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.

Pyrgomatidae homepage