A COMPARISON OF THE ECOLOGY AND LIFE CYCLES OF TWO SPECIES OF LITTORAL ISOPOD (original) (raw)

A comparison is made of the vertical distribution, life cycles, and ecology of two littoral species of the sexually dimorphic isopod, Dynamene, on rocky shores of northwest France. Adult D. bidentata (Adams) prefer middle and lower eulittoral crevices and empty .Eulunus perforutus Brug. tests, and juveniles a wide variety of algae in the same zones. Males are capable of killing B. perforutus by wedging themselves tightly in the operculum so preventing feeding and respiration. Adult Dynamene mugnitoruta Holdich prefer empty BaZunus crenatus Brug. tests, channels within sponges such as Halichondria and Hymeniacidon, and spaces within and between ascidian colonies, e.g., Dendrodoa and Didemnum, in the lower eulittoral and upper sublittoral. Juveniles are found in the same zones on a restricted number of algal species, in particular Chondrus crispus Stack. There is little overlap in the habitat preferences of the two species. Availability of food and tolerance of desiccation do not appear to be the primary factors responsible for the maintenance of zonation patterns; temperature would appear to be the main factor involved. The stages of the life cycle of the two species are the same but the sex-ratio in l)ynamene mu@-torutu is much lower, and this is possibly due to the fact that the male may only spend one breeding season in the reproductive habitat as opposed to the two spent by male D. bidentatu. Populations of 1). bidentutn from different latitudes show a tendency to increase the length of their reproductive and growth phases at more southerly locations so that ovigerous females and juveniles occur throughout the year. Winter air temperatures are a possible cause of these differences in timing. Stereoscan electron microscope observations are included to clarify the identification of the three species of Dynumene found on the Atlantic coasts of Europe and northwest Africa.

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