NICHE PARTITIONING AMONG ENDOLITHIC GASTROCHAENID BIVALVES

Endolithic gastrochaenid bivalves reveal a complex history of evolutionary adaptations for preventing predator access to their shells through the posterior, siphonal part of their borings. Mesozoic species of Gastrochaena, Carterochaena,and Spengleria demonstrate few anti-predation adaptations except for the short-siphoned endolithic life habit itself, posterior calcareous secretions which narrow the siphonal part of the boring, and,in Spengleria, separation of the siphon tubes to limit predator access. Cretaceous Kummelia and Eufistulana secondarily abandoned boring but kept a calcareous body sheath to ward off predators. During the Paleocene or Eocene, both Eufistulana and Dufoichaena added oval calcareous baffles at the base of the siphons as well as calcareous callums on the shell posteriors, both of which further limited access to the shell valves by predators. However, early Cenozoic gastrochaenids retained short to medium-length siphons. Greatly elongated siphons first appeared in the late Oligocene in Lamychaena, a genus which lacks basal siphonal baffles. Later in the Cenozoic, some Dufoichaena also greatly elongated their siphons. The Miocene and Pliocene saw the evolution of spike-like basal siphonal baffles in many relatively short-siphon species, including members of Spengleria, Gastrochaena, Dufoichaena, and the igloo-forming Cucurbitula. At least some Lamychaena have added chemical defenses against predators, such as stinging secretions. Despite this long history of escalation of defenses against predation, the plesiomorphic adaptation of relatively short siphons without basal siphonal baffles persists today in certain species of Gastrochaena and Rocellaria. The success of the Gastrochaenidae in defending against predation explains, in part, the evolutionary persistence of its genera, with only Carterochaena and Kummelia becoming extinct since the Late Triassic origin of this family.