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The paradox of Rockall: why is a brooding gastropod (Littorina saxatilis) more widespread than one having a planktonic larval dispersal stage (L. littorea)?
Johannesson, K. (1988). The paradox of Rockall: why is a brooding gastropod (Littorina saxatilis) more widespread than one having a planktonic larval dispersal stage (L. littorea)? Mar. Biol. (Berl.) 99(4): 507-513. hdl.handle.net/10.1007/bf00392558
In: Marine Biology. Springer: Heidelberg; Berlin. ISSN 0025-3162; e-ISSN 1432-1793, more
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Keywords
    Littorina littorea (Linnaeus, 1758) [WoRMS]; Littorina saxatile La Roque, 1953 [WoRMS]
    Marine

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  • Johannesson, K.

Abstract
    In benthic invertebrates dispersal of planktotrophic larvae is generally considered more effective than is, for example, the rafting of adults or egg masses. It is certainly true that over short distances, viz., in the range of tens of kilometres or less, a moderately long-lived planktotrophic larva represents an effective mechanism of dispersal. However, turbulent mixing and mortality will decrease the concentration of planktotrophic larvae, and at some distance from the ancestral population the density of settlers may be too low to enable future matings between adults of low mobility. On the other hand, adults, juveniles or benthic egg masses drifted over long distances may colonize new habitats. The crucial point is the type of larval development of the organism. If the founder group belongs to a species with direct development or which produces very short-lived planktonic larvae, the low mobility of all life-stages will maintain a population within a restricted area so that mates will be likely to encounter each other even in a small population. Even if transport of benthic stages happens very rarely, this may be more influential than larval dispersal over long distances. To show that this may be true the detailed geographical distribution of two intertidal gastropod species with contrasting modes of development is presented and further support from the literature for this hypothesis is discussed.

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