Introduction

Echinodermata are among the most familiar marine invertebrates. The phylum, characterised by the great morphological variety of its members, belongs to a branch of the animal kingdom known as deuterostomes. The Echinodermata have been extensively studied, particularly because of some aspects such as the ample fossil record extending back to the Precambrian, their ecological importance in the marine environment, the interesting mor-

F. Zito (e-mail: [email protected]), C. Costa, S. Sciarrino, C. Cavalcante,V. Poma,V. Matranga Istituto di Biomedicina e Immunologia Molecolare (IBIM) "Alberto Monroy", Consiglio Nazionale delle Ricerche,Via U. La Malfa 153,90146 Palermo, Italy

Progress in Molecular and Subcellular Biology Subseries Marine Molecular Biotechnology V. Matranga (Ed.), Echinodermata © Springer-Verlag Berlin Heidelberg 2005

Fig. 1. A Living fossil, the pencil urchin Eucidaris tribuloides. B Presumptive crinoid fossil found in 1981, in the Black Forest, Germany

phology of adult organisms and the advantage of experimentally manipula-ble embryos. Approximately 13,000 echinoderm fossil species are known (Fig. 1), while living species number about 7,000 and fall into five well-defined classes:

• Crinoidea (sea lilies and feather stars), with about 600 living species. The body is oriented with the mouth facing up and they may or may not have a stalk.

• Asteroidea (starfishes or sea stars), the most familiar forms to man, with about 1,500 living species. Typically, they have their oral surface on the ventral side and usually have multiple "arms" surrounding a central disc.

• Ophiuroidea (basket stars and brittle stars), with five arms, typically flexible, radiating from a central disc, which are used for locomotion. The class includes about 2,000 living species.

• Echinoidea (sea urchins, sand dollars and sea biscuits), with a usually globular body, formed by the fusion of skeletal plates, without distinct arms. Some of them have bilateral symmetry, which occurred secondarily during evolution. About 1,700 living species are known.

• Holothuroidea (sea cucumbers), characterised by a cylindrical shape and a relatively soft body. About 1,100 living species are known.

The Concentricycloidea (sea daisies),with only two species, is an enigmatic group of echinoderms whose phylogenetic position remains elusive, although evidence suggests a relationship with asteroids (Pearse and Pearse 1994). The phylogenetic relationships among the five classes have been extensively controversial, but it seems generally accepted that the class of Crinoidea branched first and that the Echinoidea and Holothuroidea are sister clades (Fig. 2).

Although the main character of the body plan of adult echinoderms, the pentamerous symmetry, led the father of palaeontology, George Cuvier (1769-1832), to place them in the Radiata phylum, it is now widely accepted

Fig. 2. Hypothetical phylogenetic tree of the five living classes of Echin-odermata. This hypothesis, predicting that the Crinoidea branched earlier than the other classes and suggesting that the Echinoidea and Holothuroidea are sister clades, was first proposed by Bather (1900) and is supported by morphological and molecular evidence. (Littlewood et al. 1997)

Fig. 2. Hypothetical phylogenetic tree of the five living classes of Echin-odermata. This hypothesis, predicting that the Crinoidea branched earlier than the other classes and suggesting that the Echinoidea and Holothuroidea are sister clades, was first proposed by Bather (1900) and is supported by morphological and molecular evidence. (Littlewood et al. 1997)

Ho I oth uro i dea

Crinoidea

Ho I oth uro i dea

Crinoidea that the phylogenetic proximity of echinoderms is to chordates. Among marine invertebrates, in fact, the echinoderm clade has a unique position in the evolutionary tree: during evolution it first appeared in the Metazoa phylum with features of deuterostomes, in direct evolutionary line with chordates and vertebrates, thus implying that there is less divergence between echino-derms and vertebrates than between echinoderms and other invertebrates. This is not surprising since, although the two phyla appear so dissimilar, molecular data and fossil records strongly support this grouping, even if the attempts to identify a common ancestor are highly speculative. As the eminent anthropologist Konrad Lorenz observed, "It is not a theory, but an irrefutable historical fact, that the living world - since its origin - has evolved from 'below' to 'above'". Actually, it is becoming clearer that the genes encoding developmental regulatory proteins already identified in other phyla, the modified interactions among them and changes in their expression patterns are at the basis of the evolution of animal morphology (Raff 1996; Wray and Lowe 2000).

The name echinoderm is derived from the Greek word meaning spiny skin and it has been the original denomination for sea urchins, since spines are their main external characteristic. Echinoderms are exclusively marine invertebrates and, with a few exceptions, are all benthic organisms (bottom-dwellers). A number of lifestyles are typical among different classes, e.g. sea star and crinoids are predators and filter-feeders respectively, while sea urchins scrape algae from rocks, and holothurians, sand dollars and ophi-uroids often feed on remains.

Echinoderms usually have separate sexes with no evident sexual dimorphism. Reproduction is typically achieved by external fertilisation, with eggs and sperm freely released into the seawater. The life cycle is usually complex. In most echinoderms the embryo develops into a planktonic larva with a bilateral symmetry, that in some species goes through metamorphosis, typically radical, before reaching the final adult morphology.

A number of morphological features are unique to the Echinodermata phylum, including:

• a pentaradial symmetry in adults (higher-order radial symmetry can be observed in several cases, but it is clearly a secondary modification);

• a water vascular system, consisting of a set of water-filled canals branching from a ring canal and leading to tube feet, which has many important functions, including locomotion, cellular respiration and feeding;

• a mesodermal endoskeleton composed of calcareous plates assembled in a meshwork;

• a complex subepithelial nervous system with radial nerves running under each of the ambulacra, the row of tube feet, but without a recognisable central part;

• sensory neurons located primarily within the ectoderm of podia, without specialised sense organs;

• a circulatory system, if present, consisting of a haemal system derived from coelomic sinuses.

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