There has been an ongoing census of the fish community in Lake Tanganyika for the last twenty years. During this census the Japanese scientists have taken note of some rather drastic changes in the fish populations. These findings were published in the most recent issue of the journal “Ecology of Freshwater Fish”.

The conclusions were reached by Yuichi Takeuchi and team, while surveying the density and number of the different fish in Kasenga Point, at the southern end of Lake Tanganyika between 1988 and 2008.

The investigative team, headed by Yuichi Takeuchi, took note of a total of 54 chichlid and six non-cichlid species of fish, over the period of the survey. The most abundant type of fish recorded were the aufwuchs feeders.

Even though there was an abundance of fish and no lack of number of species present, their research indicates that there was a steep decline in the density of the aufwuchs feeders and invertebrate eaters during the course of the study. In a strange turn of events, there was an increase in the density of detritus feeders during this time.

The researches theorize that the change in the fish community could be due to human activities in the area such as deforestation, and agriculture run offs, which would lead to sediment loading in Lake Tanganyika to increase.

While they could not completely rule out natural variation for these changes in the fish community of Lake Tanganyika, there is no doubt that human activity is also making a large impact on the area as well.

In several species of fish, such as the cichlid species Neolamprologus pulcher, it is common for subordinate females to help an unrelated dominant breeding pair raise their young. The reason behind this seemingly altruistic behaviour, known as alloparental care, has puzzled scientists for many years and one of the most widely spread hypotheses put forward has been the ‘pay-to-stay’ hypothesis. According to the ‘pay-to-stay’ rationale, the subordinate female helps out the dominant pair just to be able to stay in the group. Not being ostracised from the group augments her long-term survival chances, thus increasing the chance for her to live long enough to eventually obtain a breeding position.

Picture by: JJPhoto.dk

A new study carried out by Dik Heg and coauthors does however bring forth a new hypothesis: the substrate rationale. In their study, Heg and his colleges tested the hypothesis that subordinate female cichlids are helping dominant pairs in return for a more immediate direct reproductive benefit. After a series of experiments where the total number of eggs produced over a 30 day period by dominant and subordinate Neolamprologus pulcher females were carefully counted, researchers found that a subordinate female helping out a dominant pair was more likely to produce eggs herself compared to other subordinate females.

According to Heg and coauthors, the most likely reason for the increased reproductive success of “fish nannies” is that the subordinate female gains access to the breeding substrate.

If you wish to read more, see the paper “Heg, D, E Jutzeler, JS Mitchell and IM Hamilton (2009) Helpful female subordinate cichlids are more likely to reproduce”. It has been published in the journal PLoS ONE.

Here at AC Tropical Fish, we believe in the Jude Law-hypothesis. The dominant female will naturally snatch away the most prosperous male, but by posing as a benevolent nanny even a subordinate female can gain access to his home and hope for some of his triumphant DNA to eventually find its way into the genetic make up of her own offspring.

The members of the genus Synodontis, commonly known as the squeaker catfishes of Lake Tanganyika, evolved from a single common ancestor according to a paper* published in a recent issue of the Journal of Evolutionary Biology.

synodontis catfish

Researchers Day, Bills and Friel** analysed 1697 base-pair sequences consisting of nuclear (ribosomal protein-codin gene S7), mitochondrial (cytochrome b) and transfer RNA gene fragments in 65 samples (representing about 40 species) of squeaker catfishes to study the evolutionary relationships of the group.

Through their research, the authors were able to track down a single origin for the Lake Tanganyika species flock. The members of the genus Synodontis all evolved within the last 5.5 million years which makes them a comparatively new addition to this Great Rift Valley Lake which is believed to be at least 9 million, perhaps even 12 million, years old.

Day, Bills and Friel also recovered a monophyletic group of southern African riverine species which seems to have diversified very rapidly (during the last 890,000 years). This group was believed to have been formed due to adaptive radiation within Lake Makgadikgadi; a lake which is now extinct.

* Day, JJ, R Bills & JP Friel (2009) Lacustrine radiations in African Synodontis catfish. Journal of Evolutionary Biology 22, pp. 805–817.

**Julia Day, Roger Bills and John Friel

A study done on Perissodus microlepis, a cichlid from Lake Tanganyika in Africa shows that opposites attract. The cichlid is a scale eater that eats scales from other fishes. Specimens have a mouth that either open to the left or the right. This adaptation makes it easier to tear scales from other fishes when feeding. The study showed that this species prefers to pair up with a partner that has a mouth that opens the opposite way of their own. During the course of the researches who completed the study (2004-7) caught 24 Perissodus microlepis pairs. 21 of those pairs had their mouths facing opposite sides. This is the first time disassortative mating have been confirmed in the wild. (Disassortative mating is to choose partners who differ from oneself to avoid inbreeding.)

The study was done by Japanese researchers including Kyoto University researcher Tetsumi Takahashi and was announced Wednesday in the online edition of “Biology Letters” published by the Royal Society.