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The Bivalvia — Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. (Edited by Brian Morton). Hong Kong University Press, Hong Kong, 1990. FORM, FUNCTIO N AN D PHYLOGEN Y O F BIVALVE MUCIN S Robert S . Prezan t Department of Biology, Indiana University of Pennsylvania, Indiana, Pennsylvania 15705-1090 , U.S.A. ABSTRACT Many of the basic behavioural patterns found in bivalve molluscs are based on their abilities to produce mucins . These mucin s pla y pivota l role s i n activitie s a s divers e a s feeding , protection, shell formation, and dispersal. Mucins were almost certainly involved in incipient formation of calcified shells and likely were intimate components of presumed early cuticular coats of 'protomolluscs'. Similarly, initial pedal attachment structures were no more than adhesive mucins that were the evolutionary anlages of byssi. Many of the structural features of bivalves described by CM. Yonge in fact were, to some extent or another, producers of, or dependent upon mucins. In particular the mantle and mantle edge are important mucinproducing features that show distinct phylogenetic and ontogenetic trends useful in deciphering bivalve evolution. The initial lateral compression and infaunal mobility of early bivalves led to success of the class in large part because mucins 'allowed' suspension feeding, early protection in the form of an external coat, and lubrication for mucociliary activities such as locomotion. Sinc e the Precambrian, mucins have retained simpl e and taken on complex functions in the Bivalvia and well reflect their physiological, morphological, behavioural and ecological diversity. INTRODUCTION According t o Yong e (1953) , bivalves abandone d th e initia l epifauna l habit s o f th e earliest molluscs in conjunction wit h lateral compression of the still-uncalcified shell . By means of pedal muscularization and shell calcification, they soon developed the ability to move efficiently throug h soft substrata . The neotenous retention of a larval byssal gland 'allowed' many bivalve groups to resume an epifaunal habit (Yonge, 1962). We can assume that the ability to move through soft substrata was aided by mucoid lubricants in those early Mollusca. Additionally, we know that mucins are intimately involved with biomineralization of shell and with production of byssal attachment threads (in fact it is very likely that the 84 ROBERT S . PREZAN T byssal gland evolved from a pedal mucous gland). Throughout the work of CM. Yong e dealing with form and function of bivalves, there is frequent reference to the importance of mucocytes and mucins in the ontogeny and phylogeny of molluscs. Mucins, in fact, were explored by Yonge not only within the phylum Mollusca, but also in various other phyla including, and perhaps most notably as it came early in his career, the Coelenterata (Yonge, 1930,1937). Yonge was a strong advocate of linking form and function when working with any organismal system. 'It is necessary, in order properly to determine the nature of any organ, to study both structure and physiology, since it is to the physiological assumptions of the morphologist and the morphological assumptions of the physiologist that the misconceptions of the past are due.' (Yonge, 1926a) This paper is designed to examine trends molluscan mucins (essentially mucopolysaccharides and glycoproteins) and mucus-producing organs have take n i n bivalve s i n ligh t o f thei r evolutionar y importance. Man y o f th e system s examined here were first detailed by CM. Yonge in his many classic works. Functional polarity of bivalve mucins There is a distinct functional polarit y in many molluscan mucins. Some of these are briefly describe d by Prezant (1985a). Within the Bivalvia we find mucins playing pivotal roles in a variety of behaviours that could be considered functional opposites. For instance, mucins play an intimate role in various aspects of ingestion including trapping food particles on ctenidia or palp proboscides, transporting tha t food (i n suspension feeders ) dow n the ctenidia and over the palps to the mouth by mucociliary activities, and acting as a digestivetract lubrican t an d carrie r fo r digestive enzymes . Yonge (1926a ) describe d mucu s cell s within the digestive diverticula ducts of several bivalves. The importance of ctenidial mucins in filter-feeding activitie s of bivalves has been expounded upo n by several authors, e.g., MacGinitie (1945), Dral (1967), J0rgensen (1975, 1976, 1981) and Morton (1983). On the other hand mucins are involved in various aspects of egestion such as packaging faecal pellets as well...

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