Family: Schilbeidae (Schilbid catfishes)
Environment: Freshwater; demersal
Climate / Range: Tropical; 6°N – 5°N
Type locality: Above the Senchi Rapids, River Volta, Ghana (formerly Gold Coast) Syntypes (3): BMNH 1922.214.171.124–21 (3).
Distribution: Africa, lower Volta River basin.
Trewavas, E. 1943. New schilbeid fishes from the Gold Coast, with a synopsis of the African genera. Proceedings of the Zoological Society of London 113(3): 164-171.
Irvineia, gen. nov.
As Eutropius (q. v.), but with the pelvic fins 9-rayed, and with a caecal appendage of the swim-bladder extending into the tail to above the end of the anal fin. In addition the caudal vertebrae exceed in number the known range for Eutropius, and the numbers of pectoral fin-rays and of branchiostegals are usually higher than in any Eutropius.
The vertebrae (counted in two specimens) number 44 or 45 caudal and about 15 precaudal, counting the complex vertebra as numbers 2 to 4.
The branchiostegal rays have been counted in six specimens, in five of these on both sides; thus eleven series have the count is 11, in the other three 10. No species of Entropius is known to have more than 10.
The pectoral rays have been counted in 15 fins belonging to eight specimens. Twelve fins give a count of 13 branched rays, two give 12 and one gives 14. I have not found as many as 13 in any Entropius, and 12 occur only in two of the nine species tested (commonly in E. congensis, exceptionally in E. mentolis).
The last dorsal lay is divided to its base and both parts may be divided distally, whereas in Entropius this ray is usually simple or only distally divided.
IRVINEIA VOLTAE, sp. n. (Text-fig. 1.)
Greatest depth of body (at origin of anal fin) 5 to 5.2 times in the length, length of head (to end of operculum) 4.6 to 5.1. Eye lateral, 4 to 4.3 times in the length of head, 1.5 to 1.6 times in length of snout. Interocular width 2.2 to 2.7 times in length of head, 1.2 to 1.25 times in width of mouth. Lower jaw slightly projecting, its length 1.3 to 1.45 times in length of head. A continuous band of teeth on the vomer and palate, broader than that on the premaxillary. Gill-rakers on anterior arch 7 or 8+15 to 17. Maxillary barbel 1/2 length of head, extending a little beyond orbit; nasal and posterior mandibular subequal, 1/7 to 1/5 length of head; anterior mandibular absent or very short (absent in one specimen, 3 1/2 mm. in the second, absent on the right, 2 1/2 mm. on on the left side in the third). Dorsal I 6 (as well as a rudimentary spine), the last ray cleft to its base; spine feebly serrate, about 1/4 length of head *. Anal 62 to 69, the first 4 or 5 simple. Pectoral I 12-14, just reaching pelvic. Pelvics attached behind dorsal, extending to origin of anal, each of 1 simple and 8 forked rays. Caudal forked, apparently with pointed lobes. Length of caudal peduncle 1 2/3 to 1 3/4 times its least depth. Occipital spine about 0.3 times the lateral length of head, a little less than 2/5 of the mid-dorsal length of head before it, separated by a gap from the median dorsal elements supporting the front of the dorsal fin.
Colour silvery, darker above. Anal and pelvic fins clear; other fins dusky. The types are three specimens, 157 to 178 mm. in standard length, caught (together with Eutropius micropogon), in a cast-net, above the Senchi Rapids, River Volta, Gold Coast. The fishermen demonstrated the difference between the two species by showing Dr. Irvine the produced air-bladder.
In addition to the types there are six smaller specimens, 132 to 155 mm. in standard length, from the Volta estuary. These are in a poor state of preservation, but in some of them counts have been made which are used in defining the genus and species. The lowest and fin count of 62 comes from one of these. The anterior mandibular barbel is present in all these specimens and is 2 to 4 mm. long.
The size of the mouth, measured by the length of the lower jaw, is greater than in any Entropius I have examined.
The Swim-bladder. -The anterior, normal part of the bladder is of the type usual in Eutropius, a large, free, heart-shaped bag, in contact with the skin laterally in an area seen externally as a dusky patch. Inside, this is
* In the one specimen in which it has not been broken off short.
>>Text-figure 1. Irvineia voltae, one of the types. This drawing shows the proportions of the fish and the position of the caecum of the swim-bladder. The main, normal part of the swim-bladder has not been exposed. Liver and gut were removed by the collector; only the posterior end of the intestine is shown in position. The urinary bladder, wrapped in mesentery, is shown behind it, but the kidney is not shown. The swim-bladder is, of course, outside the abdominal cavity, but the pritoneum is not drawn.<<
of the usual type, partially divided into anterior and posterior chambers by a pillar-like fibrous transverse septum, immediately in front of which is the opening of the pneumatic duct. The posterior chamber is, as usual, divided into two by a membranous longitudinal septum which ends just in front of the narrow communication with the posterior caecum. Other internal ridges are confined to a few low folds of the ventral wall of the posterior chamber. The posterior caecum opens by a narrow neck out of the hind end of the main bladder and extends to the end of the abdomen, where it is more or less constricted, and then expands again to the caudal part, which extends to the end of the of the abdomen, where it is more or less constricted, and then expands again to the caudal part, which extends to the end of the anal fin, always on the right side of the basalia of the anal fin, between the bones and the thin muscle-layer. In the specimens at hand the bladder is in various states of distension, and there is probably free passage of air throughout its length. The caudal part is irregularly constricted, and the walls of the whole caecum are strengthened by internal ridges.
The caecal bladder is equally extensive in all the specimens. This is in contrast to Pangasius, in which the caecum does not extend into the caudal region until the fish is a little larger than the smallest Irvineia examined, and then reaches only a quarter of the way along the anal fin (Nair, 1937).