THE PHARYNGEAL APOPHYSIS ON THE BASE OF THE SKULL IN CICHLID FISHES

by ETHELWYNN TREWAVAS

(c/o British Mueum [Natural History]), CromwellRoad, London SW7 5BD, England)

REGAN (1920) first called attention to the differences in structure of the apophysis on the base of the skull for articulation with the toothed up- per pharyngeal bones among African Cichlidae. By its means he purged Boulenger's broad concept of genus Tilapia (1899 and 1915) of many species unrelated to the type species, Tilapia sparrmanii A. Smith, or to the mouthbrooding species long included in Tilapia, but now separated as the tilapiine genera Oreochromis and Sarotherodon. BOULENGER (1915: 134), after defining the family Cichlidae, wrote: "The division into genera, as here followed, is unsatisfactory and open to criticism, the dentition in certain species being subject to variation according to age, or even of a purely individual nature." Being unable to find a more 'natural' character, he defined Tilapia, on the dentition of the jaws, the outer teeth being bicuspid, the inner tricuspid. Nevertheless, the first dichotomy of Boulenger's key to the species of Tilapia, vague though it ways-"1. Scales cycloid (rarely in- distinctly ctenoid) II. Scales with more or less distinctly denticulate edges."-separated as Section I, 49 nominal species, 43 of which we now include in the three tilapiine genera mentioned. The remaining species, numbers 20, 42, 43, 46, 47, 48 of Section I and the whole of Section II (nos 50-94), are now placed in other genera. Boulenger intuitively understood that most of the species of his Sec- tion I constituted a natural group and Regan's discovery of the apophysial character gave the 43 nominal tilapiines a well-defined shared character-state, the apophysis being formed in all of them by the parasphenoid alone. The only close relative of the above- mentioned tilapiine genera that Boulenger placed elsewhere was O. (Neotilapia) tanganicae, which he included in Petrochromis because its outer teeth are tricuspid. This mistake was rectified by REGAN (1920), who recognized its relationship to the tilapias and proposed for it the generic name Neotilapia, but the mistake has been repeated again recently by Yamaoka (see below). The formation of the apophysis by the parasphenoid alone is, how- ever, a plesiomorph character, shared with all the Neotropical genera 717

whose skeletons I have examined, except Cichla, also with Etroplus of India and Sri Lanka and with the genera of Madagascar, as well as with some other African genera. The fact that it is also the condition in the related pharyngognath families Labridae and Embiotocidae (LIEM & GREENWOOD, 1981) is further evidence that this is the primitive state. Unfortunately the paper in which Regan announced his discovery was his work on the Tanganyikan genera. He divided these into two groups defned on the pharyngeal apophysis, one in which it was form- ed from the parasphenoid alone, the other in which the basioccipital shared in its formation. It is plain that he was unable to verify this character in all the Tanganyikan species; his paper was written before the museum acquired the large Christy collections, and some genera were misplaced (notably the closely related Perissodus and Plecodus, which were put in contrasting groups). Moreover, the Tanganyikan cichlids are more diverse than those of Lakes Victoria and Malawi and show signs of multiple origin. Not only the pharyngeal apophysis, but the lateral line pores, numbers, size and structure of scales, dentition, sex signals and modes of parental care are all more diverse than in other Great Lakes of Africa. In Lakes Victoria and Malawi Regan's division is correct. GREENWOOD (1978), realising that Regan's major dichotomy was less useful in classifying the Tanganyikan genera than was originally supposed, went to the other extreme and conclude his study of the pharyngeal apophysis thus: "The pharyngeal apophysis must be re- jected as a character of any value in formulating suprageneric relation- ships and is probably of little or restricted value in classification below that level as well. " Greenwood's paper is of value in showing, with the aid of Gordon Howes' clear drawings, the appearance of the structure in question. He refined the classification of the apophysis by dividing the derived state into two, one in which the basioccipital takes part in the facet for the toothed upper pharvngeals (Haplochromi.r type), the other in which it buttresses the parasphenoid without reaching the facet (Tropheus type). TREWAVAS (1935) had included the Tropheus-type in the Haplochromis type, recognizing it as a departure from the (primitive) TY/aj&Mtype in the direction of the extreme Haplochromis type as exemplified by the Lake Victoria haplochromines and most Malawian ones. This classification is in accord with the occasional variation within a species, or even, as in Pseudotropheus macrophthalmus, on right and left sides of the same fish (GREENWOOD, 1978: 315). The figures in GREENWOOD (1978) of lateral views of the apophysis show the similarity in the shape of the basioccipital in all species in which this bone shares in the apophysis, whether it reaches the facet