Ophiuroidea

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Introduction

The Ophiuroidea (brittle stars and basket stars) are echinoderms characterized by the disk being clearly demarked from the arms. Ophiuroids have two basic body types. Most are usually pentamerous with five slender arms, although species with six and more arms exist. The others, basket stars, also have five arms which then may branch numerous times forming a complex network.

Classification

Ophiuroids are divided into two orders: the Euryalida Lamarck 1816 which contains all the basket stars and the Ophiurida Müller & Troschel, 1840. There are about 2,000 accepted species (Stöhr, S. & O’Hara, T. (Eds) (2014) World Ophiuroidea database. Accessed at http://www.marinespecies.org/ophiuroidea on 2014-01-08). Within the Pacific abyssal system, i.e. greater than 3500 m, approximately 20 species have been recorded.

Basic anatomy

Ophiuroids have a well-developed external skeletal system comprising many small, scale-like plates of calcium carbonate. The arrangement of these plates, their shape and size are important taxonomic features (see Figure 1 and 2). The disk has a mouth on the underside (oral) but no opening on the upper side (aboral). The mouth is surrounded by five jaws in five armed species and leads to an oesophagus that connects to the sac-like stomach. The stomach fills much of the disc, but does not extend into the arms, and ends blindly; there is no anus. The water vascular system generally has one madreporite on the underside of the disk. A radial canal extends into each arm from the ring canal, and lateral canals from each radial canal supply the tube feet. The tube feet of ophiuroids lack suckers and ampullae. The arm in Ophiurida taxa consists of a series linked segments. Each segment consist of an internal ossicle, which link with those of the proceeding and following segments, and are boxed in by four plates – the dorsal arm plate, two lateral arms plates which usually also carry a series of arm spines and a ventral arm plate. In Euryalida species the arms are covered in granules, spinlets and in some groups rings of small hooks which obscure the plating and internal ossicles. The arms spines are more ventrally placed and are often hook-shaped. The articulation of the inner arm ossicles differs between the Ophiurida and the Euryalida allowing the arm in Euryalida taxa to curl vertically as well as horizontally while the arms of the Ophiurida can only move in a horizontal plane.

Ecology

Ophiuroids are carnivores, filter feeders, scavengers and detritiovores. The tube feet are often used in feeding not in locomotion; ophiuroids move by flexing their arms. Tube feet can be used directly to trap particles in the sediment or phytoplankton from the water. In other species with long arm spines, suspended particles in the water column are trapped in mucus coating the arm-spines. The trapped material is periodically gathered by the tube feet and passed down the arm to the mouth. Deposit-feeding ophiuroids may ingest sediment directly into the stomach. Basket stars are able to capture zooplankton such as small crustaceans or worms as they float through the complex branches of their arms. The flexible smaller branches can loop round the prey trapping it, the hooked arm spines lining the arm snag and hook on to ensure the prey does not escape. The arm can then be bent down and moved toward the mouth.

Species in the CCFZ

Most abyssal ophiuroids lie tantalisingly between large macrofauna and small megafauna. Some species are considered megafauna because they have long arms. No members of the Euryalida have been recorded to date from abyssal CCFZ depths.

Small macrofauna disk diameter (d.d.)<10 mm: Most ophiuroids – Ophiura loveni, Ophiura irrorata, Perlophiura profundissima, Ophiotypa simplex, Ophiocten hastatum, Ophiernus spp., Ophiacantha spp., Amphioplus daleus, Amphioplus verrilli. Most of these species will not be seen and even if they are, they would be difficult to identify.

Large macrofauna d.d. = 10-30 mm: Amphiophiura– may be seen in good resolution photographs. Amphiophiura bullata and A. convexa have been recorded in the CCFZ.

Large macrofauna/ small megafauna d.d. 30 mm +: Species of Ophiomusium, Ophiosphalma and Bathypectinura heros - can seen in photographs. Ophiosphalma armigerum has been recorded from the CCFZ. Ophiomusium glabrum, O. canaliculatum, O.granosum and O.spinigerum have been recorded from depths down to approximately 4000 m in other areas of the Pacific and so may occur in some regions of the CCFZ.

Ophiosphalma Morphotype

Morphotype description

Ophiuroid with long arms 5-7 X disk diameter. Disk is rounded to pentagonal. White in colour.

Taxonomic interpretation

Only Ophiosphalma armigerum has been recorded from the CCFZ to date.

Could be confused with

This morphotype is a distinctive shape and colour. It is possible that other species of Ophiosphalma or species of Ophiomusium could be present in which case it would not be possible to tell them apart from photographs.

Geographic range

Widespread across the CCFZ. Ophiosphalma morphotype has been recorded from German Claim area (MANGAN), French Claim area (BIONOD), Chinese claim area, Japanese claim area, IOM claim area, NORI claim area and Russian Claim area.