It is a hard life for a mussel in the deep sea. Temperatures are always a chilly 4°C or cooler, food is scarce or patchy at best, not to mention the hydrostatic pressure of thousands of meters of water constantly pressing on you. If you can't stand the cold, there is one other option. Hopping from one hydrothermal-vent to another can provide a bit of warmth, at least if you can stand the toxic hydrogen sulfide. Despite these obstacles, mussels of the family Mytilidae are thought to have recently colonized the deep sea from shallow habitats. Most of the Mytilid mussels (subfamily Bathymodiolinae)found in the deep sea harbor symbiotic bacteria that can utilize the chemical energy in compounds like hydrogen sulfide to fuel an autotrophic existence. The microbes provide the food, while the mussels provide a safe habitat for their essential microbial partners. These mussels can be found in cold-seep environments, at hydrothermal-vents, and associated with sunken wood and whale bones.
However, it is presently unclear which path the mussels might have taken to colonize the deep sea. Some speculate that shallow water mussels (which typically tolerate a wide range of temperatures) first colonized wood and whale bone habitats which brought them to the abyss, where they then colonized cold-seeps and hydrothermal-vents. This scenario requires the mussels to first lose their tolerance of warm and fluctuating temperatures in order to thrive in the constant cold of the deep sea and then regain a wider tolerance of temperatures when they subsequently colonized the hydrothermal-vents. Alternately, mussels might have colonized the cold and warm habitats of the deep sea independently. Nélia Mestre and colleagues at the University of Southampton set out to explore the possibility that a shallow water mussel could directly colonize a deep sea hydrothermal-vent environment.
The team chose to study embryonic development of Mytilus edulis, a shallow water mussel that thrives in temperate intertidal and subtidal habitats, under a variety of temperature and hydrostatic pressure regimes. They fertilized gametes in vitro, at both high and low pressures, and followed their development through time at different combinations of pressure(ranging from 1 to 500 atm) and temperature (ranging from 5 to 25°C). The embryos were allowed to develop under these conditions for a set amount of time, and then were fixed and observed using light and scanning electron microscopy.
Mestre and colleagues found that fertilization can occur at low and high hydrostatic pressures; however, the embryos' development was adversely affected by the extreme conditions. Higher temperatures and higher pressures led to a greater proportion of abnormally developing embryos. Higher pressures also led to a retardation of developmental rate, independent of temperature. Importantly, they found that at cold temperatures the embryos could develop normally at even the highest hydrostatic pressures tested, although most of the embryos developed abnormally. However, the majority of the embryos developed normally at 200 atm of pressure as long as temperatures were below 20°C.
The ability of M. edulis to develop across such a wide range of temperatures and pressures is truly impressive. It is important that embryos are able to make this journey to the deep because they represent the best chance for dispersal in these typically benthic mussels. Few other species can survive this type of stress, and many shallow water species of invertebrates die within minutes of exposure to the hydrostatic pressures employed in this study. This study suggests that shallow water Mytilid mussels could directly colonize deep sea hydrothermal-vents without the requirement of an intermediate evolutionary step via wood and whale bone.
