Form-Relations in the Context of Constructional Morphology: the Eye and Suspensorium of Lacustrine Cichlidae (Pisces, Teleostei)

Abstract

In constructional morphology - an explanatorily higher level of functional morphology - three types of relations are studied: function x function, function x form and form x form (fig. 2). In this paper methodology and techniques for the rela- tion form x form are discussed, and illustrated by the topographic relations between two joining forms, viz. the eye and the suspensorium of African lacustrine Cichlidae. Planar geometric models explain to a major extent the anatomically observed complex relation between the considered form-features of the two forms, viz. the shape, size and position of the eye (fig. 6) and the shape and structure of the suspensorium (figs. 4, 5). Although eye and suspensorium are not in close contact, it is demonstrated that the shape, size and position of the eye are geometrically closely related to the form of the suspensorium-orbit (i.e. the indentation of the suspensorium accommodating a ventral section of the eye, fig. 4). In turn, the form (viz. size and shape) of the suspensorium-orbit is geometrically related to the suspensorium-form, but this rela- tion is more complex than that between eye and orbit. Of all structural elements of the suspensorium (SESUSP'S), only the form (viz. size and shape; position was not investigated) of metapterygoid, entopterygoid and of the hyomandibula are clearly correlated with orbit-form; there is no, or only a far less evident, correlation between orbit-form and the form of the more distal SESUSP'S. This anatomically based correla- tion is mathematically explained by a direct as well as indirect geometric relation between orbit-form and the form of the three SESUSP'S; the indirect one is through the geometric relation between orbit-form and the suspensorium-shape. The observed geometric relations are general properties of the cichlid Bauplan: they are the same for cichlids from different lakes, are independent of size and hold interspecifically as well as intraspecifically. The observations and the facts refute the hypothesis that adaptive morphological differentiation of cichlids is mainly an allometric effect of overall size (contra STRAUSS, 1984).