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Oceanic islands have never been connected
to continental landmasses and represent some of the most isolated
environments on earth. Extensive stretches of deep ocean surrounding
such islands represent formidable dispersal filters and they
receive their biotas solely through dispersal from geographically
distant source populations and from subsequent in situ diversification.
This feature of their natural history makes oceanic islands ideal
habitats to test long-distance dispersal hypotheses for benthic
shallow-water marine taxa. For instance, a traditional paradigm
in marine biology has been that an extended pelagic larval phase
is a prerequisite to effective long-distance range extension
in sedentary benthic invertebrate taxa.
The figure at right (top) shows the rugged eastern coastline
of Madeira, a subtropical oceanic island in the eastern North
Atlantic. Its one of Diarmaid Ó Foighil's study sites
for a research project which aims to discover how a marine clam,
Lasaea, successfully colonized numerous oceanic islands
in the North Atlantic. The genus Lasaea is composed of
minute, crevice-dwelling, intertidal clams that have a near-cosmopolitan
distribution on rocky shores. North Atlantic lineages are exclusively
composed of direct-developers, i.e., they completely lack
pelagic larval development. See picture at right (below) of adults
(reddish dots) with a penny for scale.
Predicted phylogenetic tree topologies were generated for
4 competing dispersal hypotheses that seek to explain how direct-developing
Lasaea lineages have colonized the Bermudan, Azorean and
Madeiran archipelagoes from putative continental source populations
in North America (upstream source) and/or Europe (downstream
source). See view of the North Atlantic below indicating sampling
sites for this study.
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