Like other asterozoans, asteroids have a characteristic star-shaped body plan consisting of a central disc and multiple (typically 5) radiating arms. Asteroids are most easily distinguished from other asterozoans (the Ophiuroidea) by the structure of the arms. In asteroids, skeletal support for the arms is provided by the ossicles of the body wall, which merge with those of the central disc, giving the arm a very broad based attachment to the disc. This skeletal arrangement allows for the extension of a comparatively large coelomic cavity from the central disc into the arms, which serves to hold some of the animal's organ systems, namely the gonads and pyloric caeca. Additionally, this skeletal arrangement also limits lateral flexion of the arms. Locomotion by asteroids is accomplished almost exclusively by means of the podia (tubefeet) from the water vascular system. Differences in morphology between asteroids and ophiuroids are described further in Blake (1998) and Dean (1999).
Figure 2: A typical starfish, Asterias rubens, with tubefeet visible on the edge of the arm in the foreground. Image copyright © 2004 Kåre Telnes. Cushion stars, like this Culcita novaeguineae, may have arms so short that they look more like a ball than a star. Image copyright © 2003 Massimo Boyer.
Taxonomy of asteroids usually is based on externally observable characteristics of the skeleton, particularly the primary ossicular series which define the body wall (ambulacrals, adambulacrals, marginals, terminals, actinals, abactinals), as well as secondary ossicles such as spines, spinelets and pedicellariae. Works by Perrier (1884) and Sladen (1889) laid the taxonomic foundation of most asteroid groups. Many other authors have contributed to and/or refined the asteroid classification scheme, notably Fisher (1911, 1928), Verrill (1914), Fell (1963), Spencer and Wright (1966) and McKnight (1975). Blake and Elliot (2003) provide clear definition of ossicle terminology. Blake (1987) provides classification and diagnoses of asteroid groups.
Perhaps the most important ossicular series defining the Asteroidea is the ambulacral column, found along the oral surface of the disk and radiating arms and associated with two or four rows of podia. The asteroid ambulacrum is distinguished by erect ambulacral ossicles arranged in series along the length of the ambulacral column. Critical differences in structure and arrangement of the ossicles of the ambulacral column define two groups of asteroids: an extinct fauna restricted to the Paleozoic and the mostly extant (mostly) post-Paleozoic asteroids (Blake 1987, Gale 1987). Blake and Hagdorn (2003) recently recognized this distinction formally with diagnosis of a new subclass: Ambuloasteroidea, containing the Paleozoic Calliasterellidae and Compsasteridae in addition to post-Paleozoic asteroids (Infraclass Neoasteroidea Gale 1987).
Figure 3: Morphology of asteroids. A, aboral and oral surfaces of a generalized asteroid. Image © BIODIDAC. B, transverse section and perspective view of a generalized arm (soft tissues and spines removed); note the arched ambulacral ossicles forming the ambulacral groove and the dorsal podial pores between ambulacral ossicles. One podium (tubefoot) on the left is drawn in outline only to illustrate how the podia descend through the podial pores. Image © 2004 Emily Knott.
Application of the extraxial-axial theory (EAT) to asteroid morphology significantly aids our understanding of ossicle homologies within the Asteroidea and between asteroids and other echinoderms (Mooi and David 2000, Blake and Elliot 2003, Blake and Hagdorn 2003). According to the EAT, the ambulacral and terminal ossicles of asteroids are axial elements. These ossicles are formed according to the Ocular Plate Rule (OPR) and are associated with the developing water vascular system during ontogeny as are the axial ossicles of other echinoderms. The remaining asteroid ossicle series are extraxial elements, which can be added during ontogeny without any particular ordering system (although secondarily ordered serial homologous elements are common in the asteroids, e.g. adambulacrals and marginals). In comparison to axial elements, extraxial ossicles are prone to much more evolutionary lability (Mooi and David 1997).
Synapomorphies of the crown group: Ambuloasteroidea
Summarized from Blake (1998; 2000), Mooi and David (2000) and Blake and Hagdorn (2003).
- Deep ambulacral groove—The paired ambulacral ossicles are erect and arch across the arm axis forming a clearly defined furrow. The extent of the arch and definition of the furrow are expected to be weaker in the earliest asteroids, but these characters are difficult to observe in most fossil specimens.
- Dorsal podial pores—The dorsal podia pores are passageways between ambulacral ossicles through which the tubefeet descend. These pores allow for internal protection of the ampullae, dorsal outpockets of the podia, which contract and expand with extension and retraction of the podia. The ampullae of earlier asteroids were external, in closed, cup-like podial basins formed by the ossicles of the ambulacral column.
- Offset positioning of the ambulacral and adambulacral ossicles and differentiation of articulation structures in ossicles of the ambulacrum—These features describe a variety of related apomorphic characteristics of ambuloasteroids. Offset positioning of the ambulacral and adambulacral ossicles allows for soft tissue connections between the ambulacral and both adjacent adambulacrals which is further enhanced with differentiation of articulation structures on the ossicles. This arrangement allows more complex movement in the ambuloasteroids. In non-ambuloasteroids a single ambulacral ossicle abuts a single adambulacral.
- Presence of an odontophore—The odontophore is a small interradial ossicle associated with the mouth angle ossicle. The odontophore is expected to the homologue of the axillary in Paleozoic asteroids.
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