DIVERSITY OF LIFE

DESCRIPTION OF THE PHYLUM ECHINODERMATA (KLEIN 1734)

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Echinodermata (e-KI-no-der-MA-ta) is formed from two Greek roots that mean "spiny skin" [spiny -akanthotos (ακανθωτός); and skin -derma (δέρμα)].  The reference is to the spiny nature of most members of this phylum.  The formal name was coined by Klein (1734) and referenced by Linnaeus (1758).  
INTRODUCTION TO THE ECHINODERMATA

The echinoderms (Figure 1, Clade 1) are a large and successful group with a long and complex fossil history.  Their primary synapomorphies (after Wray 1999 and Pechenik 2005) are: a calcitic skeleton composed of many ossicles, a water vascular system, mutable collagen tissue, and a pentaradial or pentamerous body organization in adults.  

Certainly, the most obvious common feature is five-parted symmetry characteristic of some (the featherstars, sea stars, and brittle stars (Figures 2-7).  Sea urchins and sea cucumbers (Figures 8-10) also exhibit radial symmetry, but it is more subtle.  They all, except the sea cucumbers, have an armor covering made by an endoskeleton of interlocking calcareous plates, and many are covered with spines.  In fact, the name, Echinodermata, means spiny skin, a reference to the occurrence of spines on sea urchins, and to a lesser extent, on sea stars and brittle stars.  They are able to move about by flexing the armor covering or by the use of  tube feet which are controlled by a hydraulic system, the water vascular system.  The sea cucumbers (Figure 10) have highly reduced  endoskeletal elements.  All members of this phylum have a remarkable ability to regenerate whole animals from severed arms, etc.   Members of this group tend to be benthic and all are marine.  

The apparent primitive nature of the echinoderms is deceiving.  They have active, ciliated larvae (e.g. Figure 2) that settle down to a lifestyle that is either sedentary or active but benthic.  They are the most different from the putative primitive state of the Deuterostomes [i.e. similar to Cephalochordata; see Cladograms of the Deuterostome Phyla especially the theories of Putnam et al. (2008) and Delsuc et al. (2006)].  Thus, echinoderms appear to be the most derived group in the Deuterostomata.


FIGURE 2

 

MAJOR CLADES OF THE LIVING ECHINODERMS

1.  The Echinoderm Clade

2.  The Crinoid Clade

3.  The Eleutherozoan Clade

4.  The Asteroid Clade

5.  The Cryptosyringida

6.  The Ophiuroid Clade

7.  The Echinozoan Clade

8.  The Echinoids

9.  The Holothuroids

FIGURE 1. Major Clades of the Echinodermata.  This is drawn from Ruppert et al. (2004), Brusca and Brusca (2003), and Wray (1999).

 

 

The Crinoid Clade (2) 

Compared to the other echinoderms, the crinoids are very unusual and are placed in their own subphylum, Pelmatozoa.  Most are sessile as adults and are attached to the substrate by a stalk, at least for parts of their lives.  Many modern taxa (e.g. Antedon, Figure 3) are not stalked and move about over the substrate temporarily anchoring themselves by specialized cirri.  Whether stalked or mobile, the main part of the animal lies in a cup-shaped calcareous structure, the calyx, which is surrounded by 5 movable arms formed of jointed skeletal elements.  The arms may bifrucate repeatedly, have ciliated grooves, numerous side pinnules, and finger-shaped tube feet.  The ciliated grooves help to capture suspended particles and transfer them to the mouth.  Both the mouth and the anus are directed upward (at least when it is in a sessile stage).  The sexes are separate in the crinoids, as in most echinoderms.  Crinoids may brood fertilized eggs in pinnules or release them to become planktonic vitellaria larvae.  

The articulating calcareous plates which make up the arms, the calyx, and the stalk are large compared to the relative amount of tissue.  This high degree of mineralization allowed crinoids and their relatives to preserve well and leave quite a long and complex fossil histories (Figure 4).  Stalked crinoids (sea lilies) were so abundant during the Carboniferous Period that they produced deposits of limestone many meters thick.  

The crinoids are very different from the other echinoderms that are alive today.  Mostly, this is a consequence of the extinctions of the other stalked forms, which may have numbered to 14 other classes (Ruppert et al. (2004).


FIGURE 3


FIGURE 4

The Eleutherozoan Clade (3)

The Eleutherozoa are benthic but mobile.  The mouth is directed downward for feeding on the substrates that they move over.  The tube feet are for locomotion .  Furthermore, all animals in this clade have a madreporite, a specialized opening that filters the water that is being used in the water vascular system.  Some or all of the spines are movable.

