Taxon profile


Pinnidae Leach, 1819

kingdom Animalia - animals »  phylum Mollusca - mollusks »  class Bivalvia - bivalves »  order Ostreida »  superfamily Pinnoidea


Shell mytiliform, not alate, dimyarian, anterior adductor smaller; equivalve, truncate, and wholly open behind; edentulous; area linear; ligament parivincular, internal; shell structure coarsely prismatic, with a thin, partial, nacreous lining; byssiferous.

There are numerous fossil species; about 50 recent species are distributed throughout the warmer seas. Though thin and brittle these shells are notable for their length, being exceeded in this respect, among bivalves, only by the Giant Clams of the tropics. They are sedentary, secured by the byssus in an upright position, with the narrow anterior end deeply buried in gravel, muddy grit or sand, while the broad posterior end for about a quarter of the length of the shell is exposed above its burrow. The movements of the animal are confined to movements within the shell and ability to open the valves slightly. No part of the animal except the byssus normally projects beyond the shell. The mantle may entirely fill the shell, but, by means of the radiating muscle-bundles in it, this can be retracted well below the posterior end, leaving the curious pillar-shaped organ, the mantle gland, at the upper end; the function of this organ is apparently that of keeping the upper end of the shell and mantle clear of grit. The essential structure of the animal is concentrated in the lower two-thirds, between the posterior and anterior adductors; above the posterior adductor there is only the upper part of the mantle and the enormous gill, the mantle gland and the rectum; so that mechanically the support of animal and shell centres on the posterior adductor, which is consequently very large and occupies a position between one-third and one-half from the broad end. There are no siphons, but a division of the mantle by an intermantellar septum supplies a substitute for them. The gill is excessively developed and the mantle spaces correspondingly, so that water spaces are large and water currents strong. There is an exceptional development in the ciliated furrow, which forms a deep narrow channel almost the whole length of the mantle; the cilia inside this ciliated channel produce a current that rapidly removes any grit or sand that finds its way into the mantle chamber.
The young Pinnae are apparently normal equilateral bivalves. Further growth appears to be entirely posterior and the shell assumes a triangular shape, with the prodissoconch as its apex. This method of growth, unique among bivalves, is intimately bound up with the peculiar character of the animal; the animal must move bodily upward in its shell as it grows, and with it both adductor muscles move onward posteriorly. The empty anterior end of the shell is soon worn away by friction as the growing shell burrows; the mantle produces new shell ends to protect the anterior adductor muscle at each growth stage. The posterior adductor moves upward along a widening scar. The shell consists of two layers only, the inner nacreous and the outer crystalline while above the posterior adductor and also along the ventral margin, the nacreous layer is not deposited and the shell consists of one layer only, composed of numerous minute prisms each having its long axis perpendicular to the surface. In consequence the upper or posterior part of the shell is flexible and can be held com¬pletely closed by the mantle, thus offering a further protection against special irritation, e.g., if the animal is worried for some time with prods from a brush it can be induced to close its shell. The shell is peculiarly liable to distortion. Pearls of an amber colour are sometimes found in the shell. The long silky byssus, mixed with silk, has been made into gloves, as a novelty. Gibbon records that robe of this material was the gift of the Roman emperor (Justinian?) to the satraps of Armenia. A commensal crab, Pinnotheres, is sometimes associated, as noted by Pliny and other ancient writers, who called it Pinna's Friend, and invented marvellous fables of the doings of these strange partners. This crab has all its parts rounded and polished with a surface like glass. A medial groove divides the nacreous layer into twTo parts ending in two lobes posteriorly, between which the shell consists of the fibrous (or crystalline) layer only; but below the posterior adductor it is covered by a thin layer of nacre. On the outer side of the shell there is a marked keel and often a distinct cut as if the fibrous layer had formed in two halves. The keel and the slit are due to a splitting of the fibrous layer along a line of weakness. The position of the groove makes it clear that it is due to the point of attachment of the mantle which is immediately ventral to the adductor muscle, and which does not secrete nacre at this point. In general, the ventral margin of Pinna tends to be straight or evenly curved; while the ventral margin of Atrina usually bends outward in the adult, so that the shell tends to be ham-shaped rather than triangular. Distribution: Warm and temperate seas, gregarious, from low tide mark to 100 fathoms.
Cotton, B.C., 1961. South Australian Mollusca. Pelecypoda.
Author: Jan Delsing

Included taxa

Number of records: 4

genus Atrina Gray, 1842
genus Cyrtopinna O.A.L. Mörch, 1853
genus Pinna Linnaeus, 1758
genus Streptopinna E.C. Von Martens, 1880

Links and literature

CZ Pfleger V. (1999): České názvy živočichů III. Měkkýši (Mollusca), Národní muzeum, (zoologické odd.), Praha, 108 pp. [as Pinnidae]
Data retrieved on: 11 November 2013
EN Carter J. et al (2011): A Synoptical Classification of the Bivalvia (Mollusca), Paleontological Contributions 4 [as Pinnidae Leach, 1819]
Data retrieved on: 6 April 2014

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