Short
introduction to the coral-dwelling barnacles (Pyrgomatidae)
The Pyrgomatidae is one of the more interesting taxa within the
suborder Balanomorpha, which is a diverse group of symmetrical,
sessilian (=non-stalked) cirripedes that originated in the upper
Cretaceous. This is a little-known family whose members obligately
dwell on the surface of living corals (in addition, 3 species inhabit
hydrocorals and sponges). Despite their relative obscurity, the
evolutionary success of these coral-dwelling barnacles is clear: from
their origins in the late Oligocene, pyrgomatids have collectively
colonized over 200 different host species, and are very common (though
commonly overlooked) members of the tropical coral reef biota. There
are 72 extant described species distributed over 22 genera, as well as
~30 extinct species from the Atlantic and Mediterranean, all of which
fall within 4 genera (of which 2 are still extant).
The fossil evidence indicates that pyrgomatid barnacles have undergone
a recent evolutionary radiation in the Indo-Pacific that parallels the
diversification of corals within the region. In contrast, the extant
pyrgomatid fauna of the Caribbean and Western Atlantic is represented
by only 4 plesiomorphic species, relicts of the more diverse fauna that
flourished during the Miocene and Pliocene. The majority of extant
coral barnacles (20 genera, 63 described species) are classed
under the subfamily Pyrgomatinae, a wholly Indo-Pacific clade whose
distribution appears to be centered in the Indo-Malayan region. The two
other extant subfamilies are the Ceratoconchinae (W Atlantic -
Caribbean) and Megatreminae (cosmopolitan). However, doubts have been
raised regarding the monophyly of the pyrgomatids: it has been
suggested that the three subfamilies may have colonized scleractinian
corals independently. In addition, hypotheses regarding evolutionary
relationships within the Indo-West Pacific subfamily Pyrgomatinae
remain vague and informal, and have never been tested using rigorous
phylogenetic analyses. Informative morphological characters appear to
be scarce in pyrgomatids, and there is obvious potential for convergent
evolution as a result of adapations to a common ecological niche.
Molecular phylogenetics can resolve these questions by providing an
independent dataset on the evolution of the Pyrgomatidae.