Professor Jenny Day, director of the Freshwater Research Centre, shares interesting developments on Cape galaxias. Photos: Jeremy Shelton

There are real live fish in Van Blommestein Park, Zeekoevlei (Cape Town). 

Really? Fish? In Van Blommestein Park? Yes! They are tiny and rather dull-looking, but there they are. And they are biologically very interesting. At the moment they are called Galaxias zebratus, but that is about to change.

The extreme rainfall that we have experienced in the last couple of months has provided a few biological surprises, one being galaxias in both Van Blom and Kenilworth Racecourse. Of course, we would not have expected the fish – being fish – to exist in those temporary wetlands, so the real surprise is that they have appeared, seemingly from nowhere. What is the explanation? Are they able to survive long periods of desiccation, emerging only when significant wetlands appear? The answer is that they can, surprisingly, survive out of water. Where were they then? We know that there is a population in Rondevlei but how did they get to Van Blom?

It seems that this year’s very high rainfall caused Rondevlei to reach record-high water levels, flooding across the Erica Field and filling up low-lying depressions in Van Blommestein Park and elsewhere, and allowing the tiny galaxias to follow the gentle flow of water to new homes. (The situation on Kenilworth Racecourse seems to be more puzzling until one realises that there are quite a few small artificial “dams”  for watering the trackway; they clearly support a population of galaxias that also went for the gap when their “dams” overtopped.)

“They’re tough little things, able to survive relatively poor water quality and high temperatures.”

Our South African galaxiads are tiny fishes, no longer than your thumb. They are found in slow-flowing rivers and in freshwater wetlands of all kinds from the Olifants in the west to the upper Gamtoos and Krom rivers in the east. They’re tough little things, able to survive relatively poor water quality and high temperatures. They feed on minute invertebrates. Spawning takes place from spring to mid-summer, females laying 30 to 40 eggs at a time. In wetlands the eggs lie on the bottom and in rivers, between stones where they are not swept away. Having evolved where predatory fishes are uncommon or non-existent, galaxias are easy prey to introduced trout and bass.

Cape galaxia Image Jeremy Shelton

The family Galaxiidae is ancient and is thought to be related to the ancestors of the Salmonidae – trout and salmon. Today the family Galaxiidae is found in Australia, New Zealand and South America, and here in the south-western Cape. This distribution pattern is said to be Gondwanan, implying that the family originated when the southern continents formed the single land mass that we now call Gondwana, some 100 million years or so ago. About 53 species have been described in the family but only one – Galaxias zebratus – is known from southern Africa.    

Species status and genetics

Dr Keppel Barnard was a phenomenal naturalist, who described a variety of different kinds of animals from snails and crabs to dragonflies and fishes. He noted that all the galaxiads in the region had been given a single name (Galaxias zebratus), implying that they all belonged to a single species. But Barnard doubted this because the specimens he saw varied in size, shape and colour patterns that suggested to him the existence of more than one species. Nonetheless, he did not divide them up, wanting to examine more material so that he could get a better feeling for the different forms before describing and naming them.

This reluctant acceptance of a single species was challenged by Marcus Wishart, a PhD student at the University of Cape Town. By the time Marcus was working on the problem in the 1990s, a number of techniques had become available for investigating genetic similarities and differences between individuals. He therefore collected specimens from a number of riverine sites near to and around the Cape Peninsula and was able to show the existence of at least five different lineages, which he suggested might well turn out to be five different species. 

Dr Albert Chakona and his colleagues from the NRF-South African Institute for Aquatic Biodiversity (NRF-SAIAB) in Makhanda are preparing a paper on the different galaxiad lineages in the Cape and we hope to see the names of the new species soon. As yet, Albert and colleagues recognise at least eight, which they have identified merely by their origins: “Breede”, “Klein”, “Goukou”, “Riviersonderend”, “Gouritz” “Heuningnes”, “Olifants”, “Joubertina”, and “Verlorenvlei”, with others they have nicknamed “slender” and “mollis”. They intend to retain the name zebratus for the riverine species of the Cape Peninsula, while resurrecting punctifer, an old name suggested by Barnard.

“The occasional filling of wetland depressions during extreme rain events might explain the remarkable genetic diversity of fishes like galaxias.”

In addition to those from Kenilworth and Van Blommestein Park, a population of G. zebratus is known to have occurred in the lower Diep River upstream of Milnerton Lagoon. Unfortunately galaxias have not been seen there for many years and the population is thought to be extinct. Albert and colleagues will be coming to Cape Town very soon to look for specimens in the flooded Rietvlei area and the old Milnerton Racecourse to see if perchance the flooding there has flushed out some specimens. Hold thumbs that they find some. 

Albert and colleagues recently published a paper postulating that the occasional filling of wetland depressions during extreme rain events might explain the remarkable genetic diversity of fishes like galaxias. The events of this winter neatly bear out this hypothesis. But what of the little fishes that have taken up residence in the flooded wetlands? They will die when the wetlands dry and summer turns up the heat, but at least we know that healthy populations of both species are able to survive in their nearest permanent water sources.