by Matthew Norton
Most animals need to overcome a number of challenges during their life, or else it’s going to be a very short life, and one of the big ones is getting enough food to eat. Achieving this herculean feat will require exceptional foraging skills and well designed tools and weapons, with a touch of good luck every now and then. Unfortunately, most other animals in the sea have exactly the same goal in mind and because there is rarely enough to go round, intense competition between species will ensue. This can be reflected in the evolution of many species (although there are always other factors at work) who are adapted to seeing off the competition, or avoiding it altogether.
Even among a single family of fish, such as the cichlids, evolving your way out of these intense contests for food can produce some remarkable and bizarre results. It is estimated that there are at least 3,000 cichlid species worldwide, with 2,000 in the Great Lakes of East Africa alone, with a diverse range of physical features such as size, body shape and colour pattern, as well as feeding strategy. I should make it clear at this point that there are factors at work apart from the demand for different food sources, but it does play a large role in explaining why there are so many species and why some of them are so weird.
Let us start with the scale eating cichlid Perissodus microlepsis from Lake Tanganyika. As the name suggests, this particular species is a specialist in biting off the scales of other fish and are so committed to this lifestyle that their jaw is purposely twisted to one side so they can get a better grip on the side of their victim.
There are also species who target the eggs of other cichlids and employ a brutal technique to get them. Many cichlids are mouth brooders and by keeping their eggs inside their mouth (without swallowing) they are usually safe, that is until these egg hunters ram into the parent in the head and force them to unwillingly spit their precious young out to be devoured. Caprichromis orthognathus is one such egg hunter and they have developed the cunning trick of changing their body colour to blend in with their targets and line up a clear shot.
Sleeper cichlids, such as the Livingston’s cichlid, eat whole fish and also have a feeding strategy based on trickery. They lure unsuspecting scavengers to their doom by lying very still on the bottom and playing dead. To make the trap even more enticing they make their body appear bloated and blotchy, as if they were rotting away.
Finally, there is the Zebra mbuna Maylandia zebra, one of many cichlids who are highly skilled at utilizing the whole world around them for sustenance. They scrape through the thin layer of organic material that can be found on any hard surface below the water and is often rich in microscopic lifeforms as well as animal remains and tiny scavengers. It may not be the most glamorous feeding strategy, but not every species can afford to be picky with their meals and should this resource dry up M. zebra may start picking off floating plants and animals in the water as well.
Some of the feeding strategies mentioned here are not unique to cichlids, but getting them all, and then some, in this one family of fish is a testament to the incredible diversity they have achieved. It is also a family tree that is still expanding as new species are still being discovered and who knows what new secrets may be revealed.
From a human perspective
I have already mentioned that the Great Lakes of East Africa as being a hot spot for cichlid diversity and Lake Victoria is no exception, or at least it was. Cichlids did once flourish in this particular lake until we came along and introduced them to the savage Nile Perch Lates niloticus, a fish that does not belong there. It is always dangerous for native species when humanity introduces a new species into their environment, a species that they have not evolved with for the millions of years that are necessary to come up with adaptations to achieve some form of peaceful co-existence.
In the case of the Nile Perch, the consequences speak for themselves. This fish was first released into Lake Victoria in the 1950s and since then at least 200 species of cichlid have disappeared. The prevailing theory has been that the Nile Perch ate them all to extinction, but some have suggested that this invader may have out-competed some of them for food and drove them to starvation. Worse still, many of the survivors (around 300 species) are struggling to find a way of coexisting with their unwelcome neighbours.
Our carelessness with the underwater world can come back to haunt us if we start losing those species of cichlid that have value to us. Some cichlids are prized animals in the aquarium trade and for human consumption, but others provide humanity with ‘ecosystem services’ that keep us safer and more comfortable (often with little thanks). The snail eating cichlid Trematocranus placodon from Lake Malawi is a good example because they have a taste for aquatic snails that carry Schistosoma haematobium, a parasitic worm that causes Urinary Schistosomiasis (also known as bilharzia).
This condition causes relatively mild symptoms during the first few weeks of infection, but these parasites can remain in your body for years and do serious long term damage to your bladder, kidneys and other organs while also laying eggs that escape in your urine and faeces. Once hatched, these worms need a few weeks inside a snail host before they can go on to infect another human and this is where our snail eating friends can help. With fewer snails to carry the worms through this intermediate stage, there will be fewer adult worms around to infect us. But not if we wipe out the fish that keep these snail populations under control.
In the interest of balance I should mention that cichlids can also become invasive species and have the potential to cause problems if they find their way to places where they don’t belong. For example, the Mozambique tilapia has spread far beyond the waters of the country it is named after. Unlike other cichlid species, they are very flexible with their diet and feeding habits and would easily be able to exploit their new surroundings at the expense of the natives.
I admit this article has been a bit more doom and gloom than usual, but the threat posed by invasive species is serious. It is also likely to get worse if we carry on damaging the natural world that these animals live in and leave them more vulnerable to those species who will aggressively expand their borders if they get the chance. However, like any environmental issue, raising awareness of the damage it has done, and may continue to do, is the first step to finding the solutions. And finding solutions to very serious problems is one thing that we are very good at.
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