Macrosymbiotic association of the myid bivalve Cryptomya with thalassinidean shrimps: Examples from modern and Pleistocene tidal flats of Japan

Macrosymbiosis, a macroscopic-scale interspecific association, between the myid bivalve Cryptomya and thalassinidean shrimps is described. In modern tidal flats, living individuals of Cryptomya truncata are observed to protrude their posterior ends into the burrow lumen of thalassinidean shrimps such as Nihonotrypaea japonica and Upogebia yokoyai. Furthermore, a fossil impression of Cryptomya busoensis occurring in a Pleistocene tidal flat deposit of the Shimosa Group, similarly has attached its posterior end to a fossil upogebiid burrow, Psilonichnus isp. Both Cryptomya species, which are characterized by extremely short siphons, probably utilize the sediment/water interface on the internal surface of the burrow. As the fossil record of Cryptomya dates back to the Miocene and such a mode of occurrence can be recognized even in the rock record, careful observation would be expected to reveal the origin and evolution of the characteristic macrosymbiosis.