HABITAT COUPLING BY MID-LATITUDE, SUBTIDAL, MARINE MYSIDS: IMPORT-SUBSIDISED OMNIVORES

Abstract

Mysids often dominate mobile benthic epifaunas of mid-latitude continental shelves. Macquart-Moulin & Ribera Maycas (1995) reported that the six most abundant species on western and southern European shelves are all strong diel migrators. Published daytime epibenthic sledge (sled) data from the surf zone to the shelf edge matched with published behavioural data on the most abundant species were used to test, confirm and extend that relationship to other coastal regions and to identify an association of abundant migrators with species that are important in fish diets. They also reveal another pattern: a correspondence between abundant surf-zone species and species that dominate estuarine faunas seasonally. Population concentrations at estuary mouths, sills of fjords and in the surf zone suggest a lifestyle dependent upon horizontal fluxes. Marine mysids that migrate between habitats are chronically undersampled in the field, however, and are underrepresented in food-web models. Unfortunately, no single methodology samples both pelagic and benthic individuals well and nearly all shelf measurements so far reported must be considered underestimates of local abundance. Mysids are major dietary components for many benthic and pelagic fishes, mammals, cephalopods and decapods, often for key life stages, and often because mysid migrations result in encounters with predators. Mysids can be extraordinarily omnivorous, with demonstrated capabilities to digest cellulose and diets spanning macrophyte detritus, more labile detritus, large microalgae, and smaller animals and heterotrophic protists. They can be sufficiently abundant and active to play roles in sediment transport. Contributing factors to their underappreciation have been the lack of fidelity of mysids to single habitats, coupled with higher fidelity of investigators to the study of single habitats. Sampling with classical methods has been problematic because of effective evasion by mysids, compounded by extreme patchiness associated with mysid schooling. Their frequent absence from coastal and even estuarine food-web models has not been more conspicuous because the combination of their migration and omnivory spreads their feeding impacts and because they are subsidised by horizontally imported plankton and seston and are themselves horizontally exported in the form of predator gut contents and biomass. They clearly link pelagic and benthic food webs in two important and ecosystem-stabilising ways, however, by feeding in both habitats and by succumbing in both habitats to both cruising and sit- and-wait predators. Consideration of resource and predation gradients and limited data implicate horizontal, diel migrations as well, extending these linkages, especially in the onshore-offshore direction. Somewhat paradoxically, the same features that have made them difficult to study by classical means, in particular schooling, diet breadth, ontogenetic change in diet and migration between habitats, suit migrating mysids well to new, individual- or agent-based modelling approaches. Moreover, benthic observatories deploying acoustic technologies with spatial and temporal resolution sufficient to resolve individual migratory behaviours promise powerful tests of such models.