have been documented to swim significant distances. Echinoderms do not appear to have near relatives among other invertebrate phyla.
Most members of Echinodermata are dioecious and undergo sexual reproduction, with a few species reproducing asexually. Holothuroids are gonochoric (Leake 1975). Asexual reproduction through fragmentation may occur in some Asteroidea and Holothuroidea due to trauma or predation. The diet varies widely by class, with Asterozoa being carnivorous, Echinozoa and Crinozoa being vegetarian browsers and filter feeders, and Holothuroidea being detritivores. Significant conservation concerns and anthropogenic stressors include commercial fisheries, which impact diet availability, particularly clams, mussels, and oysters, and the pet trade through individual animal collection and the collection of coral and live rock causing habitat loss. Environmental concerns include habitat destruction and direct animal impacts due to ocean acidification. Population declines due to disease such as the Caribbean Diadema antillarum mortality event in 1983–1984 (Carpenter 1990; Lessios 2016) and “wasting disease” events across multiple species of asteroid (Hewson et al. 2014; Menge et al. 2016) have more recently received significant focus. Certain Asteroidea are keystone species in their ecosystems, critical for controlling prey populations and diversity. Echinoidea and Holothuroidea are of paramount importance to marine ecosystems because of respective roles in counteracting macroalgal competition with corals, and recycling nutrients from decaying organic matter.
1.2 Gross Anatomy
Uniting features of all echinoderms include radial symmetry (pentamerous symmetry), a tricoelomate body cavity, and a body wall composed of calcite endoskeletal plates (dermal ossicles) connected by “mutable collagenous tissue.” Most internal features, including the alimentary system, reproductive system, nervous system, respiratory system, and a unique water vascular system, share similar basic plans between the subphyla. The basic echinoderm body plan has 10 divisions: five radii (rays or arms) which alternate with five interradii (interrays). Typically, there is an oral surface with a central mouth and an aboral surface that contains the anus. Despite these commonalities, morphology does vary widely and thus representative examples of each subphylum are discussed separately.
The asteroid (sea star) body plan consists of a central disc with typically five but in some species (sun stars) up to 40 or more individual rays. Rays are broad based and arise from the lateral margins of the disc. They taper distally and each ray terminates in one or more tentacle‐like sensory tube feet and a red eyespot. The aboral surface is dorsal and contains the anus at the center of the central disc, which may not be grossly apparent. The madreporite, bearing openings of the water vascular system, is on one side of the disc near the interradius of the first and second rays (Figure 1.1a). The oral surface is ventral and in contact with the substrate. Originating at the mouth and extending the length of each ray is a prominent groove, the ambulacrum (ambulacral groove). Two to four rows of tube feet (podia) lie within the ambulacral groove (Figure 1.1b). The margins are lined by moveable spines that can close over the top of the groove. Ophiurids (brittle and basket stars) demonstrate similar morphology. They typically have five rays, but these are distinctly offset from a round to pentagonal central disc. The rays are typically very long, slender, and very flexible. In basket stars the rays are highly branched. The disc has a proportionally smaller diameter compared to most sea stars. Ophiurid rays lack an ambulacral groove and the tube feet lack distal suckers as they are not typically used for movement.
Figure 1.1 Representative image of the aboral (a) and oral (b) surface of a chocolate chip sea star (Protoreaster nodosus) demonstrating pentamerous symmetry. Labels include (A) radius, (B) interradius, (C) mouth, (D) ambulacral groove, (E) anus, and (F) madreporite.
Figure 1.2 Representative image of the aboral (a) and oral (b) surface of a purple urchin (Arabacia punctulata) demonstrating pentamerous symmetry. Labels include (A) ambulacral plates, (B) interambulacral plates, (C) mouth, (D) anus, and (E) madreporite.
Echinoidea lack rays and have either a slightly compressed globoid body plan (urchin) or a flattened body plan (sea biscuits, sand dollars). Similar to asteroids, they have a dorsal aboral surface with central anus (Figure 1.2a) and ventral oral surface with a central mouth (Figure 1.2b). Urchins have 10 radial sections, consisting of five pairs of ambulacral plates alternating with five pairs of interambulacral plates, which converge at the oral and aboral poles to form the test (i.e. outer shell). The ambulacral plates bear tube feet and are penetrated by pores that communicate internally with ampullae of the water vascular system, whereas the larger interambulacral plates lack tube feet (Figure 1.3). On the oral surface, the plates meet, forming a large aperture centrally that contains the mouth and peripheral peristomial membrane. Surrounding the peristomial membrane are five specialized podia (buccal podia) and five pairs of gills. At the aboral pole, the anus is surrounded by a circular membrane, the periproct. There is a ring of five specialized plates (genital plates), surrounding the periproct, one of which is modified to form the madreporite. An additional five smaller plates, ocular plates, are interdigitated with the genital plates. Together, these 10 plates form the apical system.
Figure 1.3 Image of a white sea urchin (Tripneustes ventricosus) demonstrating the distinction between ambulacral (arrow, inset right) and interambulacral (arrowhead, inset left) plates; tube feet are lacking in the latter where black‐pigmented pedicellariae predominate.
Spines are arranged symmetrically in meridional rows along both ambulacral and interambulacral areas with the longest spines near the equator and shortest near the poles. Most urchins have long primary spines and shorter secondary spines equally distributed over the surface. Some species only have primary spines. Spines are cylindrical, taper to a point, and attach to the plates by a tubercle, resembling a ball and socket joint. Sand dollars and sea biscuits have a dorsoventrally compressed body plan compared to urchins, but similar anatomic features. The ventral ambulacral areas are called phyllodes and have tube feet modified for feeding and adhesion. The dorsal ambulacral areas are called petaloids (or petals) and tube feet are broad, flat, and specialized for respiration (gills).
In sea cucumbers, the main body axis is long and the oral surface including the mouth is at the anterior end of the animal and the body axis is parallel to the substrate (Figure 1.4a,b). The mouth is often surround by specialized tube feet (buccal podia) that are large and highly branched. The side of the body that lies on the substrate (ventral surface) contains three ambulacra that are referred to as the sole. The dorsal side contains two ambulacra. Some burrowing species lack this differentiation. Tube feet can be arranged in prominent rows, be spread uniformly over the surface, or may be absent. When present, those on the ventral surface typically have suckers.
