MEPS 287:189-199 (2005)  -  doi:10.3354/meps287189

ABSTRACT: Dark submarine caves are an extreme and fragmented habitat in which mysids (Crustacea: Mysidacea) of the genus Hemimysis can be found. Hemimysis speluncola has long been the dominant mysid species of the NW Mediterranean caves, but with the recent warming of this region, its congener H. margalefi has replaced it. Nothing is known about the biology and ecology of H. margalefi and here, we provide the first information about its demographic structure and reproductive biology in a cave recently affected by the species shift. We conducted monthly sampling for 4 yr in the Jarre Island cave, near Marseilles (France), where a population of H. margalefi established itself in the late 1990s. Population dynamics were followed by monitoring length-frequency histograms and the influence of temperature on several life-history traits was investigated. H. margalefi reproduces all year round and the brood size depends on female size. Recruitment is discontinuous and occurs when post-nauplioid larvae are 1.3 to 2.0 mm long. Four annual and overlapping cohorts (about 6 mo long) underline more intensive periods of breeding. Females can produce up to 2 successive cohorts during their life. Whatever the maturity stage, females present larger sizes than males. Seasonal variations of water temperature have a strong influence on different life-history traits of H. margalefi, such as mean sizes of the different cohorts, length of intermoult stages and growth rate. Some population dynamics features of H. margalefi are a further indication that it is a species with an affinity for warm water, which has recently benefited from the warmer conditions in the NW Mediterranean, colonising new areas and replacing its congener H. speluncola in most of its former range.

KEY WORDS: Population dynamics · Reproduction · Mysidacea · Hemimysis margalefi · Marine caves · NW Mediterranean · Regional warming