Astroboa nuda Lyman, 1874 (width ca. 100mm tip to tip)

Featured Invertebrate Data


Frequency on Okinawa: Collection / Photo Data:
[brackets indicate range for all Okinawa-collected / photographed specimens of the species]

Species Account:

        Astroboa nuda is considered to be uncommon on Okinawa. Although I've seen quite a few animals, but only during night dives (in all fairness I should mention that all were seen at Horseshoe Cliffs, where the great majority of my night dives were experienced). I've collected / photographed only a few individuals over a period of many years of Okinawan diving.

I've added the following information concerning the Class Ophiuroidea from Stöhr, O'Hara and Thuy (2012):


General background
The Ophiuroidea or brittle stars, basket stars (euryalids with branching arms) and snake stars (euryalids with non-branching arms), are the largest group among extant echinoderms, with 2064 described species, found in all oceans from the intertidal to the greatest depths. The name Ophiuroidea is derived from the Greek words ophis, meaning snake, and oura, meaning tail, in reference to the often thin, snail-like winding or coiling arms. The discovery of the currently recognized extant species began with two descriptions, published in the Systema Naturae (Asterias caput-medusae Linnaeus, 1758), now in Gorgonocephalus, and Asterias ophiura Linnaeus, 1758, now in Ophiura). From the mid-eighteenth century, the discovery rate accelerated and remained relatively high for about a century, when it leveled-off to today's lower rate. Remarkably, the first deep-sea animal ever to be reported on was the brittle star Gorgonocephalus caputmedusae accidentally dredged up by Sir John Ross in 1818 while sounding the bottom of Baffin Bay in his attempt to find the North West passage. The first fossil ophiuroid was described as early as 1804 from the Middle Triassic of Göttingen, Germany (Asterites scutellatus Blumenbach, 1804; now in Aspiduriella). The description rate for fossils has remained relatively low and constant since that date. The use of isolated skeletal elements as the taxonomic basis for ophiuroid palaeontology was systematically introduced in the early 1960s and initiated a major increase in discoveries as it allowed for complete assemblages instead of occasional findings to be assessed.

The typical ophiuroid body plan shows a pentagonal to round central disc that is offset clearly from the five arms; but a considerable number of species depart from this generalized shape. Species with six, seven and up to ten arms are known. In basket stars the arms branch once or multiple times. Most species are moderate in size with disc diameters between 3 mm and 50 mm; the largest species of basket stars may have discs of 150 mm diameter. The length of their arms is usually measured in relation to their disc diameter and varies from about 2-3 times the disc diameter to 20 times or more (e.g. Macrophiothrix, Amphiodia).
At first glance, ophiuroids may resemble certain seastars, but a number of unique features set them apart. The ambulacral groove, found on the underside of the arms, is completely closed over by hard skeletal parts (lateral and ventral plates), whereas in asteroids it is an open furrow. Ophiuroids lack an anus and the madreporite that connects the water vascular system (often through one or several hydropores) with the surrounding ocean water is part of the mouth skeleton (one of the oral shields), instead of a plate on the dorsal surface as in asteroids. The ophiuroid mouth opening is closed by a number of jaws that corresponds to the number of arms. The jaws or oral plates are hypothesised to have evolved from ambulacral plates and are homologous to another ophiuroid specialisation, the arm vertebrae. Ophiuroid tube feet lack suction cups and are rarely used for locomotion. Instead, ophiuroids move by twisting and coiling their arms, pushing against the surface like a snake or gripping objects and pulling themselves forward. Swimming has been reported in some species. No eyes have been found in ophiuroids, but arm plates, functioning as calcitic microlenses above light sensitive tissues have been identified in several phototactic species in the genus Ophiocoma. Brittle stars easily fragment (autotomize arms) when stressed (Stöhr & O'Hara, personal observations), a property of the mutable collagenous tissue, found in all echinoderms.

        I've added a second page w/ a series of images of the species.

Literature Cited:

Page Date: 01 Sep '17
Page Modification Date: 01 Sep '17
Digitally manipulated photo
Copyright © 2017 Robert F. Bolland