Ilze K. Berzins1, Roy P. E. Yanong2, Elise E.B. LaDouceur3, and Esther C. Peters4

1 One Water, One Health, LLC, Golden Valley, MN, USA

2 Tropical Aquaculture Laboratory, Fisheries and Aquatic Sciences Program, School of Forest Resources and Conservation, Institute of Food and Agricultural Sciences, University of Florida, Ruskin, FL, USA

3 Joint Pathology Center, Silver Spring, MD, USA

4 Environmental Science and Policy, George Mason University, Fairfax, VA, USA

3.1 Introduction

The phylum Cnidaria contains an estimated 11 000+ living species (World Register of Marine Species 2018) and is almost exclusively an aquatic group, primarily marine but with some freshwater species and a few terrestrial parasitic species (Atkinson et al. 2018). There are two distinct body forms: the polyp, which is often sessile, and the medusa, which is normally free‐floating (Figure 3.1). Familiar groups include corals, sea fans, jellyfish (sea jellies), sea anemones, and the common laboratory hydra. The name cnidarian means “stinging creatures.” Cells called cnidocytes – stinging cells – are unique to this phylum.

The phylum is divided into multiple classes. The four main classes are the Anthozoa (sea anemones, hard or stony corals, and soft corals such as sea fans or sea whips), Scyphozoa (jellyfish or sea jellies), Hydrozoa (hydra and fire corals), and Cubozoa (box jellies). Newer classes have been acknowledged including Staurozoa (stalked jellyfish) and, within the intracellular parasitic subphylum Myxozoa, the classes Myxosporea and Malacosporea (Fiala et al. 2015). Older classifications combined cnidarians with ctenophores (comb jellies) in the phylum Coelenterata, but currently they are recognized as distinct phyla.

Life expectancy in cnidarians is often difficult to ascertain. Some colonial corals have been estimated to exceed 4200 years (Devlin‐Durante et al. 2016; Roark et al. 2009). Medusae of many species are fast growing, mature within a few months, and die after breeding. They are prone to damage from wave action, currents, and predators. There are anecdotal reports of individual sea anemones and jellies living up to 50 years.

Cnidarians reproduce both sexually and asexually, most species incorporating both methods. They produce gametes (eggs and sperm), can be monoecious, producing both eggs and sperm, or dioecious, with individuals of separate sexes (gonochoric). Monoecious species are also referred to as hermaphroditic, although they are usually not self‐fertilizing even if they are simultaneous hermaphrodites; other species can be sequential hermaphrodites, switching the type of gametes produced. Gametes are usually released into the water column, known as spawning, though some species brood fertilized gametes within the body cavity or within external structures on the body wall. Spawning depends on various environmental conditions such as water temperature and light, and often occurs at specific times of the year. Fertilized eggs develop into planulae, larvae that can swim or crawl using cilia and eventually attach to a substrate. The attached end becomes the aboral end of a polyp.

In many Scyphozoan species, the polyp absorbs the tentacles and begins to split horizontally into a series of discs that become juvenile medusae (ephyrae). This process is known as strobilation. Cubozoan polyps undergo complete metamorphosis (do not undergo strobilation), and each polyp transforms into a single medusa. Hydrozoa have a variety of life cycles, some with no polyps and some with no medusae. Anthozoans have no medusa stage, and polyps are responsible for sexual reproduction. Cnidarians can also reproduce asexually by various means. New polyps can bud off from parent polyps or split in half to expand or begin new colonies. These polyps are genetically identical to each other, or clones. Some hydrozoan medusae can also divide in half. Many cnidarians (in particular the stony corals) can be fragmented, where a portion of a colony is broken or separated off from the main colony (parent colony), to form a new colony. This technique is being used in many restoration efforts (Berzins et al. 2007, 2011).

Schemati illustration of cnidarian body forms: polyp and medusa.

Figure 3.1 Cnidarian body forms: polyp and medusa.

Source: Kevin Seline, artist.

