C.D.N. Barel’s research while affiliated with Leiden University and other places

An exploratory analysis was made of the head shape of cichlid fishes. A 3-D head truss was used, which allowed approximations of the volumes of three compartments of the head. We applied the method to six species of rock-dwelling haplochromine cichlids, of which we studied 12 populations from four rocky islands in the southern part of Lake Victoria. Head shape was correlated with eight environmental variables. Truss distances and compartment volumes correlated with these variables, e.g. volumes of the compartments containing the gills correlated (negatively) with oxygen levels; truss distances, including the oral jaws and their musculature, correlated with food composition; and eye size correlated with width of the transmission spectrum. Another finding was the likely architectonic interactions between anatomical elements. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76, 39–48.

Among teleost fishes, suction feeding is the most frequent mode of obtaining food. Using a 3-D kinematic model of the suction apparatus, it is calculated for what quantitative combinations of the model's static parameters three hydrodynamic demands pertinent to suction feeding could be optimized under certain anatomical constraints. Based on these calculations and constraints each of the considered parameter value sets can be categorized as optimal, suboptimal or overruled. The morphospace thus subdivided is called the evaluated morphospace. It demonstrates, for instance, that optimal sets are not randomly distributed in the morphospace but exhibit a certain pattern implying an interdependency of the parameter values. Of 73 East-African, lacustrine cichlid species the parameter value sets were plotted in the evaluated morphospace where these species-specific sets appear to be located in a narrow zone around the border between optimal and suboptimal sets but quite far away from the overruled sets. Remarkably, of all optimal value sets theoretically feasible, a relatively small range is actually used by cichlids and possibly by teleosts in general. This and the border position of the value sets in the optimal domain suggest a constraint on optimizing the suction apparatus, but the nature of the constraint could not yet be detected. The importance of the results is based on the following considerations: (1) The 73 species investigated constitute a representative sample of a speciose and ecological diverse group, viz. the ca. 1500 cichlid species that make up the independently evolved flocks in the Great East-African Lakes. (2) Preliminary investigations indicate that the results hold also for non-cichlids with a similar way of suction feeding. (3) The model may well explain the deviating value sets as observed among fishes with a different type of suction feeding.

During suction feeding teleost fish have to start mouth opening prior to other expansion movements of the head such as operculo-suspensorium abduction. The distribution of the input force over the various expansion movements is determined by the position of the hyoid in the expansion apparatus. Based on a three-dimensional (3-D) kinematic model of this apparatus it can be calculated at which positions of the hyoid operculo-suspensorial abduction is precluded. For 73 cichlid species from various African lakes and covering a wide array of feeding types and adult sizes, it is demonstrated that these optimal positions for the onset of suction feeding can be attained or closely approached by species whose regular diet only requires suction feeding but not by species whose regular diet is dominated by items requiring forceful biting. It is argued that the suboptimality of the biters is due to an architectonic constraint, viz. an increase in head width necessary to accommodate their enlarged m. adductor mandibulae. Although it is theoretically feasible to optimize the model's parameters for every head width, the biters apparently have not achieved such an adaptive change. As these parameters also feature in the execution of other functions, it is likely that conflicting demands on their optimal value overrule their optimization for the starting position of the hyoid of biters. The results hold for cichlids of independently evolved species flocks and therefore concern general rules for biter-sucker transformations in cichlids. © 1996 Wiley-Liss, Inc.

