1. Acknowledgements & Contact
Asynchronous growth in Amphiura ungulata (Echinodermata: Ophiuroidea) Sabine Stöhr1 & Øydis Alme2, 3 1Swedish Museum of Natural History, Dept of Zoology, Stockholm 2Uni Research AS, P.O Box 7810, 5020 Bergen, Norway 3Bergen Museum, Univ. of Bergen, Nat. History Collections, 5020 Bergen, Norway
Abstract
Methods
Results & Discussion
Norwegian ecosystem surveys to West Africa (Angola - Morocco) collected benthos samples in the years Among the obtained brittle stars we found 124 specimens with two long arms and three extremely short or absent arms and an elongated, narrow disc. These were identified as juvenile Amphiura ungulata by comparison with adult specimens. The most distinguishing character of this species are arm spines with a hoof-shaped transect. The arms grow strongly asynchronous in a pattern with large delay between the groups of arms. The disc grows at first along with the first two arms, later with the remaining three arms. The oral frame has its normal round shape from the earliest stage. The large number of specimens with unequal arms suggests that this condition is not the result of damage and regeneration, but a normal ontogenetic stage in this species. We document the morphology during ontogeny by SEM, and amend the species description. This is the first time such a highly unusual stage is reported.
157 specimens of Amphiura ungulata were collected along the west coast of Africa (northern Angola to Morocco), from 2005 to 2012, using a 0.1 m2 van Veen grab, sieved with mesh size 0.5 and 1 mm, and fixed in 96% ethanol or 4% borax buffered formaldehyde solution, respectively.
We selected specimens of several sizes for scanning electron microscopic (SEM) examination. They were treated with diluted (50:50) household bleach (NaOCl) to remove the outer skin layers. After washing in tap water they were left to dry, then mounted on aluminium stubs with spray glue and examined in a Hitachi FE-SEM 4300.
The smallest specimens in our material has a 1.25 mm disc long, 0.5 mm wide disc, with numerous disc scales and two long arms. A primary rosette is absent. In addition to the ventral arm spine, also the middle spine is hoof-shaped in transect, as revealed by SEM. The smallest stage immediately after metamorphosis has not been found yet, but it would be highly interesting to know if the elongated disc shape is present in the earliest postlarva or develops gradually. In the first postlarval stages of other species of Amphiura the arms are all represented by terminal plates and the first segment that is incorporated under the disc (Stöhr 2005). In A. ungulata, the first segment is represented by the first ventral arm plate, following this general pattern. The oral frame in all specimens has the typical round shape, which suggests that the disc grows independently, at first in opposite directions together with the first developing arms, later adding width along with the other three arms to obtain a round shape. This is an extreme case of asynchronous development that has not been observed before.
Our material includes 124 specimens with two long arms and three short or absent ones in various stages of development. Explaining this remarkable and unique condition is difficult, but the large number of these specimens suggests that this is the normal ontogeny of this species rather than some kind of accidental damage. We know very little about the ecology and biology of A. ungulata, but most species of Amphiura have an infaunal life-style, burrowing in soft bottoms (Woodley 1975),feeding on particles. Perhaps, the larvae of A. ungulata live under energy limiting conditions and cannot produce a postlarva with five arms. Possibly, the postlarva invests its energy reserves into quickly growing two arms to be able to feed efficiently. Then the remaining three arms are grown with the energy obtained by the first two. A
Introduction
Amphiura ungulata Madsen, 1970, was described from West Africa, based on few damaged specimens. The distinguishing feature of this species is the long ventral arm spine which is in transection hoof-shaped. This shape is so unique among amphiurid brittle stars that damaged and juvenile specimens can be identified beyond doubt by this spine.
Since 2005, the Institute of Marine Research (Bergen, Norway) has been running several ecosystem surveys in the Guinea Current and Canary Current Large Marine Ecosystem areas as part of the EAF-Nansen program. These surveys have resulted in a rich collection of ophiuroids, among them a number of unusual specimens. They have an elongated disc and only two arms. Close examination revealed the typical hoof-shaped arm spines of A. ungulata. This is the first species of ophiuroid known that has two-armed juveniles (Sumida et al. 1998; Stöhr 2005). 
Here we present a first description of the unusual ontogeny of A. ungulata and additional observations on its morphology by scanning electron microscopy. It is also the first report of the species since its original discovery. B
References
Stöhr, S. (2005) Who’s who among baby brittle stars (Echinodermata: Ophiuroidea): postmetamorphic development of some North Atlantic forms. Zoological Journal of the Linnean Society 143, 543–576.
Stöhr, S., O’Hara, T.D. & Thuy, B. (2012) Global diversity of brittle stars (Echinodermata: Ophiuroidea). PLoSOne 7, 1–14.
Sumida, P.Y.G., Tyler, P.A., Gage, J.D. & Nørrevang, A. (1998) Postlarval development in shallow and deep-sea ophiuroids (Echinodermata: Ophiuroidea) of the NE Atlantic Ocean. Zoological Journal of the Linnean Society 124, 267–300.
Woodley, J.D. (1975) The behaviour of some amphiurid brittle stars. Journal of Experimental Marine Biology and Ecology 18, 29–46. C
Acknowledgements & Contact
We thank Jon Kongsrud and the late Christoffer Schander for their support. Thanks to the cruise leaders and ship crews that helped collecting the specimens.
Sabine Stöhr,
Øydia Alme
Fig. 2. A. ungulata juvenile, dorsal disc (A), ventral disc with 3 terminals (B),
hoof-shaped arm spines (C). SEM images. Arrow indicates terminal plate.
Fig. 1. Dorsal view of juvenile A. ungulata