Evolution and systematics

As for most soft-bodied animals, the fossil history of holothuroids, or sea cucumbers, is threadbare. Only 19 species have been described from body fossils, although one form, an Achistrum species from the Middle Pennsylvanian Mazon Creek Formation in North America, is known from study of several thousand, often quite well-preserved, specimens. Most ancient species are known from study of isolated fossils of their ossicles. These microscopic skeletal elements, found in the body walls and internal organs of most taxa, are an important feature in defining extinct and living species. However, because ossicle form varies even within a single animal, most fossil ossicles are classified as paraspecies on the basis of unique morphological features. Largely on the basis of this record of ossicles as well as the few known body fossils, approximately 12 of the 25 living families of holothuroids have been found to have ancient representatives.

The earliest undisputed fossils of holothuroids are of isolated ossicles from the Middle Silurian Period circa 425 million years ago (mya). Plate ossicles attributed to holothuroids are known from the Ordovician Period 450 mya, but their identity as holothuroid is uncertain because they resemble the plates of other echinoderms. Holothuroids appear to have evolved perhaps 480 mya from a poorly known group of extinct burrowing echinoderms called ophiocystioids, which resembled spineless sea urchins with a reduced number of large, plated tube feet. The oldest described body fossil is of Palaeocucumaria hunsrueckiana from the Lower Devonian Period 395 mya. This species is unique among known holothuroids in having plated tentacles, a feature that suggests in part a link to the ophiocystioids. Holothuroids continued to diversify during the Paleozoic Era, when members assigned to the orders Apodida, Elasipodida, Dendrochi-rotida, and Dacytlochirotida first appeared.

Holothuroids were decimated 250 mya by the end-Permian mass extinction event, as were nearly all other marine organisms. During this time, other classes of echinoderms either became extinct or were reduced to representatives from one or two genera. Holothuroids, however, survived as several divergent groups, perhaps aided by a deep-water, burrowing, or detritus-feeding lifestyle. By the Middle Jurassic Period, approximately 180 mya, holothuroids had diversified considerably, and several new important groups arose, including the living orders Aspidochirotida and Molpadiida, as well as the family Synaptidae within Apodida. In addition, several still-living taxa, now known only from deep water, disappeared from the fossil record. This disappearance may indicate that these organisms invaded the deep sea around that time. By the Jurassic, the family Achistridae, one of the most successful holothuroid groups from the late Paleozoic, had again become a dominant component of the fossil fauna. It met its demise in the Lower Cretaceous 140 mya. Holothuroid fossils younger than 65 million years old are surprisingly scarce, possibly because of a lack of collecting effort. Still, several groups at the family and subfamily levels appear to have arisen during this time, including the family Molpadiidae and two subfamilies within Synaptidae—the Leptosynaptinae and Synaptinae.

The class name Holothuroidea is derived from the term holothourion used by Aristotle to describe an animal that was only "slightly different from the sponges," "without feeling," and "unattached," but "plant-like." This puzzling description resulted in use of the latinized term Holothuria, and its variants from at least the early sixteenth century until the late eighteenth century for siphonophore jellyfish, sea squirts (Tunicata), and priapulid worms, in addition to what we now call holothuroids. Linnaeus originally referred most holothuroids to Fistularia, a name preoccupied by a fish and

A sea apple (Pseudocolochirus violaceus) with its feeding tentacles extended. (Photo by ©A. Flowers & L. Newman/Photo Researchers, Inc. Reproduced by permission.)

subsequently abandoned. However, by the nineteenth century, Holothuria referred almost exclusively to certain holothuroids. Nevertheless, the historic uncertainty in ascription engendered several alternative names for the entire class that gained some currency. Arguments over the provenance of the term continued into the twentieth century until Holothuria was formally assigned to the eponymous class of echinoderms in 1924.

