrum of the cephalothorax. The first and second abdominal segment are modified for reproduction. The remaining abdominal segments together form a broad flap (in females) or wedge‐shaped abdominal flap ending in a pointed telson (in males) (Figure 11.9). A thin cuticle covers the retroflexed ventral surface of the abdomen. Through this cuticular surface, muscle can be noted running parallel and on either side of the intestine, which empties at the anus on the telson. Pleopods are not present on segments 3–6 of the male, but are present in the female and are used for egg attachment.

11.2.2 Internal Gross Anatomy

The body wall (the thin membrane lining the carapace) can be noted upon dissection and can be easily removed from the inner surface of the carapace. Chromatophores are often noted in the membranes underlying cuticles (Figure 11.10). Internally within the cephalothorax, the heart is noted dorsally and posteriorly. Underlying it are the gonadal tissues (Figures 11.11 and 11.12).

The ovaries of female decapods are bilateral and may be composed of both anterior and posterior lobes (Figure 11.12). In the lobster, the green anterior and posterior lobes are connected by a bridge underlying the heart forming an H‐shaped organ. When the ovary is filled with ripe eggs, the lobes of the gonad extend into the dorsal abdomen. Oviducts move eggs ventrally to the gonadopores. In males (Figure 11.11) the white, knobby string‐like lobes of the testes, as in the female, extend anteriorly and posteriorly and connect to form an “H” ventral to the heart. Convoluted, long, white, bilateral vasa deferentia move sperm to the gonadopores. The number of lobes of ovary and testes vary between types of decopod crustacean. Shrimp, crabs, and some lobsters have multiple lobes.

Ventrally, the esophagus opens into the cardiac stomach. The cardiac stomach is lined externally by a membrane (similar to a serosa) and internally by a thin layer of chitin. Laterally in the gastric stomach wall, oval white gastroliths, which can store calcium before molting, can be identified (soft if postmolt and mineralized if premolt) (Figure 11.13). At the dorsolateral side of the junction between the cardiac stomach and the pyloric stomach are the two parts of the gastric mill (described later) (Figures 11.13 and 11.14). Ventrally within the pyloric stomach is the winged portion (pyloric filter press) of the ampullary filter complex, which conducts fine particles of food to the hepatopancreas. Ventral and lateral to the gonadal tissues is the large lobulated light green hepatopancreas. The dorsal chamber of the pyloric stomach leads to the midgut which continues dorsally through the abdomen to the anus on the ventral surface of the telson. In the head portion of the cephalothorax, just underlying the antennae and applied to the carapace membrane is the flattened, discoid dark green (in the American lobster) antennae gland (kidney). Ventromedial to the green gland and antennae is the supraesophageal ganglion and the circumesophageal connectives that lead around the esophagus to the subesophageal ganglion. The neural cord extends from the subesophageal ganglion and continues along the ventral body wall parallel and adjacent to the ventral thoracic and abdominal arteries (Figure 11.15).

11.3.1 Cuticle

The cuticle (carapace, shell) forms the outer surface of the animal as discussed above. The carapace is a complicated structure that undergoes molting (ecdysis) periodically in order for the internal tissues to grow. It acts both as an external skeleton (hard carapace) to which muscles and other connective tissues attach on the inner surface and a softer “rubber” like skeleton (arthrodial membrane) that provides for joint function (Figure 11.16). Both types of cuticle are formed by a simple columnar epithelium. The epithelium is stimulated to produce layers of carapace by endocrine hormones. Underlying the carapace epithelium is the “spongy” hypodermis composed of large vacuolated cells (the glycogen contents are removed during histologic processing). Also found in the hypodermis are sinusoids, arterioles, nerves, and other connective tissues (fibrocytes, muscle cells, etc.) (Johnson 1980; Roer and Dillaman 1993).

Cuticle formation has been studied extensively in H. americanus. The production of the hard carapace occurs in layers (see below) and production of each layer is associated with parts of the molt cycle. The entire cuticle is produced by the underlying epithelial cells, which can be cuboidal to tall columnar, depending on the stage of molt. The epithelial cells are affected by endocrine hormones that instruct the cells as to the type of cuticular layer to produce. The layers of the cuticle are composed of tightly adhered cuticular cylinders produced by the epithelial cell. The cuticle is composed of chitin crystals with associated proteins (Aiken 1980; Neville 1975). The crystals orient in a helical fashion as the chitin is produced by each epithelial cell, resulting in a histologic appearance of successive lamallae of chitin in each layer of the cuticle. Tiny extensions of the epithelial cells (pore canals) originate from the epithelial cell edges and form cellular canals that traverse the cuticle above each cell in newly molted animals (Halcrow 1993). It is through these canals that mineralization of the chitin/protein deposits occurs post molt. The pore canals collapse as the chitin is mineralized.

A fully formed carapace is designated as stage C₄ and includes the occurrence of the membranous layer. It is also termed the “resting stage,” the intermolt stage, or anecdysis (Figure 11.17). This stage can be very short in young larvae and juveniles (just a few weeks to months) and years long in large, older decapods. It is during this stage that the animal goes about its daily functions and grows/expands its internal tissues. The first part of stage D, termed D₀ (passive premolt), is often included with stage C₄ (usually written as C₄/D₀). During D₀, autotomized limbs begin to regenerate and gastrolith mineralization begins. As the internal tissues grow and become constrained by the carapace, molting hormones are produced. Molting starts with stage D₁, active premolt. During stage D₁, a new epicuticlar layer is produced by the epithelial cells. The new epicuticle is deposited between the membranous layer and the now hypertrophic (tall columnar) cuticular epithelium. During D₂, a new exocuticle (not yet mineralized) is formed by the epithelium on the inner surface of the new epicuticle. In stage D₃, absorption of mineral from the old cuticle’s inner surface and breakdown of the membranous layer occur, creating a space throughout the body between the old carapace and the newly formed epicuticle (Skinner 1985).

The act of molting (ecdysis) occurs in two phases. In the first (passive) phase, the lobster absorbs water and becomes diffusely edematous. In the second phase, the arthrodial membrane between the dorsal portion of the cephalothorax and the dorsal portion of the first somite of the abdomen ruptures. The lobster then begins to withdraw from the old carapace through this hole, first by withdrawing the cephalothorax, then the abdomen. The carapace of the newly molted animal consists of the newly formed epicuticle and the exocuticle. Immediately post ecdysis (4–8 hours), molting stage A₁, the animal continues to absorb water into its tissues. Because the animal’s tissues are edematous, the soft carapace stretches and takes on a larger shape. These layers have not yet been mineralized so the animal is “soft” and easy prey for other carnivores. Newly molted crustaceans usually hide in a burrow of some type to avoid predation. In stage A₂, the animal begins to mineralize the exocuticle. As the animal mineralizes the new exocuticle, this larger “house” is maintained and the animal’s tissues begin to eliminate the widespread edema. Once the exocuticle is mineralized, the animal will begin foraging again.

In stage B, secretion of the new endocuticle begins and the outer portions of the endocuticle are mineralized (calcified endocuticle). In stages C₁ to C₃, further production of the endocuticle occurs and calcification of the outer layers of the endocuticle continues. The inner layers of the endocuticle (uncalcified endocuticle) are never strongly mineralized.

The stages leading up to ecdysis (D₁ to D₃), ecdysis, and stages immediately post ecdysis (A₁ to B) occur relatively quickly (1–2 days). Ecdysis itself should not take more than an hour even in very large animals.

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