Cnidarians are considered carnivorous animals. Smaller species feed on plankton, whereas larger forms can capture and ingest larger prey items, such as fish, worms, and crustaceans (Meredith et al. 2016; Milisenda et al. 2018; Sullivan et al. 1994). Certain groups, in particular most of the reef‐building or stony corals, have a symbiotic relationship with single‐celled dinoflagellates known as zooxanthellae (mostly in the genus Symbiodinium) (LaJeunesse et al. 2018). The algal cells undergo photosynthesis and exchange nutrients and waste molecules with the cnidarian host cells. They also often provide color to the host and can be expelled from the host cell or die within the host cell in reaction to various environmental stressors (e.g., temperature, light, pH, and salinity changes), resulting in a loss of color, often referred to as coral bleaching. The coral can be recolonized by other subtypes or clades of dinoflagellates that may be better adapted to the new environment but the coral may die before that can happen.

From a human point of view, the most noticeable aspect of cnidarians involves coral reefs. Reefs are located around the world in shallow, deep, warm, and cold waters. They are often referred to as the “rainforests” of the ocean. They contain some of the most biodiverse and oldest ecosystems on earth, providing home to more than 25% of all marine life, including one‐third of all marine species, yet comprise less than 1% of the earth’s surface ( Millions of people and thousands of communities in more than 100 countries depend on reefs for food, protection (barrier reefs are natural storm barriers), and jobs, which generate billions of dollars of resources and services (for food, tourism, shelter). Cnidarians are also an important part of food webs, being both predators as well as prey. For example, as predators, they can have a significant impact on zooplankton and larval populations (Crum et al. 2014; Meredith et al. 2016; Purcell and Arai 2001); flamingo tongue snails graze on sea fans; parrot fish scrape up stony coral polyps including some of the supporting exoskeleton (calcium carbonate particles later excreted contribute to sandy beaches); and medusoid forms are often ingested by other jellies, sea turtles, and fishes.

New discoveries in biochemical and biomechanical properties of cnidarians are leading to new tools for our medicine cabinets. Bone graft materials from stony coralline species, due to chemical and structural characteristics similar to those of human cancellous bone, have been investigated (Pountos and Giannoudis 2016). Some cnidarians possess bioluminescence or fluorescent proteins that are used as gene markers (Prasher 1995) and their mucus contains antibiotic compounds (Ritchie 2006). From an adverse perspective, while most nematocysts cannot penetrate human skin, there are some species of cnidarians whose nematocysts can be large with long skin‐penetrating tubules and whose toxins/venoms can pose serious threats (Bentlage et al. 2010). However, scientists are looking into puncture mechanics of nematocysts that can be utilized to deliver therapeutics in a wearable drug delivery patch (Oppegard et al. 2009).

Threats to cnidarians include climate change, infectious diseases, development of shorelines, increase in surface runoff, land‐based sources of pollution, ship groundings, and damaging fishing techniques (dynamite, nets, etc.). As a possible indicator of some of these detrimental changes, there has been an increase in the frequency of jelly blooms (Purcell 2012). Changes in currents, nutrients, light, and temperature from agricultural or urban runoff can increase the growth of the plankton on which sea jellies feed. Jellies can also tolerate the low oxygen levels caused by eutrophication, itself a result of pollution by runoff; however, their predators may not survive these conditions. As another example, the underlying support system of a coral reef is a calcium carbonate structure that accretes slowly over many years. Ocean acidification poses a threat by altering the concentration of carbonate ions required to maintain this process (Albright et al. 2018).

3.2 Gross Anatomy

3.2.1 General Characteristics

Cnidarians are soft‐bodied, diploblastic (two cell layers) metazoans (animal kingdom), with primary radial symmetry (although with some variations). The two adult cellular layers, the epidermis and gastrodermis, are separated by a nonliving gelatinous layer, the mesoglea, which ranges from a thin sheet to a thick, mucoid (“jelly‐like”) material. They are basically sac‐like organisms, with a single opening leading into a body cavity, the gastrovascular cavity, and can have one of two basic body types, polypoid or medusoid (Figure 3.1).

