Ecology and Evolution of the Gastrochaenacea (Mollusca, Bivalvia) with Notes on the Evolution of the Endolithic Habitat

The Gastrochaenacea are a compact group of mechanically and chemically boring bivalves that comprise a major but commonly overlooked element of tropical and subtropical endolithic faunas. As shown by their representatives in Diploria skeletons at Soldier Key, Florida, Spengleria rostrata, Gastrochaena (Gastrochaena) hians and Gastrochaena (Rocellaria) ovata are well suited to life in thin and rapidly eroded substrata. They have adapted to this habitat by evolving an unusual capacity for siphonal retraction and extension, a unique mechanism for directional boring, and the ability to secrete thick calcareous burrow linings and to rapidly repair damaged burrows. The Gastrochaenacea probably evolved in the Triassic or Lower Jurassic from shallow infaunal burrowing permophorids or grammysiids through an intermediate semiendolithic nestling stage. The evolution of endolithic lithophagids (Mytilacea) and gastrochaenids may have occurred in response to an expansion in the endolithic habitat accompanying the Triassic and Jurassic proliferation of scleractinian corals. Gastrochaenids underwent a secondary adaptive radiation in the Cretaceous and Tertiary, resulting in the tube-dwelling Kummelia and Eufistulana and the "igloo"forming Gastrochaena (Cucurbitula). Tube-dwelling gastrochaenids and clavagellids represent a classic example of synchronous evolutionary convergence from ecologically and phylogenetically distinct ancestors. The Clavagellacea probably evolved from deep-burrowing representatives of the Pandoracea. The primary nutrition of Soldier Key gastrochaenids apparently does not come from diatoms, the likely dominant representative of the phytoplankton, because diatom tests are rarely encountered in intestinal contents. Future investigations should explore the possibility that gastrochaenid nutrition comes primarily from the microbiota of resuspended sediment or from planktonic organisms lacking mineralized tests. Efficient size sorting of ingested particles correlates with ctenidial plication and smaller and more numerous major siphonal tentacles in Spengleria rostrata.