Functional morphology and cirral activity are described for two species of Armatobalanus and 15 species of pyrgomatine pyrgomatids in relation to their inhabitation of living scleractinian and hydrozoan corals. New details are given of the opercular anatomy of Megatrema anglicum. Armatobalanus allium and A. arcuatus abrade overgrowing coral coenosarc mechanically, using tergal beaks and cirri during cirral activity. Pyrgomatids erect an aperture frill (modified tergoscutal flaps). The frill is secretory and appears to be a source of chemical inhibition of coral overgrowth. Armatobalanus allium performs a strong pumping beat. Cirral activity in A. arcuatus is more varied and includes a faster dipping beat. Dipping beat is also characteristic of pyrgomatines. Opercular structure and function indicate that Armatobalanus allium, A. arcuatus, Megatrema anglicum and Cantellius euspinulosum form a functional evolutionary series, supporting the derivation of pyrgomatids from Armatobalanus. Within the genus Cantellius, a basic (type 1) and two modified (types 2 and 3) aperture frill mechanisms occur. Type 1 retains large tergal depressor muscles inserted on tergal wings on either side of the infolded aperture frill (C. euspinulosum, C. Septimus, C. gregarius). Type 2 is similar except that the muscle insertions are shifted mainly into the folds of the (larger) frill (C. pallidus, C. acutum). The type 3 frill is located more apically in a low operculum with reduced tergal depressor muscles and enlarged lateral scutal depressor muscles (C. secundus). More modified pyrgomatines can be variously derived from the three groups of Cantellius on the basis of aperture frill mechanisms, cirral activities and differences in cirral morphology. The type 1 frill occurs in Nobia projectum, N. conjugatum and Pyrgoma cancellata. The type 2 frill is strongly developed in Nobia grandis. Creusia spinulosa and four species of Savignium have a type 3 frill. The cirral activities of species with frill types 1 and 2 are based on vertical dipping beat. Species with a type 3 frill have rostrocarinal dipping beat. In Savignium milleborum this is fast beat, in S. elongatum, ‘normal’ beat, accompanied by carinal exhalent jets. Functional morphology thus reveals three main lines of evolution in the pyrgomatine pyrgomatids based on divergences within Cantellius. Each line has yielded specialized species with a fused wall and highly modified operculum, exemplified by Pyrgoma cancellata (very reduced orifice). Nobia grandis (very large aperture frill) and Savignium milleporum and S. elongatum (double adductor scutorum). Nobia and Savignium as previously defined are polyphyletic. Four new genera are proposed. The evolution of an aperture frill and chemical inhibition of coral overgrowth is associated with a reduced orifice and often a reduced cirral fan. In some species there may be nutrient uptake from the coral host, but experimental tests are needed.
Zoological Journal of the Linnean Society – Oxford University Press
Published: Dec 1, 1992
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