Before the decline of the species flock of haplochromine cichlids of Lake Victoria due to the Nile perch upsurge, there were many co-existing haploehromine species such as the taxonomically and ecologically well-studied zooplanktivores of the Mwanza Gulf. In spite of the scarcely separated niches of some of these species, no sign of competition for space or food could be demonstrated. As is argued in this paper, optical differentiation could well be an aspect of adaptive radiation of these zooplanktivores, particularly among the highly sympatric species. Our hypothesis is based on the morphological modifications of retinal structures in nine zooplanktivorous species. Interspecific variation was observed in composition, size and density of the photoreceptors and ganglion cells. The analyses included the intraretinal variation and size dependency of some of the structural parameters. The optical functions deduced from retinal structure indicate distinct interspecific differences in sensitivity thresholds and a slight differentiation in visual resolution. These functions correlate poorly with the photic conditions of the species-specific habitats. The optical properties can, on the other hand, be connected with the more subtle differentiation in food items and feeding behaviour among these species. It is our concluding hypothesis, that the optical differentiation among the haplochromine zooplanktivores primarily served resource partitioning by different modes of visual prey detection rather than niche partitioning by habitat.

This paper is about a general methodology for pattern transformation. Patterns are network representations of the relations among structures and functions within an organism. Transformation refers to any realistic or abstract transformation relevant to biology, e.g. ontogeny, evolution and phenotypic clines. The main aim of the paper is a methodology for analyzing the range of effects on a pattern due to perturbing one or more of its structures and/or functions (transformation morphology). Concepts relevant to such an analysis of pattern transformation are reviewed and several new ones introduced: pattern unit; direct and indirect functional demands; compatibility and trade-off; integrating, adding and decoupling; functional effectiveness; spatial, profile and other architectonic constraints; domains of structure-function relations; goal and process adaptability; multiple pathways. The paper is written from the the perspective of architectonic morphology, viz. functional morphology focusing on the relation between anatomical coherence and the compatibility of functions. The advantages and disadvantages of inductive and deductive approaches are discussed.

This paper analyses the role of constructional morphology in explaining the limitations on the interactions between an organism and those factors in its environment which are potentially relevant to its inclusive fitness. Constructional morphology deals with the relations between functionally relevant anatomical units (apparatuses or functional components) and thereby demonstrates what quantitative and qualitative constraints there are on combining units necessary for environmental interactions. It is argued that investigations on the relations between form and environment (ecological morphology) should (1) consider three types of relations: form-form, form-function and function-environment factor, (2) include behavioral and physiological ecology and (3) not be limited to a particular stage, but include as much of ontogeny as possible.

Ecomorphologists have thus far paid little attention to phenotypic plasticity of anatomical structures. In this paper we present four cases of phenotypic plasticity in African cichlids, which we suppose to be adaptive responses to environmental changes. On the basis of these cases we hypothesize that the response of a plastic anatomical structure to an environmental change may be constrained by surrounding structures. Alternatively, the surrounding structures may be affected by a plastic structure which changed as a direct response to environmental alterations. We also analyze the morphological and ecological implications of the potentials of growth retardation and accelaration on the phenotypic plasticity in organisms. The flexibility of the reaction norm of an organism may depend on these potentials of heterochrony.

In external appearance and in standard taxonomic analysis Haplochromis iris and H. hiatus, are closely resembling species, which morphologically mainly differ in the live colours of sexually active males (Plate I) and, with marginal overlap, in the number of gill-lamellae, the size of the swimbladder and fat content. Ecologically the two species demonstrate (again with overlap) differences in bathymetric distribution, breeding periods, relative contributions of their (otherwise same) food items and in nematode infection. The relation between these ecological differences and the relation between ecological and morphological differences is analysed. The results corroborate the biological validity of the species rank, which hitherto was based on morphology only. The data and their interpretation lead to a discussion on bathymetric segregation as a speciation mode for lacustrine cichlids. Finally the possible role of trophic polymorphs in the radiation of Lake Victoria haplochromines is considered.

Based on ca 100 lacustrine cichlid species and following a holistic procedure of functional morphology, this paper analyses what constraints spatial relations of structures may set on the compatibility of functions. Among the many feeding behaviours of cichlids two groups are distinguished: (1) those in which powerful biting with the oral jaws are involved and (2) those in which the food is directly sucked into the buccal cavity without prior manipulation by the oral jaws. Related to the core functions