The single orifice functions as both a mouth and an anus. The polyp is normally a sessile form and the main body is a tubular or cylindrical column with the oral end, or oral disc, directed upward. The oral opening, or mouth, is surrounded by tentacles, which are extensions of the body wall. The opposite side, the aboral end, is usually attached to solid surfaces (although some species can burrow into soft sediment and in other species, polyps are surrounded by connective tissue or exoskeleton) and is often referred to as the base or basal plate. The medusa is usually a free‐swimming “umbrella” or bell‐shaped form, with the convex side (exumbrella) directed upward. The mouth is located in the center of the concave undersurface (subumbrella) and the tentacles hang down from the margin of the bell. Some cnidarians pass through both forms in their life cycle, while others exhibit only the polyp or the medusa form. Cnidarians can be found in colonies or exist as individuals and exhibit a wide range of sizes – microscopic to massive reef formations to medusae measuring several meters in diameter and with tentacles 20+ meters long (Brusca et al. 2016).

The gastrovascular cavity is often subdivided by mesenteries, infoldings of the mesoglea and gastrodermal layers. Mesenteries provide structural support and increase the surface area of the gastrodermis. In many species, mesenterial or gastric filaments are located on free margins of mesenteries, and the cnidoglandular band epithelium of the filament is armed with abundant numbers of nematocysts and cells producing enzymes for digesting prey. Gonadal tissue is usually located within the mesenteric mesogleal regions of polyps or in the gastric space of medusae.

In medusae, the mesoglea is the main supporting structure, whereas in polyps, water in the gastrovascular space can act as a hydrostatic skeleton. Epitheliomuscular cells are cuboidal to columnar to flattened cells (depending on the species) that span the thickness of the epithelium, with their bases concentrated into attachment sites (myonemes) along the mesogleal pleats (Leclère and Röttinger 2017). The musculature is mostly diffuse but there are distinct tracts of muscles such as the circular bands in mouths of anemones (acting as a sphincter muscle) or along bell margins in medusae (facilitating water movement for locomotion).

There are no discrete respiratory, excretory, or circulatory structures. Nutrient uptake in cnidarians can occur by endocytosis of particulate food, absorption of dissolved organic compounds or utilization of organic compounds leaked from the symbiotic dinoflagellates. Respiration and excretion occur by direct exchange through cell membranes using passive and active transport processes. There is no centralized nervous system, only a diffuse network of nerve cells, the nerve net. Some species have ocelli that are light‐sensitive structures (Martin 2002), statocysts for detection of gravity and orientation, and primitive chemosensory pits. The cnidarians’ immune system is also very basic. They do not have an acquired immune system but several innate mechanisms provide protection. The mucus contains bactericidal and other antimicrobial activity, colonial organisms can recognize self from nonself, and most (if not all) possess amoebocytes, pleomorphic cells with phagocytic and biochemical properties that function as the principal cell of the innate immune system in cnidarians (Bosch and Rosenstiel 2016; Ocampo and Cadavid 2014; Ritchie 2006). Anthozoan Specifics

Anthozoans are exclusively marine, do not have a medusa stage, and can be solitary or colonial. The subclass Hexacorallia contains the anemones and hard or stony corals (order Scleractinia) and the subclass Octocorallia contains the octocorals, soft corals and gorgonians such as sea fans and sea pens. In the Hexacorallia, the mesenteries are usually paired and in multiples of six, whereas in octocorals there are just eight pinnate tentacles (with side‐branches or lateral outfoldings) and eight complete mesenteries.

In colonial stony corals (scleractinians) and the octocorals (all octocorals are colonial), the polyps are connected and the organisms secrete several different types of support structures. The stony corals secrete an aragonite (calcium carbonate) exoskeleton. The entire skeleton is known as the corallum and develops into many different growth forms depending on species of coral – including massive, laminar, branching, and encrusting (Peters 2016). These large exoskeletons form the familiar coral reefs. Figures 3.2 and 3.3 help provide a visual guide to the following anatomic descriptions of a scleractinian coral.

The polyps occupy only the surface of the corallum. The size of the corallum expands in size as the polyps increase in number by budding. Each polyp sits in a calcium carbonate cup known as a corallite. The wall of the corallite is the theca and the floor is the basal plate. As the overall complex increases in size, the bottoms of the corallites are sealed off by transverse calcareous partitions called dissepiments, each becoming the new basal plate for the polyp. Spacing between dissepiments varies. Some species have very narrow spaces with dense partitions, often in the slower growing corals. In faster growing, often branching species, the spaces are large with thinner partitions, and can fragment very easily. The spaces are often colonized by other organisms (primarily fungi and protozoa) (Marcelino et al. 2018). Little is known about these communities and whether they affect the overlying coral tissues. Calcareous partitions, septae, radiate from the theca and basal plate toward the center and provide support to the mesenteries. The size and shape of these calcareous septae are distinctive for each species and are often used in species identification.

