<strong>Form, Function, Food and Feeding in Stellate Echinoderms*</strong>

Abstract

The stellate echinoderms (crinoids, ophiuroids, asteroids) have arms that are involved in feeding. Arms are extensions from the central body and affect the form of the organism. The arms of crinoids, ophiuroids and asteroids differ in structure and have different characteristics. Crinoids are pentaradiate and primitively pentabrachiate, with an internal skeletal support that permits up and down movements. Although branching of the arms occurs, the basic structure remains unchanged. This, along with the mouth on the upper surface, i.e. directed away from the substrate, means that the food of crinoids has always been microplankton. The mouth of ophiuroids is directed towards the substrate. The internal skeletal support of their arms permits both up and down and lateral movements. Like crinoids, most ophiuroids are pentaradiate and pentabrachiate, with supernumerary arms being limited in number. Branching of ophiuroid arms occurs only in some euryalids. Ophiuroid arms also have spines, which do not occur in the arms of crinoids. With these characteristics, food of ophiuroids includes not only microplankton but also nekton, benthic particles and small prey. The major distinction of asteroids from crinoids and ophiuroids is that skeletal support of asteroid arms is external. Like ophiuroids, the mouth is directed toward the substrate. Unlike crinoids and ophiuroids, digestion in asteroids involves direct contact between the stomach and the prey and is extraoral in most asteroid taxa. Expansion of the oral surface increases the area over which the stomach can be extended and increases the capacity for feeding. This has occurred in two ways in stellate echinoderms: extension of the interradii in five-armed species and connection of the proximal parts of the rays in multiarmed species. Asteroid arms are not branched. Modified spines (pedicellariae) are used for feeding. The array of food is more extensive in asteroids because of these features including surface prey, particles, multiple small prey, macroprey and nekton. The form of the arm of each stellate group has constraints and potentials that determine food and feeding.

A Sister-Group Comparison of Branching and Pedicellariae in Brittlestars (Echinodermata: Ophiuroidea)

“…Basal branching probably gives biomechanically stronger support for further branching of the arms because the first fork and sometimes the second or third forks ( Astroboa nuda, Astrocaneum herrerai, Astrocyclus somaliensis, G. eucnemis ; Clark 1919 ; Döderlein 1927 ; Baker et al 2018 ) are integral with the disc, at least in mature animals with large discs. Basal branching also occurs in crinoid rays, in which the first axillary (fork brachial) is usually part of the calyx ( Clark 1921 ; Lawrence 1987 , 2012 ). Incorporation of basal forks into the central body (disc or calyx) of these two diverse taxa indicates a need for a stable basis for branching of arms.…”

“…Addition of rays occurs by insertion of new rays between or among the original five rays from the ring canal in some seastars (Asteroidea; Lawrence and Komatsu 1990 ) and some brittlestars (Ophiuroidea; Okanishi and Mah 2020 ). The addition of rays increases the length of usable ambulacra and the podia (tube feet) that line them to magnify the functions of podia for feeding, locomotion, gas exchange, or other roles ( Lawrence 2012 ). Alternatively, other echinoderms increase the length of usable ambulacra by branching of the original five rays.…”

“…Alternatively, other echinoderms increase the length of usable ambulacra by branching of the original five rays. Branching occurs only in suspension-feeders: many feather stars and sea lilies (Crinoidea; Messing et al 2021 ) and some ophiuroids ( Lawrence 2012 ). The ophiuroids with branching rays (or “arms” in ophiuroids) are found exclusively in the order Euryalida, a monophyletic taxon ( Okanishi and Fujita 2013 ; O'Hara et al 2014 , 2018 ; Thuy and Stöhr 2016 ).…”

“… Fedotov (1926) described the progression of branching in young growth stages of G. eucnemis , demonstrating the gradual growth of the disc almost to cover the first fork. Only more recently ( Strathmann 1975 ; Lawrence 2012 ) has branching been viewed in more functional and theoretical contexts. The morphological, histological, and genetic foundations for the proclivity to branch in the Euryalida, and not to branch in the other ophiuroids, has not been addressed.…”

Annotated checklist of Echinoderms from Araçá Bay, Southeastern Brazil

“…Echinoderms are benthic and exclusively marine animals with broad diversity inhabiting rocky shores, beach, bays and subtidal zones including the deepsea (Pawson 2007). All of them are free-living and present a huge variety of food strategies and ways of life (Hickman 1998;Lawrence 2012). The phylum consists of approximately 7,000 living species and 13,000 extinct species, comprising five monophyletic classes: Ophiuroidea (2136 species), Asteroidea (1800), Holothuroidea (1400), Echinoidea (1000) and Crinoidea (700) (Alvarado and Solís-Marin 2013;Kroh and Mooi 2015;Mah 2015;Stöhr et al 2015).…”