Southern California 2013, Leg 2: Molecular Ecology
May 18-24, 2013
Our expedition aims to sample invertebrate animals living on a whale carcass and at hydrocarbon seeps in the Southern California region. It was hypothesized that deep-sea hydrothermal vent fauna can disperse between widely separated sites by exploiting intermediate stepping-stones formed around whale-falls and other chemosynthetic habitats. We suspect that this popular hypothesis is mostly incorrect due to species misidentifications and undersampling of cold-water hydrocarbon seeps along continental margins. Instead, we propose that cold seeps provide the primary habitat for many opportunistic organisms that can also invade nearby habitats such as whale-falls and sedimented hydrothermal vents.
Hydrocarbon seeps found along the western North America margin are primary habitats for a diverse assemblage of vesicomyid clams and other chemosymbiotic animals such as the siboglinid tubeworms, Lamellibrachia barhami and Escarpia spicata. Our ongoing molecular phylogenetic studies strongly support the hypothesis that these animals first evolved in seep-like habitats and more recently invaded organic food-falls and vents. For example, Akebiconcha (=Vesicomya) gigas is a widespread eastern Pacific species that lives in methane seeps from the Oregon coast to the Costa Rica margin. It also occurs in sedimented areas near vents along the Juan de Fuca Ridge and in the Gulf of California. That it also occupies chemosynthetic whale-falls along the California margin is not surprising. The tubeworms, L. barhami and E. spicata, also occupy seeps along this margin, and they are rarely found in sediments near vents and at a whale-fall. These opportunistic worms and clams have never been found on basaltic vent sites of the Northwest pacific ridge systems, the East Pacific Rise, or the Alarcón Rise.
Given the rarity of these species at sedimented vents and whale-falls, it is more likely that these alternative habitats represent demographic sinks rather than sources of dispersing larvae that propagate throughout widely dispersed chemosynthetic systems. New collections on this expedition from a Southern California whale-fall and cold seeps should fill a significant sampling gap between the Northwestern Pacific vents and seeps, Monterey Bay, and the Gulf of California. High-resolution maps from MBARI’s mapping AUV will help us to locate new cold seeps and characterize the sulfide levels that support chemosynthetic microbes and chemosymbiotic invertebrates.