Schematic illustration of the gross anatomy of a scleractinian polyp from an imperforate coral.

Figure 3.2 Gross anatomy of a scleractinian polyp from an imperforate coral.

Source: Reprinted with permission from Corals of the World (Veron 2000). Painting by Geoff Kelly.

The part of the polyp that can extend above the theca is the column, and portion below the surface, sitting in the cup, is the polyp base. The skeleton between the polyps is known as the coenosteum. The polyp epithelium lining the cup assists in the building of the calcified exoskeleton and is known as the calicodermis. The tissue connecting the polyps is known as the coenenchyme and consists of two full sets of tissue layers, separated by gastrovascular canals. In imperforate corals (usually the dense, slower growing corals such as Orbicella spp.), this network extends over the surface of the skeleton. In perforate corals, the connecting tissue is both on the surface and embedded within the skeleton, with canals piercing through the walls of the corallites and surrounding skeleton (e.g., Acropora spp., Porites spp.). The lumens of the gastrovascular cavities are connected via the canals to adjacent polyps and fluid, food, molecules, and disease agents can be spread to adjacent polyps through these tubes.

Octocorals do not secrete the large exoskeleton of the stony corals but most contain small, often microscopic calcium carbonate pieces called spicules or sclerites embedded in the coenenchyme (Figures 3.4 and 3.5). Scleroblasts secrete these minute structures into various shapes, sizes, and colors giving the octocorals their texture, and at times color, and protect the tissue from predators. They often are used in species identification. Corticocytes, also in the coenenchyme, form an axis epithelium to secrete a flexible or stiff internal axial rod as a supportive base. These rods are composed of an organic material, a protein‐mucopolysaccharide complex, called gorgonin. Desmocytes, interspersed with corticocytes (Figure 3.29), anchor the tissue to the gorgonin. Around the rod is a layer of coenenchyme of variable thickness connecting polyps and perforated by gastrodermal epithelium‐lined tubes (gastrovascular canals and smaller tubes, solenia.) which are continuous with the gastrovacular cavities of other polyps. The portion of the polyp wall adjacent to the coenenchyme is often thickened and reinforced with sclerites and is known as the anthostele.

Schematic illustration of an overview of key structures in a polyp from the scleractinian coral Orbicella faveolata. SK, skeleton.

Figure 3.3 Overview of key structures in a polyp from the scleractinian coral Orbicella faveolata. SK, skeleton.

Source: Reprinted with permission from NOAA Coral Disease and Health Consortium (Galloway et al. 2007). Section prepared and stained by the Medical University of South Carolina’s Histotechnology Program 2007 students. Photomicrograph prepared by James H. Nicholson.

Schematic illustration of the gross anatomy of an octocoral.

Figure 3.4 Gross anatomy of an octocoral (gorgonian polyp).

Source: Reprinted with permission from NOAA Coral Disease and Health Consortium (Galloway et al. 2007). Adapted from an illustration in Bayer et al. (1983). Jennifer Clark, artist.

Photo depicts the subgross photomicrograph of a sea pen. The epidermis (inset, E) is subtended by coenenchyme-containing amoebocytes and sclerites (inset, S). Vertically oriented mesenteries extend down from the superficial coenenchyme mesoglea and contain mesenteric filaments and gonads (G; sperm). A gastrovascular canal (inset, GV) is lined by gastrodermis.

