Comparative Functional Morphology of the Gills of African Lacustrine Cichlidae (Pisces, Teleostei)

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Abstract
In gross-morphology and finer details, the gills of African lacustrine cichlids (studied from a representative sample of more than 80 species) demonstrate a considerable morphological differentiation. The frequency and dimensions of the secondary lamellae (among others constituting the form of the pores through which the respiration-water passes), the number of filaments and the gill-area of the cichlids range as widely as those of other teleosts of the same size. An explanation for this form-differentiation of the gills is sought in comparative functional morphology (including ecological and constructional morphology). Morfihology of the secondary lamellae: HUGHES' model for the relation between pore- form and waterflow also holds for cichlids. This relation provides a major clue for analysing the functional morphology of cichlid gills. In the present paper the need for a certain water-flow (suggested by the lamella-frequency) could be further related to information on the habitus, (functional) anatomy, feeding habits and (natural) habitat of the fish: e.g. small pores correlating with a streamlined body- form → fast swimmer → fast waterflow; space limitation for the gills correlating with relatively small pores suggest compensation by faster waterflow; (temporarily) frequenting hypoxic waters correlates with small pores and suggests an increased waterflow. Spatial relations: The filament-lengths and number are "plastic" form-features of the gills, viz. they are only partly explained by the functional requirements of the gills; given a certain pore-shape only a certain total length of the filaments is needed to provide the required gill area (= total gas-exchange surface). The number of filaments (as counted in this paper) is related to the height of the secondary lamellae and to the length of the ceratobranchial which seems to be determined mainly by the trophically required shape of the buccal cavity. The lengths of the individual filaments (on the ceratobranchial) are related to the size and shape of the sternohyoid muscle which belongs to the expansion apparatus. Gill area: In contrast to data for other teleosts studied so far, the gill-area in cichlids can be realized in various ways, i.e. there is no simple relation between the relative size (compared to the volume of the fish) of this area and the pore-dimen- sions. For a considerable number of cases, an explanation for the relative size of the gill-area could be derived from a comparison with the available morphological and biological data. Morphology of the filaments: Filaments are considered as the available area on which the gas exchange surface (i.e. the secondary lamellae) is to be constructed. A definite pattern of allometric relations between the size of the filament area, the shape of the filaments and the shape plus frequency of the secondary lamellae could be demonstrated. Again the pore-dimensions appear to be important. The form- features of the filaments are best understood as adaptations (1) to the functionally required dimensions of the secondary lamellae (HUGHES' model) and (2) to the available space (see below). Filaments with approximately the same area-size are relatively broader (greater width/length ratio) in fishes with (absolutely) wider pores (i.e. with a lower number of secondary lamellae/mm). Intrabranchially the differences in the pore-dimensions are proportionally smaller than the differences in filament areas. This means that (intrabranchially) filaments with a smaller area are relatively broader and bear (absolutely) fewer secondary lamellae; especially the frequency of the secondary lamellae differs intrabranchially only very little among filaments with considerably different areas.