The distribution of hydroid sibling species on hermit crabs in estuaries in the Gulf of Maine

Abstract

Estuaries in the Gulf of Maine contain disjunct, isolated populations of a number of invertebrate taxa typically distributed only sound of Cape Cod. These estuarine populations may either be remnants of a southern fauna that were stranded by Quaternary changes in water temperature, or they may have been established more recently via larvae that entered the estuaries from the open ocean. We tested these hypotheses by examining the distribution of sibling species of hydroids that are symbiotic on paguriid hermit crabs. These crab-hydroid species pairs differ greatly in their ability to colonize new areas through the water column. The hermit crabs possess a planktonic larva that can disperse great distances, while the hydroids have a crawling planula larva that is extremely limited in its dispersal capabilities. Consequently, estuarine hermit crab populations that lack their native hydroid symbiont are likely to have originated by larval colonization. Hydroid sibling species were identified on the basis of MDH electromorphs. Species identifications were confirmed through a limited number of test matings with colonies of known species identity. In three out of the four estuaries that we examined, shells inhabited by the southern hermit crab,Pagurus longicarpus, were occupied only by the local northern hydroid,Hydractinia polyclina. This pattern is consistent with a more recent establishment of the hermit crab populations through larval dispersal or transport. However, in Casco Bay, shells inhabited byP. longicarpus were occupied by a mixture of the northern hydroid,H. polyclina, and the southern hydroid,H. symbiolongicarpus, that is associated withP. longicarpus throughout the southern part of its range. Thus, theP. longicarpus population in Casco Bay may either be a relict of a former, more broadly distributed population of this crab in the Gulf of Maine, or if recently established, is likely to have originated by the introduction of adult crabs with hydroid-covered shells.