Figure 3.5 Subgross photomicrograph of a sea pen (order Pennatulacea). The epidermis (inset, E) is subtended by coenenchyme‐containing amoebocytes (not visible from this magnification) and sclerites (inset, S). Vertically oriented mesenteries extend down from the superficial coenenchyme mesoglea and contain mesenteric filaments and gonads (G; sperm). A gastrovascular canal (inset, GV) is lined by gastrodermis. Larger gastrovascular spaces are gastrovascular cavity (inset, GC). 200× (inset 1000×). HE. Scyphozoan Specifics

The predominant body form of scyphozoans is the medusa and species in this group are commonly referred to as jellyfish or sea jellies. The bell of the medusa is typically a thick mesogleal layer and differences in the bell margin often differentiate species. Tentacles may be present or absent and the margin is often scalloped or lobed. Sensory centers are situated on club‐shaped structures called rhopalia found along the bell margin in the notches between lobes, and can contain chemosensory pits, statocysts, and ocelli (photosensitive structures of differing complexity). The mouth is in the center of the undersurface of the bell and may be suspended on a tubular extension called the manubrium. Portions of the mouth edge may be drawn into four or eight elongate structures known as oral arms, which aid in the capture of prey. The gastrovascular cavity is divided by four mesenteries (referred to as “septa” in some sources) into four gastric pouches or canals, resulting in a tetramerous or quadriradial symmetry. Most cnidae are located in tentacles and oral arms but can also occur in all types of epithelial tissues, including epidermis and gastrodermis (Ruppert et al. 2003). Gametes arise along the mesenteries or on folds of the gastrodermis. Gametes can sometimes be observed grossly as four horseshoe‐shaped organs suspended in the bell, with the “open” side of the horseshoe facing the oral cavity; this is most obvious in Aurelia aurita. Cubozoan Specifics

The Cubozoa have cube‐shaped medusae and include sea wasps and box jellies. The venom from their nematocysts is very toxic and, in some cases, can be fatal to humans. From each corner, short stalks called pedalia bear one or more tentacles. The margin of the bell is folded inward to form the velum (velarium). This structure restricts the aperture of the bell, helping to create a more powerful flow of water when the bell contracts. On the underside of the bell, in the center, is the mouth on an elongate structure, the manubrium. The mouth leads into the gastrovascular cavity which is partitioned by mesenteries into four gastric pockets (again with tetramerous or quadriradial symmetry) extending to the tentacles through canals. Along the bell margin, between the pedalia, are sensory structures, rhopalia. Cubozoans are unique because of the possession of complex eyes (Coates 2003). In each rhopalium are two complex eyes with a lens, retina, and cornea. Statocysts and ocelli are also present. Hydrozoan Specifics

In most hydrozoan species, the polyp is the predominant body form. The medusa stage is often small, and a few species never go through a medusoid stage. The medusa is the sexually reproducing stage in most hydrozoans. Medusae form by budding from polyps and have a velum inside the bell of the medusa. Around the margin is a ring of tentacles. Inside the umbrella, the mouth can be supported by an elongate tubular manubrium. Gametes form on the sides of the manubrium. Some forms, such as Hydra spp., exist as solitary polyps but most hydrozoans are colonial. The initial polyp produces new polyps by budding and develops a network of interconnected tubes, called stolons. The lumen of the stolon is connected with that of the gastrovacular cavities of the polyps (Buss et al. 2013). In some species, the polyps secrete calcareous coatings, such as in the fire coral, Millepora spp. This exoskeleton, called a coenosteum, is perforated by pores for the polyps. Some hydrozoans have developed pelagic colonies (siphonophores) made up of interconnected polyps and medusoids forming polymorphic structures with different functions called zooids. These pelagic forms are often confused with the medusae of scyphozoans. The “Portuguese man‐of‐war” is an example of a pelagic colonial hydrozoan.

3.2.2 Keys for Dissection/Processing for Histology

Collecting specimens from the field should include gathering as much information as possible such as environmental conditions, condition of tissues, and gross photographs. Thorough protocols have been developed for obtaining stony coral samples (Price and Peters 2018). Consult papers that have used histology in studies on other cnidarians to develop the best procedure to use.

Multiple options for fixation have been used with cnidarians; most commonly these include: (i) a 10% formalin‐seawater fixative, (ii) Helly’s Modified Fixative, (iii) Bouin’s, and (iv) Z‐Fix Concentrate®, a zinc formaldehyde solution that must be diluted with ambient sea water (see Appendix 3.1 at the end of this chapter). Routine sea water‐formalin is an easy fixative but provides less cellular detail because it does not fix the membranes well. Helly’s is very good for preserving cellular structure and intracytoplasmic granules but may cause some shrinkage. Bouin’s is excellent for preserving cellular structure, but the sample should remain in the Bouin’s for only a few hours or the tissue will become hardened; the excess fixative must be removed by multiple changes of 70% ethanol. Z‐Fix Concentrate diluted with sea water provides better detail than regular formalin and specimens can remain in this fixative indefinitely. The diluting sea water should be of the same or nearly same salinity as the water from which the cnidarians were collected, because they do not osmoregulate.

