We compare the cranial morphology of four fish species with an increasing anguilliformism in the following order: Clarias gariepinus, Clariallabes melas, Gymnallabes typus, and Channallabes apus. The main anatomicalmorphological disparities are the stepwise reduction of the skull roof along with the relative enlargement of the external jaw muscles, which occurred in each of them. Gymnallabes typus and C. apus lack a bony protection to cover the jaw muscles. The neurocranial bones of C. gariepinus, however, form a closed, broad roof, whereas the width of the neurocranium in C. melas is intermediate. Several features of the clariid heads, such as the size of the mouth and the bands of small teeth, may be regarded as adaptations for manipulating large food particles, which are even more pronounced in anguilliform clariids. The jaw musculature of G. typus is hypertrophied and attached on a higher coronoid process of the lower jaw, causing a larger adductive force. The hyomandibula interdigitates more strongly with the neurocranium and its dentition with longer teeth is posteriorly extended, closer to the lower jaw articulation. The anguilliform clariids also have their cranial muscles modified to enable a wider gape. The adductor mandibulae and the levator operculi extend more posteriorly, and the anterior attachment site of the protractor hyoidei dorsalis shifts toward the sagittal plane of the head. A phylogenetic analysis of the Clariidae, which is in progress, could check the validity of Boulenger's hypothesis that predecessors of the primitive fishes, such as Heterobranchus and most Clarias, would have evolved into progressively anguilliform clariids.

Within clariid fishes several cranial morphologies can be discerned. Especially within anguilliform representatives an increase in the degree of hypertrophy of the jaw adductors occurs. The hypertrophy of the jaw adductors and skeletal modifications in the cranial elements have been linked to increased bite force. The functional significance of this supposed increase in bite force remains obscure. In this study, biomechanical modeling of the cranial apparatus in four clariid representatives showing a gradual increase in the hypertrophy of the jaw adductors (Clarias gariepinus, Clariallabes melas, Channallabes apus, and Gymnallabes typus) is used to investigate whether bite force actually increased. Static bite modeling shows that the apparent hypertrophy results in an increase in bite force. For a given head size, the largest bite forces are predicted for C. apus, the lowest ones for C. gariepinus, and intermediate values are calculated for the other species. In addition, also in absolute measures differences in bite force remain, with C. apus biting distinctly harder than C. gariepinus despite its smaller head size. This indicates that the hypertrophy of the jaw adductors is more than just a correlated response to the decrease in absolute head size. Further studies investigating the ecological relevance of this performance difference are needed.

During ontogeny, larval fish have to deal with increasing nutritional and respiratory demands as they grow. As early ontogeny is characterized by an increasing complexity of moving structural elements composing a fish skull, some constraints will have to be met when developing mechanisms, which enable feeding and respiration, arise at a certain developmental stage. This article focuses on the presence/absence of a possible functional response in mouth opening during ontogeny in Clarias gariepinus. Some reflections are given, based on morphological data, as well as related function‐analysis data from the literature. Starting shortly after hatching, a total of up to five different mouth opening mechanisms may become functional. Of these, three may remain functional in the adult. As could be expected, the apparatuses that enable these mechanisms show an increase in complexity, as well as a putative improvement in mouth opening capacity. Initially, two consecutive mechanisms may allow a restricted depression of the lower jaw (both passively and actively). Synchronously, two more mechanisms may arise, which involve the coupling of the hyoid depression to the mouth opening. At about 11 mm SL a fifth mechanism becomes established, better known as the opercular mouth opening mechanism. An overlapping chronology of functionality of the different mechanisms, as well as differences in efficiencies, could be an indication of the absence of a true critical period in C. gariepinus (at least in relation to mouth opening), as well as the possible presence of a shift in feeding type. Finally, the coupling of the chronology of the shift in mouth opening mechanisms and several morphological, behavioral, and physiological changes during ontogeny, related to feeding and respiration, make it possible to distinguish five important phases in the early life history of C. gariepinus. J. Morphol. 247:197–216, 2001. © 2001 Wiley‐Liss, Inc.

The ontogeny of the chondrocranium of 31 different stages of the African catfish Clarias gariepinus (Siluroidei: Clariidae) was studied, both from cleared and stained, and sectioned material. The fish ranged from 4.1 (1 day post‐hatching) to 127.0 mm SL (100 days post‐hatching). The chondrocranium of C. gariepinus seemed to correspond to the general adaptive trends in siluroids, especially in relation to the reduction of eye size and the dorso‐ventral flattening of the skull. The platybasic neurocranium involved several modifications related to the trabecular bars, the hypophyseal fenestra, the ethmoid region and even the olfactory nerves. Certain reductions were present, which have been observed in all siluroids (e.g. absence of the pila lateralis, the commissura lateralis, the myodomes) or are part of a variable trend within siluroids (e.g. reduction of the taenia marginalis anterior and the tectum synoticum). Compared with some other siluroid species, the neurocranium of C. gariepinus is well developed, for example in the otic region. The same was observed in the splanchnocranium where some general siluroid trends persist (e.g. isolation of palatine from pterygoquadrate, presence of ‘hyo‐symplectic‐pterygoquadrate’ plate). Some trends, as observed in other siluroids, were present also (e.g. interhyal continuous with suspensorium and ceratohyal, Meckel's cartilage initially continuous with the suspensorium). The branchial basket is well developed as all expected elements are present (basibranchials I‐IV, hypobranchials I‐IV, ceratobranchials I‐V, epibranchials I‐IV). Based on the observed ontogeny of C. gariepinus and data from the literature, a hypothesis was formulated which indicated the presence of a general reductional trend within siluroids. In C. gariepinus, all four (I‐IV) infrapharyngobranchials develop, although the anterior two are much reduced and fused with each other.

Suction feeding in Astatotilapia elegans occurs by a series of rapid, coupled movements of various head parts. The lower jaw rotates with respect to the neurocranium through an angle of 62° in less than 15 ms. The power requirements for jaw depression are calculated from a mathematical model and may reach a peak of ±4 watt in a 12-cm-long specimen. Data from the literature on mechanical output of fish muscle suggest that a muscular volume equal to 44% of the volume of the fish (= 45 cm ) should be required, if it is premised that movement and muscle shortening are directly coupled. Therefore, we assumed that storage and release of strain energy must be considered. The work demands for depression equal 0.014 J. The amount of energy storable in only three pairs of head ligaments can be estimated between 0.008 J and 0.05 J. The use of strain energy implies initial blocking and subsequent triggering of the movement. The position of the hyalomandibular connection, dorsal to the mandibular-suspensorial articulation, appears to be of crucial importance in balancing the forces of sternohyoideus and the body muscles. Triggering at the onset of jaw depression occurs by the contraction of the levator operculi. The line of action of the hyalomandibular connection is lowered beneath the jaw suspension, which raises the equilibrium of forces. Elastic recoil can occur thereafter.