Feeding strategies and resource partitioning ­mitigate the effects of oligotrophy for marine cave mysids

Abstract

We investigated how large populations of several mysid species can coexist in oligo - trophic underwater marine caves and their relationships in the marine cave food web using car- bon and nitrogen stable isotopes. Isotopic signatures indicate food partitioning among the 5 spe- cies of cave-dwelling mysids from the northwest Mediterranean Sea we studied. Hemimysis speluncola feeds mainly on phytoplankton and zooplankton from outside the caves, Siriella gra- cilipes on sedimentary organic matter and zooplankton from outside, Harmelinella mariannae on small cave-dwelling crustaceans, and Hemimysis margalefi and Hemimysis lamornae mediter- ranea on sedimentary particulate organic matter. These differences in diet could promote coexis- tence of such diverse and abundant mysid faunas in marine caves by reducing interspecific com- petition for scarce resources. The analysis of both seston and cave sediments revealed that the quantity and quality of organic matter are strongly reduced in marine caves, suggesting that cave- dwelling mysids find most of their food elsewhere. This inference agrees with documented distri- butions of some of these mysids outside of caves at night where they can feed in the rich littoral zone. These migrations of some species make cave-dwelling mysids important vectors of organic matter transfer from the outside euphotic littoral zone to various locations inside caves. Outside organic matter is then made available to other cave dwellers through mysid fecal pellet production and predation, as suggested by the isotopic compositions of predators like teleost fishes, decapod crustaceans, and carnivorous cladorhizid sponges.