The biology and functional morphology of Philobrya munita (Bivalvia: Philobryidae)

Abstract

Philobrya (Philobrya) munita was originally described as P. (Hochstetteria) munita Finlay 1930, but the features of the shell and ligament clearly ally it with the former subgenus. P. munita, unlike the majority of its closest relatives which are benthic, lives intertidally attached by a stout byssus to the holdfasts of algae on moderately exposed shores in New Zealand. P. munita is monomyarian though it possesses a shell which has the basic heteromyarian form with an inflated posterior slope (and correspondingly enlarged posterior adductor and posterior byssal retractor muscles) and a reduced concave anterior slope (with a reduced anterior byssal tetractor inserted on the posterior slope of each valve). The umbones are beak‐like and there is a distinct byssal notch. Internally a septum or modified hinge plate is located under the opisthodetic alivincular ligament. There are no hinge teeth or denticles. The significance of the shell and ligament of P. munita is discussed in relation to the basic duplivincular ligament of other arcoids and the ligament of other bivalves. P. munita possesses a posterior inhalant aperture only. This is unusual since some other members of the Arcoida have both anterior and posterior inhalant streams. The posterior inhalant stream in P. munita results from the very great reduction of the anterior slope of the shell. The ciliary currents of the mantle cavity are described; the ctenidia are of type B(la) and there is a single cephalic eye located just anterior to the anterior filament of each ctenidium. The roof of the supra‐branchial chamber above the inner demibranch possesses a hypobranchial gland. The labial palps are small and there is a labial caecum located within the inner lip of the mouth. The mantle comprises three folds, with the outer fold possessing a secondary fold. The outer mantle fold bears a row of pallial eyes; light reception is thus through the periostracum. The structure of the stomach has been reconstructed from serial sections and the basic histological features of the alimentary canal are described. The pericardium possesses a heart with a single ventricle and a pair of auricles which communicate with each other posteriorly. The kidney and gonads discharge via a common urino‐genital aperture. P. munita incubates fertilised eggs within the infra‐branchial chamber of the mantle cavity; incubation within the supra‐branchial chamber being impossible with a filibranch ctenidium. P. munita is compared with other philobryids, other members of the Arcoida and with other bivalves. It is concluded that P. munita is a very unusual bivalve–the morphological eccentricities it possesses resulting from its primitive arcoid origins, the evolution of the heteromyarian form, and specialisations not seen in other bivalves.