Sunday 9 September 2012

Symbiosis and the success of Galeommatoid Bivalves.

Galeommatoid Bivalves are small (under 20 mm, mostly under 10 mm) nondescript Clams found in both soft and hard bottomed marine environments (which is unusual, as most Bivalves specialize in one or the other). They are thought to be one of the most successful groups of Bivalves, with over 500 species named, which is thought to be a fraction of the total number. Despite this the group have a very limited fossil record, suggesting their success and diversity is a recent accomplishment. Many species of  Galeommatoids are known to be commensals, living on the bodies of a very wide range of invertebrate hosts, but not gaining any nutrition from them.

In a paper published in the journal PLoS One on 8 August 2012, Jingchun Li and Diarmaid Ó Foighil of the Museum of Zoology and Department of Ecology and Evolutionary Biology at The University of Michigan, Peter Middelfart of the Australian Museum in Sydney cary out a review of the success of the Galeommatoids, in which they conclude that the habit of forming commensal relationships is the key to the groups success.

The non-commensal Galeommatoid Bivalve, Scintilla strangei, in a rock crevice. Li et al. (2012).

Bivalves have two basic defenses against predation, namely armor, in the form of a shell, and hiding, generally by burying themselves. Being very small Galeommatoids cannot grow particularly heavy shells to fight off predation, as many larger Bivalves do. Many non-commensal Galeommatoids live in crevices in rocks, suggesting this may be their ancestral habitat, but only two species of are known to live in soft sediments, both within the top few millimeters of sediment. One of these, Mysella charcoti, has been shown to be able to survive passage through the intestine of carnivorous Fish, suggesting that this species has been able to use armor as a primary defense. However Mysella charcoti is restricted to Antarctic waters, where it encounters a very limited range of predators, and it is unlikely that this strategy would be of much use in a more diverse community where the Clams might encounter other feeding techniques, such as the drilling of some Gastropods and Octopus.

Bivalves that live in soft sediments have to be able to keep in contact with the surface in order to be able to be feed and breath. This is usually done by means of a siphon that reaches from the buried shell to the surface, and which can be withdrawn when danger threatens. Thus for most Bivalves the length of the siphon dictates how deep into the sediment the animal can hide. The record for the deepest known burrowing Bivalve currently goes to the Geoduck (Panopea generosa), a very large burrowing Clam, from the west coast of North America, which can reach siphon lengths of up to a meter. However the tiny (under 5 mm) Galeommatoid Clam Mysella vitrea can live deeper than this; Mysella vitrea lives on the bodies of Ghost Shrimps (Trypaea australiensis) which dig deep burrows then pump water through them with their tales, a resource that the Clam is also able to share.

(Top) The Geoduck(Panopea generosa), the largest known burrowing Bivalve, can grow up to one meter in length and burrow as deep as its siphon is long. Biggest Menu. (Bottom) The commensal Galeommatoid Clam Neaeromya rugifera, lives on the body of a Mud Shrimp, Upogebia pugettensis, and can live as deep as the Shrimp does. Li et al. (2012).

Free living Galeommatoids are most abundant on hard substrates, such as rocks or corals, where they gain protection by growing wedged inside cracks and crevices, but a number of species also live commensally on rock-or-coral boring organisms, such as boring Bivalves, Sipunculan Worms and Slow Shrimps. While these forms can often also live free of their hosts, the commensal relationship does open up a large number of additional colonization sites, giving these species an advantage over similar clams that do not do this. It is likely, therefore, that these clams developed the habit of living commensally on burrowing organisms in hard surfaced environment, but that once they had acquired the ability it enabled them to spread rapidly into soft bottomed environments and to undergo an evolutionary radiation which has led to the success of the modern group. 

The commensal Galeommatoid Clam Scintillona bellerophon attached to its host, the burrowing Sea Cucumber Leptosynapta clarki.

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