Speciation versus phenotypic plasticity in coral inhabiting barnacles: Darwin's observations in an ecological context.
Speciation and phenotypic plasticity are two extreme strategic modes enabling a given taxon to populate a broad ecological niche. One of the organismal models which stimulated Darwin's ideas on speciation was the Cirripedia (barnacles), to which he dedicated a large monograph. In several cases, including the coral-inhabiting barnacle genera Savignium and Cantellius (formerly Pyrgoma and Creusia, respectively), Darwin assigned barnacle specimens to morphological "varieties" (as opposed to species) within a genus. Despite having been the subject of taxonomic investigations and revisions ever since, the significance of these varieties has never been examined with respect to host-associated speciation processes. Here we provide evidence from molecular (12S mt rDNA sequences) and micromorphological (SEM) studies, suggesting that these closely related barnacle genera utilize opposite strategies for populating a suite of live-coral substrates. Cantellius demonstrates a relatively low genetic variability, despite inhabiting a wide range of corals. The species C. pallidus alone was found on three coral families, belonging to distinct higher-order classification units. In contrast, Savignium barnacles exhibit large between- and within-species variations with respect to both micromorphology and DNA sequences, with S. dentatum "varieties" clustering phylogenetically according to their coral host species (all of which are members of a single family). Thus, whereas Savignium seems to have undergone intense host-associated speciation over a relatively narrow taxonomic range of hosts, Cantellius shows phenotypic plasticity over a much larger range. This dichotomy correlates with differences in life-history parameters between these barnacle taxa, including host-infestation characteristics, reproductive strategies, and larval trophic type.
Mokady
O
O
Institute for Nature Conservation Research, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Israel. mokady@post.tau.ac.il
Loya
Y
Y
Achituv
Y
Y
Geffen
E
E
Graur
D
D
Rozenblatt
S
S
Brickner
I
I
D000818:Animals; D001483:Base Sequence; D003063:Cnidaria / Q000469:parasitology; D004272:DNA, Mitochondrial / Q000235:genetics*; D004275:DNA, Ribosomal / Q000235:genetics*; D019143:Evolution, Molecular; D008855:Microscopy, Electron, Scanning; D008957:Models, Genetic; D008969:Molecular Sequence Data; D010641:Phenotype; D010802:Phylogeny; D012335:RNA, Ribosomal / Q000235:genetics*; D001468:Thoracica / Q000145:classification / Q000235:genetics* / Q000648:ultrastructure
D004272:DNA, Mitochondrial; D004275:DNA, Ribosomal; D012335:RNA, Ribosomal; C075526:RNA, ribosomal, 12S