The size or thickness of the specimen must be thin so the fixative will penetrate it easily, and 10–20 times the volume of fixative per volume of tissue is required. Jellyfish, anemones, and large fleshy corals need to be trimmed into slices no thicker than 5 mm. Most specimens will retract their polyps or tentacles when handled. They can be anesthetized with a magnesium sulfate solution (see Appendix 3.1) to help relax tissues. Using the technique of agarose preembedding (termed “enrobing”) prior to decalcification of the exoskeleton from scleractinian corals and sclerites from octocorals is recommended for preserving spatial orientation and overall ease of sectioning (Bythell et al. 2002) (see Appendix 3.2). Sections can be cut without decalcification with a lapidary trim saw (Figure 3.6). After fixation and decalcification (if needed), tissue samples must be rinsed to remove excess solutions, trimmed to fit into tissue‐processing cassettes (no thicker than 3 mm), and processed to embed the sections in paraffin or plastic (e.g., JB‐4 methylmethacrylate, epoxy, other resins), depending on the microtomy equipment available for sectioning the resulting tissue blocks. The sections are then mounted on glass microscope slides, dried well, and stained (Figure 3.7).

Photo depicts undecalcified Acropora cervicornis section prepared using resin embedding and petrographic thin section techniques with the blue-gray aragonite crystals of the skeleton still present.

Figure 3.6 Undecalcified Acropora cervicornis section prepared using resin embedding and petrographic thin section techniques (Price and Peters 2018) with the blue‐gray aragonite crystals of the skeleton (S) still present. The yellow‐orange staining areas are the coenenchyme tissue and one coral polyp is contracted into its corallite, showing tentacles (T), actinopharynx (AP), gonad (G), gastrovascular cavity (GC), and gastrovascular canal (GV). 100×.

Source: Photo credit: Kathy L. Price.

Hematoxylin and eosin (HE) stain is generally used for routine identification of tissues. Nuclei will stain blue with hematoxylin, mucus in mucocytes will stain pale blue/purple, and proteins, especially those in the mesoglea, granular gland cells, and myonemes, stain pink with eosin. Special stains such as Masson’s trichrome can distinguish collagen in the mesoglea (blue) and the contractile myonemes (red). Pentachrome stains are useful for observing changes in the mucopolysaccharides of the mucocytes. Alcian blue (AB) and periodic acid–Schiff reagent (PAS) will stain carbohydrates and using them together can distinguish between neutral mucins and acidic mucins. A thorough discussion of staining techniques can be found in Price and Peters (2018).

Photo depicts acropora cervicornis tissue section with aragonite skeleton dissolved.

Figure 3.7 Acropora cervicornis tissue section with aragonite skeleton dissolved (arrows pointing to some of the spaces where the skeleton used to be present). 40×. HE.

3.3 Histology

Although similarities and differences among the four major groups of the phylum Cnidaria were introduced, most images in this section are from members of the class Anthozoa, subclass Hexacorallia, order Scleractinia (stony corals). This reflects their recent use in restoration efforts (Berzins et al. 2007, 2011) and in long‐term monitoring programs, as indicators of environmental stress and in disease evaluation (Bythell et al. 2002; Peters 2016). Several excellent references on anatomy and histology are available including Fautin and Mariscal (1991), Galloway et al. (2007), Lesh‐Laurie and Suchy (1991), Peters (2016), and Thomas and Edwards (1991). Other papers have been published on particular cnidarian taxa, including octocorallia (Bayer et al. 1983; Fabricius and Alderslade 2001; Goldberg 1976; Moore et al. 2016), corallimorpharia (Fautin 2016), and cerianthid anemones (Peters and Yevich 1989). Table 3.1 provides a list of the tissue types evaluated histologically in cnidarians.

