On Etheria elliptica Lam. and the course of evolution. including assumption of monomyarianism. in the family Etheriidae (Bivalvia: Unionacea)

Abstract

The family Etheriidae (Unionacea) consists of four apparently monotypic genera, Etheria, Bartlettia, Acostaea and Pseudomulleria . All inhabit moving, often turbulent, river waters, almost entirely within the tropics in South America, Africa and India. Etheria elliptica , found in the basins of the Niger, Congo and Nile and also in Madagascar, was studied living at Jinja, Lake Victoria. It is cemented indifferently by either valve (i.e. attachment by way of the mantle/shell) becoming bilaterally asymmetrical. It is also very irregular where attached to an uneven surface. The mantle margins are singularly free, being briefly fused, by way of the inner mantle folds only, at the posterior end of the ctenidia. Unlike other etheriids, and the Unionacea generally, the mantle/shell is elongated dorso-ventrally with consequent effects on the adductors. The ligament, as probably throughout the Unionacea, has no fusion layer, consisting (apart from a periostracum which is early worn away) of anterior outer, inner, and posterior outer, ligament layers. Owing to a localized transverse ‘pinching’, the last named is compressed and extended to a corresponding extent laterally into the valves. This feature is characteristic of all the Etheriidae. Consequences of these changes in form of the mantle/shell are the dorso-ventral extension of the visceral mass, also of the ctenidia the anterior end of which is withdrawn far from the mouth with correlated enlargement of the palps. Ciliary currents on the ctenidia resemble those in Anodonta and other Unionidae but with an additional oralward current along the margin of the outer demibranch, possibly of advantage to animals living in water with usually little suspended material. Pseudofaeces collect widely around the margin of the inhalant chamber. Despite the absence (certainly in the adult) of a foot, there are well-developed posterior pedal retractors and very small anterior retractors. The oesophagus is unusually long, the stomach resembles that of the Unionidae, the midgut is extremely wide and the rectum passes dorsal to the pericardium. Pedal ganglia and statocysts are retained. Although the course of development is unknown, cementation and change to adult asymmetry occur during early growth, the youngest animals being apparently typical bilaterally symmetrical (i.e. equivalve although inequilateral) unionids. Possibly they possess a foot and, in view of the habitat (on hard substrata in running water), also a byssus. The elliptical cemented shell shows surprising development in E. elliptica var. cailliaudi where the dorso-ventrally elongated shell is strikingly asymmetrical with a dorsal beak prolonging the under valve which, in all, may be four or more times longer than the upper, free valve. The great part of the beak is composed of valvular, as distinct from ligamental, material owing to extension of the region between the anterior adductor and the ligament. The almost straight ligament, which runs down one side or other of the beak according to which valve is attached, is probably unequalled in depth throughout the Bivalvia. In this variety (as taxonomists are agreed in regarding it) the tendency towards dorso-ventral extension which appears shortly after initial attachment in E. elliptica , continues throughout life. Bartlettia stefanensis , an inhabitant largely of the Amazon basin, has the same basic type of ligament but, as in the unionids generally, this is opisthodetic. The shell is markedly inequilateral with the posterior end enlarged and globular, the anterior end reduced but capable of localized extension, the animal being heteromyarian. Little is known about the animal which is reported to retain the foot. There is no evidence of cementation although the animal is attached to the substratum (typically in turbulent waters) apparently by insertion of the anterior end of the shell into a crevice and its subsequent growth within the space available. The adult shell is invariably most irregular. Here also early attachment by byssus is a probable precursor to the adult habit. Acostaea and Pseudomulleria , which closely resemble each other, are inhabitants respectively of South America and India and represent the culmination of evolution within the Etheriidae. Like Etheria they are cemented indifferently by either valve. Beginning life as ‘Anodon-like, equivalve and dim yarian’ (Jackson), they attain great bilateral asymmetry with accompanying loss of the anterior adductor. Monomyarianism is here attained by exclusively posterior growth accompanied by major bilateral asymmetry due to change in the disposition of the mantle margins following cementation of one valve. The early symmetrical valves are retained (in Acostaea ) at the tip of a ‘talon’ attached to the lower valve. No account of the animal of Acostaea exists, but that of Pseudomulleria resembles that of Etheria with loss of the anterior adductor and pedal retractors. Assumption of monomyarianism in these genera is associated with cementation and a horizontal posture and here again it may possibly be preceded by a short phase of byssal attachment (when still in the vertical posture). Comparison is made with conditions in the monomyarian Anisomyaria and in the Tridacnidae. The three different manners in which the anterior adductor has been lost are clearly indicated in the nature of the pallial attachments. The process of evolution within the Etheriidae is discussed and is illustrated in table 1. Although all species live in turbulent water, the diversity of adaptive radiation is surprising, the constituent genera being isomyarian, heteromyarian or monomyarian while cementation is associated with both dorso-ventral and posterior extension of the mantle/shell. In the Etheriidae cementation occurs later in the life history than in any other bivalve apart...