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A new study has shown that rock-boring bivalves are more diverse in morphology than other bivalve groups. Rock-boring does not require specialised physical traits, but instead is determined by availability of habitat. Most bivalve species can only transition from non-rock boring to rock boring, which could suggest an evolutionary dead-end for this trait if species fully transition.
Bivalves are a group of molluscs which live inside a hinged, two-part shell. These include a variety of species varying in shapes and sizes from giant clams to small scallops. They are known as important ecosystem engineers, providing food for a wide range of species and filtering the water they inhabit. Some species are also known to pose problems, causing damage to telecommunications cables, concrete foundations and even ship hulls.
Endolithic bivalves are those that have evolved either to bore directly into rocks or to modify existing cracks, dissolving sediment using a protein secretion or by mechanically grinding. In many examples of specialised functions, different species evolve similar features to better perform their function. For example, fast-swimming aquatic predators such as dolphins and tuna display a torpedo-like shape to streamline their bodies through water.
As rock boring is an energetically demanding behaviour, scientists had predicted that bivalves which bore would display specialised and distinctive shell shapes and therefore a lower diversity of forms. However, following analysis of three-dimensional morphometric data, scientists found that rock-boring bivalves displayed the highest level of morphological diversity when compared to other groups of non-boring bivalves.
Dr Katie Collins, Curator of Benthic Molluscs at the Natural History Museum and lead author says, 'What we found was totally unexpected. While we assumed boring molluscs would converge on a small range of forms that were really optimised for this function, this is not the case.'
'Instead, rock-boring bivalves have the greatest range of forms among their relatives, ranging from spheres to cigar and spoon shapes. It seems as though there is no single optimal form for boring, allowing these endoliths to maintain a high diversity of forms.'
Rock boring has many ecological advantages helping to protect the organism from predators and environmental conditions. Bivalves can transition from non-boring to boring but only the wood-boring bivalves known as shipworms, have transitioned the other way, and one species, Lithoredo abatanica, has even transitioned back to rock-boring. This study shows that the rock boring trait can be accessed from many different morphologies, and that it can provide great advantages and safety for species that adapt to it, but endolithy can be considered an evolutionary dead-end in that endolithic lineages never seem to diversify if species transition completely.
‘Convergence and contingency in the evolution of a specialized mode of life: multiple origins and high disparity of rock-boring bivalves’ will be published by Proceedings of the Royal Society B at 00.01 GMT on Wednesday 8 February 2023. https://doi.org/10.1098/rspb.2022.1907
Press pack including images can be found here.
Notes to editors
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