Nutritional ecology of suspension feeders
Gitai Yahel Lab
My research focuses on the diverse processes that link the seafloor to the overlying ocean. These processes range from feeding mechanisms of individual suspension feeders such as sponges and bivalves through the behavioral pattern of groundfish and migratory zooplankton to the interplay between hydrodynamics and the benthos (organisms that inhabits the bottom of the sea). Fieldwork and the development of new methods and instrumentation for underwater studies are major themes in my work.
Research Interest
Nutritional ecology of suspension feeders
Capturing particles suspended in the water is a unique feeding strategy for aquatic animals. My studies focus on measuring the metabolism, diet composition, feeding preferences, and feeding rates of marine suspension feeders. I am particularly interested in developing underwater (in situ) methods that facilitate the study of undisturbed animals in their natural environment.
Many benthic suspension feeders feed on micron and submicron scale particles such as bacteria, small protists, and phytoplankton. However, despite a century of extensive lab research, only few studies addressed basic questions such as metabolic rates, biomechanics of submicron biological filtration, filtration rates, selectivity, and community scale fluxes in the field. To address these questions I have developed methods for the in situ study of grazing rates and metabolism of benthic suspension feeders. These methods include new scuba-based sampling techniques such as the InEx (L&O Methods 3, 46) and the ROV (Remotely Operated Vehicle) compatible water sampler for deep sea work, named SIP (L&O 52, 428). These methods have allowed us (and subsequently many others) to sample point sources in the deep sea much more efficiently. We used SIPs to study the enigmatic deep dwelling glass sponges of the Canadian Pacific continental shelf (L&O 52, 428; Fraser Ridge, submitted; PLOS One 6, 12). We have recently developed a shallow version of the SIP, which we call VacuSIP, that allows efficient and prolonged sampling with much greater spatial resolution.
The ability to sample undisturbed organisms, at the field, was a key to the discovery that dissolved organic matter (DOM) is the major carbon source for tropical reef sponges (L&O 48,141). This finding ended a century-old debate over the existence of dissolved organic feeding in metazoans, and since our original 2003 paper, it has been corroborated for many sponges in a number of follow up studies in different marine habitats. Using our new in situ sampling technique (VacuSIPs), as well as controlled laboratory experiments, we are now studying DOM and nitrogen transformation, nitrogen and carbon budgets, respiration, and selectivity in over 25 suspension feeder taxa, including bivalves, scidians, sponges, and most recently also pelagic tunicates (salps and appendicilurians). A comparison of 15 species of marine sponges from contrasting environments (tropical reefs, boreal waters, and the Mediterranean) revealed a unique, yet not a trivial connection between the microbial populations within sponges and the metabolic pathways that they mediate (Environ Microbiol. 14, 1924).
For many years active suspension feeders were considered non-selective. Using direct sampling techniques, we discovered that some benthic suspension feeders are in fact highly selective. Surprisingly, this selectivity is size independent in coral reef bivalves (Aquat Biol 6, 235) as well as in boreal sponges (AME 54, 181) and Mediterranean ascidians and bivalves (Yahel, unpublished data). To better understand these processes we are now using next generation genomic techniques that will help us elucidate the identity of the microbes that are not retained by the filter feeders and describe the underlying selectivity mechanisms. Our preliminary data (ISM 2013) suggest that Pelagibacter ubique, the most abundant marine bacterium in the ocean, as well as other phylotypes belonging to the SAR 11 clade, can usually evade filtration. We are currently applying advanced molecular techniques to study species specific and size independent filtration by both benthic and pelagic suspension feeders in order to better understand predator-prey relationships that underlie the marine food-web. This study may also shed new light on the phenomenal success of the SAR 11 clade in the ocean.
The nutrition of benthic suspension feeders is ultimately controlled by the amount of water they can process. So far, the energetic cost of pumping was assumed to be negligible (0.1-4% of total metabolism), implying that water processing is not a limiting factor for pumping in suspension feeders. Using careful measurements of a sponge aquiferous system, we have challenged this paradigm. Our measurements revealed that a tiny proteinaceous mesh, that has been previously overlooked, is responsible for ~50% of the sponge hydraulic resistance, and that the overall cost of pumping is at least 28% of total metabolism in these sponges (PLOS One 6, 12). Combining in situ and laboratory measurements of sponge pumping, as well as modeling of hydraulic resistance, we have also demonstrated, for the first time, that sponges can harness the ambient flow to enhance flow through their bodies. These results call for a new look at the mechanisms underlying current-induced flow and for reevaluation of the cost of biological pumping and its evolutionary role. We are currently trying to expand on this research, with a focus on solitary ascidian as models.