Living holothuroids are divided into six orders. Ordinal assignment is based largely on the form of the calcareous ring and tentacles as well as the presence of certain organs, such as respiratory trees and the muscles that retract the oral region. Each order is described according to its taxonomic diversity and major diagnostic features:

APODIDA. Footless sea cucumbers. The order contains approximately 269 species in 32 genera and three families. Tentacles are digitate, pinnate, or, in some small species, simple. Respiratory trees are absent. Tube feet are completely absent. The calcareous ring is low and bandlike, without posterior projections. The body wall is very thin and often transparent. These sea cucumber are found in both shallow and deep water.

Elasipodida. Deep-sea sea cucumbers. The order contains approximately 141 species in 24 genera and five families. Tentacles are shield shaped and used in shoveling sediment. Respiratory trees are present. The calcareous ring is without posterior projections. With the exception of one family, Deimatidae, the body wall is soft to gelatinous. All forms live in deep water.

Aspidochirotida. Shield-tentacle sea cucumbers. There are approximately 340 species in 35 genera and three families in this order. Tentacles are shield shaped, that is, flattened and pad-like. Respiratory trees are present. The calcareous ring is without posterior projections. The body wall is generally soft and pliant. Most forms live in shallow water, although one family is restricted to the deep sea.

Molpadiida. Rat-tailed sea cucumbers. Approximately 95 species compose 11 genera and four families in this order. Tentacles are digitate to simple. Respiratory trees are present. The calcareous ring may have short posterior projections. The body wall is generally soft and pliant. Most forms live in relatively shallow water, although one family is restricted to the deep sea.

Dendrochirotida. Suspension-feeding sea cucumbers. The order contains approximately 550 species in 90 genera and seven families. Tentacles are highly branched. Respiratory trees are present. Some members have a calcareous ring composed of numerous small pieces or have long posterior extensions. These animals have muscles for retracting the oral introvert. in a few species, the body is hardened from enlarged plate-like ossicles and is U shaped. These sea cucumbers live either attached to hard bottoms or burrow in soft sediment. Most species live in shallow water.

Dactylochirotida. U-shaped sea cucumbers. The order contains approximately 35 species in seven genera and three families. Tentacles are simple or have a few small digits. Respiratory trees are present. The calcareous ring is without posterior projections. These sea cucumbers have muscles for retracting the oral introvert. All members have a rigid body encased in enlarged flattened ossicles. The body usually is U shaped. All members live burrowed in soft sediment. Most live in deep water.

compared with that of the other four living classes of echinoderms, the phylogeny of holothuroids was poorly known. These animals lack the integrated skeleton that provides an extensive fossil record and do not have the numerous morphological characteristics of other groups of echinoderms. The first speculations about evolutionary relations appeared in a tree figured in 1868 by the German zoologist Carl Semper. Several of Semper's suggestions have been corroborated with formal comparative analyses of morphological features and DNA sequences. The morphological work shows that apodans (members of Apodida) branched off quite early from the other holothuroids, which are united by the presence of hemal vessels and tube feet on the body wall. Among the latter forms, the elasipodans diverged next. Holothuroids in the remaining sister group are united by the

Sea cucumber anatomy. (Illustration by Laura Pabst)

presence of respiratory trees. This group radiated in the Upper Triassic to Lower Jurassic into the aspidochirotes and a group united by several characteristics, including posterior projections on the calcareous ring. The latter group diverged into the molpadiians and another lineage, comprising the den-drochirotes and dactylochirotes, the members of which have a retractible oral region called an introvert. Evidence from DNA sequences is largely congruent with the morphological data. However, several important points of disagreement remain: The arrangement of family-level branches within the dendrochirotes is poorly supported, and several families with hardened skeletons may turn out to be distantly related or subsumed within soft-bodied groups. The family-level arrangement within Elasipodida is still uncertain. In addition, the aspidochirote family Synallactidae may comprise two or more independently evolved lineages. Finally, the phyloge-netic affinities of the enigmatic families Eupyrgidae and Gephyrothuriidae are unknown.

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