Pycnogonids – sea spiders – are, of course, not spiders. Like spiders they are, however, chelicerates, because they possess clasping appendages called chelicerae on either side of the mouth. (Irritatingly, some sea spiders have lost these structures…) These bizarre marine animals are basically several pairs of legs stuck together. They have no body to speak of, no respiratory system and no digestive system (that doesn’t stop them sucking the juices from their food…) They have close external fertilization and males carry the embryos on a specially adapted appendage, the oviger.

Until recently, little was known about their feeding habits. A study that has just appeared in Invertebrate Biology, by Braby, Pearce, Bain and Vrijenhoek, reveals exactly what two species eat – sea anemones.

The pycnogonids and their prey were found about 3000m down in the Monterey Submarine Canyon, at the site of whale fall (ie a dead whale), a tree fall (a sunken tree trunk) and a clam field. The sea spiders were observed eating sea anemones, including the bizarre pom-pom anemone, Liponema brevicornis.

Fig. 1. A. Whale-fall habitat, with an individual of Colossendeis gigas climbing over whale vertebrae and several small white anemones, Anthosactis pearseae, attached to the bones. Scale bar=10 cm. B. Wood-fall habitat, with a pom-pom anemone, Liponema brevicornis, nestled against the wood. A pycnogonid, C. gigas, holds a tentacle of L. brevicornis in its proboscis. Scale bar=10 cm. C. Cold-seep clam field habitat, with unidentified pycnogonid (arrow) and an individual of L. brevicornis among clams. Scale bar=10 cm. D. Individual pom-pom anemone, L. brevicornis, rolling in benthic current, illustrating its other common name, "barrel anemone." Scale bar=10 cm. E. Individual of C. japonica perched in feeding position on a large individual of L. brevicornis, at wood fall. Scale bar=10 cm. F. Individual of C. gigas perched in feeding position over a small individual of L. brevicornis, which appears to be in transition between barrel-shape and sedentary posture; at whale fall. Scale bar=5 cm. G. Individual of C. gigas holding two tentacles of L. brevicornis with its proboscis; at whale fall (note baleen strewn across sediment). Scale bar=5 cm. H. Individual of C. gigas holding an individual of A. pearseae in its pedipalps and proboscis; at whale fall. The pycnogonid is in the grasp of the remotely operated vehicle's manipulator arm. Scale bar=5 cm.

The authors conclude:

Although deep-sea sediment-dominated habitats are considered to be food limited and low in biomass, organically enriched oases such as whale falls, wood falls, and clam beds are scattered unpredictably in these nutritional deserts. Our observations indicate that two species of sea anemones potentially provide a constant food supply for pycnogonids in close proximity to this whale fall. Other potential prey items for pycnogonids at the whale- and wood-fall sites include a few other anthozoans (sea anemones and sea pens), hydroids that rapidly colonized settlement plates and other deployed equipment, holothurians, and abundant sessile polychaetes in the genus Osedax, which occur on sunken bones all over the world (Rouse et al. 2004; Glover et al. 2005; Fujikura et al. 2006; Vrijenhoek et al. 2008). Our time-series studies of these deep-sea oases (Braby et al. 2007), and of the pycnogonids and anemones that aggregate there, provide a glimpse of the interactions that shape these rarely observed communities.


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