3.3.1 Epithelium

The outer cell layer in adult cnidarians has been referred to as ectoderm or ectodermis and the inner layer as endoderm or endodermis. However, in recent discussions (Peters 2016) the terms epidermis and gastrodermis refer to adult epithelia that are derived from embryonic ectoderm and endoderm, respectively. The simple embryonic ectoderm and endoderm layers differentiate into different cell types whose exact origins often are still being investigated. The epidermis (ectoderm) and gastrodermis (endoderm) are separated by a nonliving gelatinous layer, the mesoglea (derived primarily from the ectoderm), which ranges from a thin sheet to a thick, mucoid, gelatinous material (Figures 3.83.14).

Table 3.1 Tissues, structures, and cells for histologic evaluation in cnidarians.

Tissue type/system Tissue name Structure/cell name, type
Epithelium/Integument Epidermis (embryonic ectoderm) Simple columnar epithelium/pseudostratified columnar

Ciliated or flagellated columnar cell (also known as supporting cell or collar cell; can be difficult to appreciate under light microscopy)


Cnidocyte (nematocyst, spirocyst, ptychocyst)

Granular cell (pigment, glandular)
Epithelium/Exoskeleton Calicodermis (Scleractinia) Calicoblast (squamous to columnar, pleomorphic; secretes organic matrix which assists in the deposition of aragonite crystals)



Axis (Alcyonacea) Corticocyte (squamous to columnar; secrete gorgonin or antipathin)

Epithelium/Digestive Gastrodermis Body wall


Mesenterial filament/cnidoglandular band

Cuboidal supporting cell (+/‐ symbiotic dinoflagellates [zooxanthellae] in vacuoles)

Cuboidal to columnar absorptive/storage cell


Cnidocyte (nematocyst/spirocyst)

Ciliated columnar supporting cell (difficult to appreciate on light microscopy)

Collar cell (difficult to appreciate on light microscopy)

Granular cell (pigment and glandular)
Connective Mesoglea Collagen matrix

Fibroblast (difficult to appreciate on light microscopy)

Mesogleal pleat
Epitheliomuscular cell/myoneme
Nervous Nerve Net Neurons (difficult to appreciate on light microscopy), ocelli

Interstitial cella
Reproductive Gonad Ova

Spermatozoa (in spermaries)

a These are considered to be multipotential cells that can migrate through the mesoglea and between epithelial cells and differentiate into other cell types (i.e., epithelial, amoebocyte).

Note: Not all cell types are found in all species and variations occur.

In anthozoans, at the oral end of the polyp, tentacles surround a relatively flat oral disc. In many scleractinian corals, the tips of the tentacles have a bulbous structure, the acrosphere, which has a concentrated number of cnidocytes (containing nematocysts or spirocysts) used both in prey capture and defense (Figures 3.203.22). The tentacle‐bearing portion of the octocoral polyp is called the anthocodium. Octocorals typically lack nematocysts and spirocysts on the tentacles. The polyp may possess an operculum made up of eight calcareous scales that can cover withdrawn tentacles. Polyps of some octocoral species can be dimorphic. Larger, normally developed polyps are autozooids, and smaller, less well‐developed polyps are siphonozooids (not to be confused with hydrozoan siphonophores, described later).

Schematic illustration of the surface body wall of a scleractinian polyp, showing cell types of the epidermis, mesoglea, and gastrodermis.

Figure 3.8 Diagram showing the surface body wall of a scleractinian polyp, showing cell types of the epidermis, mesoglea, and gastrodermis.

Source: Reprinted with permission from NOAA Coral Disease and Health Consortium (Galloway et al. 2007). Jennifer Clark, artist.

Schematic illustration of the key features of a scleractinian polyp mesentery. Features often change, moving from oral to aboral regions.

Figure 3.9 Diagram of the key features of a scleractinian polyp mesentery. Features often change, moving from oral to aboral regions.

Source: Reprinted with permission from NOAA Coral Disease and Health Consortium (Galloway et al. 2007). Adapted Fautin and Mariscal 1991. Jennifer Clark, artist.

Photo depicts body wall from a scleractinian coral, Orbicella annularis. The surface of the coral in contact with sea water is covered by a simple columnar or pseudostratified columnar ciliated epithelium which may contain scattered columnar epitheliomuscular cells, and plump mucoctyes.

