Mussel Identification, or A Bathymodiolin mussel by any name…
Cheryl Morrison, PhD and Katharine Coykendall, PhD
Bathymodiolin mussels, all in the genus Bathymodiolus, evolved in environments very different than the ones we land-dwelling organisms, that get our energy indirectly from the sun through plants, did. Bathymodiolus species live in areas of the sea floor where fluids of chemicals, such as methane and sulfide gasses, are escaping through the sea floor. Not only can these mussels tolerate the chemicals that would be toxic to most marine organisms, they thrive here! They can filter feed, like many sedentary marine organisms (including intertidal mussels) but they also have unique microbes that live in their gills as symbionts. The symbiotic microbes can convert methane and/or sulfide to energy (chemosynthesis), and pass it along to their mussel hosts. It is such an efficient relationship that the mussels get most of their nutrition from the microbes, and have a reduced digestive tract.
Animals that have chemosynthetic bacterial symbionts acquire them in one of two ways: vertically or horizontally. Vertical transmission means that adult host animals pass their symbionts to their offspring. Like a favorite piece of jewelry handed down through generations, the symbiont is transferred to the young mussel. Some species of deep sea clams acquire their symbionts this way. Horizontal transmission means that at some point early in their development, host organisms capture symbionts from free-living bacteria in the environment. This would be more akin to going and buying a new piece of jewelry. Studies have shown that Bathymodiolus mussels acquire its symbionts this way.
Scientists can determine which mode of transmission a host uses by looking at the DNA of the host and the DNA of the chemosynthetic bacteria that live inside them. If vertical transmission is the norm, then one would expect that the pattern of evolution of the chemosynthetic bacteria to mirror that of the host’s. The closest relatives of the bacteria would be found in the closest relatives of the hosts. After many generations of being housed within the host, bacteria tend to lose some functional genes as they rely more and more on the host, so they have a reduced genome.
On the other hand, in horizontal transmission, every new generation of host captures a sample of bacteria from the environment. The evolutionary history of the bacteria and the host would not be parallel. Also, because the bacteria have to survive out in the environment and as a symbiont within a host, it has to have a complete, fully functioning genome. Interestingly, some species of Bathymodiolus have just methanotrophic symbionts, while others host both methanotrophs and thiotrophs (sulfide utilizers).
These mussels are often the first species to arrive at a new gas seep site and can cover large areas of the sea floor, as we saw on our dives. Finding a vast mussel bed without other characteristic seep animals like tubeworms may indicate that the seep is relatively young. The mussels may also actively exclude other potential settlers to the seep site by feeding on their larvae.
There are 35 species of Bathymodiolus mussels described to date, and 9 of these occur in the Atlantic, with the majority (7) found at cold seep sites. Similar shell morphology (shape) and soft tissue anatomy means that traditional taxonomy based on appearance has proven difficult as the sole means to distinguish among species.
We can delineate species by sequencing a section of DNA and comparing these sequences among species. From such analyses, two major groupings within the Bathymodiolus genus have been detected: The B. childressi group that includes some Pacific species and tends to be at shallower sites; and the B. boomerang/heckerae group, with both hydrothermal vent and cold seep species and tends to be found at greater depth. B. heckerae is found at Blake Ridge, a cold seep off of the Carolinas, as well as a seep off the west coast of Florida. We found bathymodiolin mussels at cold seeps in both Norfolk and Baltimore Canyons. By sequencing a barcoding gene (COX1) from several mussels from Baltimore Canyon collected last year, we know that the mussels from that seep site are B. childressi, a species that has one type of symbiont that oxidizes methane. We have been discussing which species of mussels we found – childressi, boomerang, or heckerae, or perhaps a combination! Once we get back to the lab, we will sequence this same stretch of DNA from our samples from the deeper seep off of Cape Henry,Virginia and get a molecular identification of this species.
Although the genus Bathymodiolus was described only 27 years ago, many new species of Bathymodiolus mussels are being described from hydrothermal vent and chemosynthetic communities. Our sampling will help build a richer understanding of these unique bivalves.