Wednesday, 5 November 2008

New fish species

Since writing my earlier account of the first few days of trawling we have been very busy sorting and identifying catches from the 24 trawls carried out to date. We are now reaching the stage where, although every trawl has brought up at least one extra species for the collection, we are becoming very familiar with the fauna as a whole. I am therefore able to spend more time on collecting, pinning out fins, and preserving fishes to obtain good representative samples of a size range of each species. I have been in regular email contact with Phil and Elaine Heemstra, the editors of the forthcoming Fishes of the Western Indian Ocean books, and delight in presenting them with photos of fish that are proving difficult to identify. They, in turn, are sending these photos to their various expert contributors to get their opinions. It certainly looks like there will be several new species to be described, and here are just a few of the more interesting fishes we are struggling with.

Nemipterus sp., resembles Nemipterus bipunctata which occurs off Mozambique, but that species lacks the distinctive yellow facial markings of this fish. These markings are reported as important distinguishing characters of this genus in the Indonesian/Australian seas where numerous species occur.

A species caught in one trawl only, does not resemble any known species from the Indian Ocean.

A large (50 cm+) snapper, Lethrinus sp., with very distinctive lips. We caught two in one trawl. The species does not appear in the FAO guide to the snappers of the world and the photo has been sent to the expert on these fishes for comment.

A stingray with completely smooth skin and uniform colouration (markings on back are trawl damage). These features do not fit any known Dasyatis species.

A large, 40 cm+, sparid bream provisionally assigned to the genus Dentex. We caught three of these from two trawl stations.

This very delicately coloured flatfish belongs to the genus Parabothus, but will have to be sent to an expert to determine its true identity. There is a strong possibility that it is a new species. These are the only two specimens collected.

In addition to apparently new species, we have lots of interesting species to sort later in the laboratory back in Grahamstown. In pelagic trawls near the surface we catch lots of small juvenile fishes that are difficult to link with adult stages. The remarkable photo is of a completely transparent flatfish caught in a trawl at the surface, where the total depth was 2800 m. Flatfishes are bottom-dwelling species once they have metamorphosed into the adult flatfish form with both eyes on one side of the head, but here we caught several of these fishes completely metamorphosed into adult forms and still living a pelagic life at lengths of 3-4 cm.

The snake blenny, Xipharias stelifer, is a remarkable fish that lives in burrows in the sand. We caught four at one site.

These are just a few examples of the biodiversity in this part of the Indian Ocean. We haven’t counted the number of fishes collected on this survey yet but the total is now well in excess of 150 species.
By Denis Tweddle & Oddgeir Alvheim

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