 

The Asteroid Clade (4)

The Asteroidea, or sea stars, are a sister group to all other eleutherozoans.  Typically, they have five arms (Figure 5), each with an open groove, similar to that of the crinoids.  The central disk has an obvious madreporite on the dorsal side, slightly off-center.  The animals have five arms, the base of which surround the central disk, and the terminus of each arm has an eyespot. In some taxa, the distal portions of the arms may divide several times.   The gut also branches with a pocket going into each arm.  Sea stars have single-lobed ampullae (internal bulbous swelling associated with the external tube foot), bilobed in one species.  The moveable spines are short and they have pedicellariae.  Like the crinoids, the skeletal elements articulate making the animal very flexible.  

As a group, sea stars are usually predators, often generalists, and sometimes detritivores.  They will eat almost any animal that they can catch, usually clams.  The can "grasp" the shell with their tube feet and use the water vascular system to exert a continuous pull, against which the adductor muscles of the clams usually tire and the shell begins to open.  At the first opportunity, the sea star everts a portion of its stomach into the clam shell and begins to digest it.  Others like the Crown of Thorns (Figure 6), feed on coral polyps and have been a major problem on coral reefs of the Pacific and Indian Oceans.


FIGURE 5


FIGURE 6

The Cryptosyringida (5)

Members of this Cryptosyringida share the synapomorphy of having radial nerves that are within the epineural canal.  This clade includes the Ophiuordia, Echinoidea, and Holothuroidea.

 

The Ophiouroid Clade (6)

The Ophiouroidea are the brittlestars and they resemble the sea stars.  However, there are subtle differences aside from the placement of the radial nerves.  The arms are like tentacles that emerge from the central disk (Figure 7), and they have no ventral groove.  The arms are very mobile and easily broken, thus giving them the common name.  The madreporite is located ventrally.  The brittlestars have a branched stomach that is similar to that of the sea stars, even to the point of extending into the tentacles.  They are secretive animals and rarely seen, but there are nearly 2,000 described species.


FIGURE 7

The Echinozoa Clade (7)

The Echinozoa are covered by the ambulacra.  They have tube feet with sucker discs and internal ossicles (Ruppert et al. 2004).  Ossicles also form a ring around the pharynx.

 

The Echinoidea (8)

The Echinoidea or sea urchins typically have a compact, nearly spherical body (Figure 8) that is held rigid by the fused ossicles to form a test, on which the ambulacra form a nearly symmetrical 5-petaled star.  Tube feet emerge from a pair of holes in the test.  They have a specialized 5-parted "jaws" called Aristotle's Lantern, which can allow them to erode soft rock.  I recall seeing sea urchins on the Pacific coast of Costa Rica that had chewed away the igneous rock to make protective cavities for themselves.  The most obvious protection for them, however, is the spines, which can be long, thin, and very painful if imbedded in the skin of a swimmer.  Sea urchins are grazers, detritivores, and opportunistic predators of animals that cannot get away.  

Aside from the typical spherical tests, some, like sand dollars and sea biscuits,  produce tests that are flattened and decidedly bilaterally symmetrical (Figure 9).


FIGURE 8


FIGURE 9

The Holothuroidea (9)

The Holothuroidea are the sea cucumbers (Figure 10), in which the ossicles have become very reduced.  The elongate animal retains the pentamerous symmetry internally with five longitudinal bands of muscles.  They also have a five-parted pharyngeal skeleton.  They are benthic scavengers and detritivores.  They have the curious habit of eviscerating themselves when threatened and then regenerating the lost organs. 


FIGURE 10
SYSTEMATICS OF THE ECHINODERMATA

Almost all taxonomic systems (e.g. Brusca and Brusca 2003; Tudge 2000; Margulis and Schwartz 1998) recognize that the echinoderms have 5 natural groups (classes) within them.  An additional group, the tiny "sea daisies" known by only two species may represent a sixth living class.  Even with the wealth of living and fossil material and the general agreement on the class structure, there is still controversy concerning the evolutionary relationships between the major taxa.  Brusca and Brusca (2003) and Wray (1999) indicate the two major alternative phylogenies: the Asterozoan Hypothesis (Figure 11) and the Cryptosyringid Hypothesis (Figure 12).  Both date to the beginning of the 20th Century (Wray 1999), and both have molecular and morphological support (Brusca and Brusca 2003 and Wray 1999).  The Cryptosyringid Hypothesis (Figures 1 and 12) proposes that the clades are nested from Crinoidea to the most derived Echinoidea + Holothuroidea. The Asterozoan Hypothesis (Figure 11) also considers the Crinoidea to be sisters of all of the other groups of echinoderms; however, two terminal clades emerge: Echinoidea+Holothuroidea and Asteroidea+Ophuroidea.

 

 

FIGURE 11. ASTEROZOAN HYPOTHESIS
FIGURE 12. CRYPTOSYRINGID HYPOTHESIS

 

 

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By Jack R. Holt and Carlos A. Iudica.  Last revised: 08/30/2020