Figure 3.10 Body wall from a scleractinian coral, Orbicella annularis. The surface of the coral in contact with sea water is covered by a simple columnar or pseudostratified columnar ciliated epithelium which may contain scattered columnar epitheliomuscular cells (difficult to differentiate with HE stain), and plump mucoctyes (thin arrow). An eosinophilic granular cell containing fluorescent protein pigment is visible in the midsection of the epithelial band (thick arrow). A narrow band of mesoglea (asterisk) separates the epidermis from the gastrodermal cells, which have numerous Symbiodinium in vacuoles (arrowhead). 400×. HE.

Photo depicts body wall from a scleractinian coral, Orbicella cavernosa.

Figure 3.11 Body wall from a scleractinian coral, Orbicella cavernosa. The mesogleal layer (asterisk) is much thicker in this species. Zooxanthellae in the gastrodermis are highlighted with arrowheads. HE. 400×.

In anthozoans, from the mouth, a short, muscular tube, the actinopharynx, leads into the gastrovascular cavity. The mouth opening can be circular, oval, smooth or ridged. The tissue next to the mouth sometimes forms a raised rim around it, known as the peristome. In some species (although scleractinian polyps lack this) there are one to several grooves along the length of the actinopharynx known as siphonoglyphs, which are lined by epithelial cells bearing elongate cilia to assist with moving water and food into the gastrovascular space and waste material out. When siphonoglyphs are present, the animal is classified as having biradial or bilateral symmetry. Tissue partitions, the mesenteries (Figure 3.9 and section, extend from the body wall into the middle of the sac with gastrodermal tissue on either side of a mesogleal layer. Gastrodermal cells often contain cytoplasmic vacuoles that harbor single‐celled symbiotic dinoflagellates, formerly known as zooxanthellae, now referred to by their genus names, for example Symbiodinium. Some mesenteries connect with the actinopharynx and are considered “complete” mesenteries, those not connected are “incomplete.” The mesenteries increase the surface area of the gastrodermis and also provide some structural support. The free edges of complete mesenteries are modified into thickened coils of white mesenterial filaments (also known as gastric filaments; Figure 3.9). The filament itself is covered by an epithelium of thin columnar ciliated cells for water circulation, acidophilic granular gland cells secreting digestive enzymes, and cnidocytes used in food capture or defense. Adjacent to the band, the gastrodermal cells of the mesenteries are often columnar, phagocytic, and can degrade particulate matter. The mesentery portion near the body wall contains epitheliomuscular (Figure 3.49) cells used to extend or contract the polyp. General organization of these epithelia is displayed in animals from multiple orders in Figures 3.103.16.

Photo depicts the body wall from a branching perforate scleractinian coral, Acropora cervicornis, showing the ciliated columnar epidermal cells with cnidocytes containing developing or mature spirocysts (thick arrow), a scant mesogleal layer (thin arrows) and the gastrodermis containing numerous zooxanthellae.

Figure 3.12 Body wall from a branching perforate scleractinian coral, Acropora cervicornis, showing the ciliated columnar epidermal cells with cnidocytes containing developing or mature spirocysts (thick arrow), a scant mesogleal layer (thin arrows) and the gastrodermis containing numerous zooxanthellae (arrowhead). 1000×. HE.

Photo depicts the body wall from an aggregating anemone showing the epidermis that is composed of elongate cells on a basement membrane.

Figure 3.13 Body wall from an aggregating anemone (Anthopleura sp.) showing the epidermis (E) that is composed of elongate cells on a basement membrane. Subjacent to the epidermis, the mesoglea (asterisk) is a loose collagenous tissue. At the bottom of the image, part of the gastrodermis (G) and a mesentery (M) can be seen. 200×. HE.

Photo depicts the cross-section of a moon jelly. The epithelial layer at the top of the image represents the exumbrella, which is composed of a simple epidermis (E). The epidermis contains rosettes of nematocysts (N) and occasional pigment cells.

Figure 3.14 Cross‐section of a moon jelly (Aurelia aurita

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Nov 28, 2021 | Posted by in INTERNAL MEDICINE | Comments Off on Cnidaria

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