No abstract

Within the clariids (air-breathing cat®sh), a complete range of fusiform to anguilliform species can be observed. This study deals with the cranial morphology of Channallabes apus, an extreme anguilliform (eellike) species, compared to the anguilliform Gymnallabes typus and the more fusiform Clarias gariepinus. The overall morphology of the head of Channallabes apus shows a hypertrophied adductor mandibulae complex, with the corresponding substantial narrowing of the neurocranium, seen in the frontals, the sphenotics, the pterotics and the posttemporo-supracleithra, as well as the reduction and displacement of the eyes and some canal bones, such as the infraorbitals and the suprapreopercles. The presence of a hypertrophied muscle complex possibly indicates that a more powerful bite may occur. This implies that adaptations can be expected in several parts of the skull. On the lower jaw of C. apus a higher coronoid process is found, and on the suspensorium, two sets of three processes are present on the hyomandibular bone, indicating a stronger connection to the neurocranium. Several of the observed features, such as the elongation of the body, the reduction of the eyes, the increase in vertebrae number, limblessness and the increasing rigidity of the skull, may also be related to a process of miniaturization.

An important step towards understanding the evolution of terrestriality in vertebrates is to identify how the aquatic ancestors of tetrapods were able to access ground-based prey. We have discovered that the 'eel catfish' Channallabes apus, an inhabitant of the muddy swamps of tropical Africa, has a remarkable ability to forage and capture prey on land. The animal's capacity to bend its head down towards the ground while feeding seems to be an essential feature that may have enabled fish to make the transition from an aquatic to a terrestrial mode.

An ontogenetic series of in-captivity bred Corydoras aeneus was used, in order to study the developmental changes in the external morphology. Allometric growth of several body parts was studied, attempting to reveal important steps in the species' early life history. Based on the external morphology, the different stages during early development of C. aeneus were identified, according to Balon (Journal of the Fisheries Research Board of Canada 32: [1663][1664][1665][1666][1667][1668][1669][1670] 1975). After hatching, at a SL of 3.5 mm, the developmental state corresponded to an eleutherembryonic phase, followed by the protopterygiolarval phase (4.4-5.7 mm SL), the pterygiolarval phase (5.7-14.0 mm SL) and the juvenile period. In addition, an overall growth curve and inflexion points were determined. As such, ontogenetic changes in growth coefficients k (in SL = b age k ) were determined. Log transformed data were used for a piecewise linear regression method, as per regression spline smoothing procedures. This way, the growth curve could be divided into six different intervals of growth rate. Initially, the slope was 0.05 until 0.7 dph, then increasing to 0.18 until 4 dph, and 0.36 until 10 dph. After this, growth rate reached a maximum of 0.76 until 24 dph, slowed down to 0.47 until 37 dph and then finally again slowed down to 0.36. A similar growth analysis was also done on the different body parts and these results were compared to both morphological and data from literature. This led to the conclusion that the inflexion points found during the early development of C. aeneus matched the different key-events known in teleost early life history and development. The transition from endo-to exogenous feeding, at the moment a functional branchial respiratory system becomes increasingly important, was the first point at which allometries changed together with functional demands. A second, similar congruence occurred at the transition to the pterygiolarval phase, when priorities shift towards locomotory needs. Finally, our results also indicated a transition to a carangiform swimming mode at approximately 8 mm SL.

Clariid catfishes span a broad range of body forms ranging between fusiform and anguilliform morphotypes. Although such variation in body shape has been observed in other families of teleost fishes, amphibians and reptiles, it is rarely as extreme as within the Clariidae. Although the Clariidae were thought to have undergone anagenetic evolution (i.e. progressive evolution within a lineage), more recent studies indicate that anguilliformity evolved several times through a process of cladogenesis (i.e. branching of evolutionary lineages). In this study, it is shown that the phylogenetic analysis of morphological data mainly gives a reflection of the cranial evolution in the Clariidae despite the use of 18 post-cranial characters (out of a total of 53 characters). A combined phylogenetic analysis of both morphological and molecular data rather suggests the derived nature of body elongation. The corresponding morphological changes that co-occur with this elongation can be regarded as an extreme case of convergent evolution at the genus level within the Clariidae.