Friday, December 2, 2011

Beautiful sea spiders!

By David Staples, Marine Sciences, Museum Victoria, Australia

© David Shale

Sea spiders, or pycnogonids as they are correctly known, are a most unconventional group of marine arthropods numbering about 1,300 species worldwide. They are particularly unusual because there is invariably an exception for every diagnostic character and aspect of their life cycle. For example they include forms with 4, 10 and 12 legs, leg spans that range from a few millimeters to 74 cm and species that are blind or have one two, four or eight eyes! The males (usually) carry eggs on special appendages whilst in two families males have never been recorded carrying eggs. Some species brood their young and others parasitize an invertebrate host. In some parts of the world they commonly form part of the intertidal (shoreline) fauna yet they are also found at abyssal depths.

The sea spider’s body is typically no thicker than its legs and often look more like a collection of drinking straws than anything else, none more so than members of the large deep-sea genus Colossendeis. Colossendeis species are the giants of the pycnogonid world with leg spans commonly in the range of 30-50 cm.
Distribution records of Colossendeis are often widely spaced and without any apparent connection. In common with many of their smaller counterparts they are very capable ‘swimmers’ in the sense that by rapidly treading water they can lift themselves off the sea floor and into the water column. Assisted only by the occasional kicking action they can then be carried over long distances by the currents.
The deep sea provides a unique environment. The temperature at depth is relatively constant, so too are salinity levels and of course light never reaches these depths so it matters little to a deep-sea specimen whether it is in the tropics or southern Ocean. Considering the slow moving deep-sea currents that meander around the globe perhaps it should not be surprising that species recorded from the tropical central Atlantic should also be found in the temperate waters of southern Australia and possibly southern Indian Ocean.

© David Shale