In the West Cameroonian crater lake, Barombi Mbo, and its inflowing stream there are 17 species of fishes, of which 12, including the 11 cichlids, appear to be endemic. We give a systematic account of the endemics, including four new species and two new genera, Konia and Myaka. The ecology of the lake is described in relation to the feeding biology of the fishes, studied by underwater observation in the lake and the examination of stomach contents. The cichlids in Barombi Mbo probably evolved there from two or three ancestral populations and now show clear ecological separation in their feeding and breeding. The problem of speciation within the lake is of particular interest because of the apparent absence of physical barriers to account for the genetic isolation of incipient species.

The West Cameroonian crater lake Barombi Mbo is stratified, with no detectable oxygen below 20 m. Of the endemic cichlid fishes in this lake Konia dikume is exceptional in having a mean haemoglobin concentration of 16.55 g/100 ml, while the other ten endemic cichlids range from 5.55 to 8.70 g/100 ml blood. The erythrocyte count is also higher in K. dikume than in the other species. The phytoplankton in the lake is most abundant between 10 and 15 m from the surface. The single species of cyclopoid copepod and the rotifers in the zooplankton remain in the top 20 m throughout the day and night, but the larvae of Chaoborus spend the daytime at depths below 20 m, mostly between 30 and 70 m. At night these larvae ascend into the upper layers, and feed on rotifers. Konia dikume feeds on the larvae of Chaoborus, and the high concentration of haemoglobin in the blood of this fish, coupled with a large blood volume, appears to be an adaptation for the storage of oxygen. This would enable the fish to dive into the deoxygenated hypolimnion and extend the time available for feeding when the Chuoborus larvae are ascending at dusk and descending at dawn. Konia dikume appears to be a visual feeder, and in the short tropical twilight any extension of the feeding period could be of survival value.

The lorica of the rotifer Brachionus calydflorus Pallas is variable in form. The most striking aspect of this variation is the presence or absence of posterolateral spines. Beauchamp (1952a,b), and, in more detail, Gilbert (1967), have shown that the presence of postero-lateral spines can be related to the presence of carnivorous rotifers ofthe genus Asplanchna. A substance liberated by Asplanchna causes females of Brachionus calydflorus to produce offspring with postero-lateral spines. Gilbert & Waage (1967) studied a natural population of 5. calydflorus and found that the length of the postero-lateral spines varied directly with the abundance of Asplanchna brightwelli Gosse. The greatest mean spine length that they found was 93-7 /xm when the density of Asplanchna was seventeen per litre. In Gilbert's (1967) laboratory experiments he recorded some individuals with postero-lateral spines up to 126 ^m in length.In plankton samples from two sewage ponds near Bandung in Java the posterolateral spines of B. calydflorus were extraordinarily long (Plate I). Measurements of 100 individuals from each pond are shown in Table 1. The samples had been taken quantitatively by filtering a known volume of water, so that we were able to estimate the density of the population and the density of Asplanchna brightwelli.The Javanese sewage ponds are extremely rich in zooplankton, and the density of Table 1. Population density of Asptanchna bn'ghtweltt and dimensions of Braehionus ealyciflorus in two sewage ponds No. Brachionus ealyeiflorus per litre No. Asplanchna brightwelli per litre Mean length of lonca of B. catyeiftortis Mean length of postero-lateral spines Mean length of postero-median spines Mean length of occipital spines Correspondence: Professor J. Green,

A new species, Oncocypris debundshae, is described from the crater lake on Cape Debundsha.