Bullheads and Black Gobies: A remarkable case of Convergent Evolution
What is Convergent Evolution?
Convergent evolution is when two species, though not being closely related, develop similar traits and features. One of the most famous examples is of bats and birds - two animals that, despite being only very distantly related and having no flying common ancestor, developed flight independently. In fact, it's thought bats first took to the skies around 50 million years ago whereas birds did so 100 million years earlier. The reason for similar adaptions like this are often the same - in the case of bats and birds, it was both that prey were easier to hunt aerially and the fact that it was harder for terrestrial predators to catch them while airborne. However, in this edition of Pernature we'll be focusing on a less well-known case of convergent evolution on our shores: the European bullhead and the black goby.
Bullheads
 |
| A European bullhead caught in the River Great Ouse |
To understand our strange case of convergent evolution, lets meet our first fish - the European bullhead.
Bullheads, also known as miller's thumbs, tommy's logge, freshwater sculpins or European bullheads to avoid confusion with bullhead catfish of North America, are small, bottom dwelling river species in the UK. Growing no larger than 1 ounce or 28 grams, these micro fish are found across streams, rivers and lakes of Britain and prefer areas with lots of rocks to hide under. Unlike other fish species, they lack a swim bladder - the organ vital to other fish for buoyancy - as they live almost exclusively on the bottom (being known as benthic) and have no need for such buoyancy. Their prey is dominated by invertebrates such as crustaceans (such as small crayfish and shrimp) and benthic insects (including caddisfly and mayfly larvae), but small fish are occasionally taken too as well as detritus. When it comes to the breeding season, bullheads are much more interesting and peculiar. First of all, males turn much darker - in some cases almost completely black - and his first dorsal fin will develop white edging. Whilst this transformation is underway, the male finds a territory - most usually a large rock - and aggressively defends it from other males, even snapping at other harmless fish like stone loaches. Once he has his territory set up, the male has a unique way of enticing females to him - by singing. While it may not be as melodious as a robin or song thrush, it is still impressive for a bottom-dwelling British fish, and - if successful - draws in females. To carry out this (though untuneful) 'singing', the fish rubs his pectoral girdle against his skull - producing pulses of sound between 50 - 500Hz. If he is victorious in courtship, one or multiple females will deposit batches of eggs after mating in a small hole he will dig under his rock, where he guards the eggs for around a month until they hatch. Breeding occurs from Late Spring to Mid Summer, and after the up to 400 eggs hatch they will live around 5 years (or 10 if they are lucky and don't get eaten by predators such as kingfishers) and reach sexual maturity at 2-3 years. If you want to find them yourself, they are easily located by carefully turning over rocks in streams and river, or caught by rod with a maggot or chopped worm as bait. They are widespread across England and Wales, and are also found in Scotland, Ireland, and most European countries excluding the far North and South.
Did you know...
Bullheads are crepuscular - meaning they are most active at dawn and dusk though may feed at other times of day too. In the day, they generally stay hidden under rocks or deadwood in the river or lake.
Black gobies
 |
| A black goby caught in Plymouth |
Inhabiting shallow coastal waters of between 1 and 50 metres of depth, the black goby is a small benthic marine fish growing to a maximum of around 60 grams or 2 ounces. They are commonly found in lagoons, estuaries, harbours and inshore waters. Like the bullheads discussed earlier, these gobies also lack a functioning swim bladder as they also spend most of their time resting or shuffling across the substrate of the sea floor. Also similar to miller's thumbs, black gobies have a large bulky head, a strange tapering body and large fins, and have similar brown-black colouration. Unlike tommy logges - though of course it is still made up of benthic invertebrates - their diet consists of saltwater species, some quite different to the species of a freshwater sculpins diet. It includes small crabs, shrimp, burrowing worms, gastropods (aquatic snails) and various bivalves (tellinids, small clams, cockles and mussels) whereas small fish and fry also make up a small part of their diet along with organic matter. During a Tunisian stomach-content study, scientists found around 46.75% of the black goby diet is made up of molluscs - particularly bivalves and gastropods. While some goby species, especially the painted goby of Atlantic estuaries and lagoons, sing akin to that of bullheads, male black gobies rely on visual and behavioural cues to attract a mate, including an elongated dorsal fin and defending areas around rocks or holes as a nesting site. If a female goby approves after inspecting his nest and appearance, she will lay potentially hundreds of eggs under a rock, under a shell or in a hole or crevice. Remarkably, back gobies have been known to have six individual clutches in a year. Their have a wide range of predators and so must keep their wits about them, ranging from larger fish such as cod and whiting to large crabs, diving ducks such as goldeneye, shags and even kingfishers that migrate to the ice-free coastal waters in Winter. If you want to find them for yourself, you may spot them while snorkelling of UK shores, or caught with a rod with mussels or ragworm as bait. However, when fishing for them don't expect to catch them straight away; other species such as corkwing wrasse, tompot blennies or scorpion fish (the bullhead's closest relative) all feed on similar food sources and hence may be caught first, as well as other species.
Did you know...
Black gobies are primarily diurnal but have crepuscular activity peaks, and unlike other gobies aren't known for common night-foraging. Their large eyes, comparable to that of the tommy logge, are relatively large and aid dawn and dusk foraging.How are they related?
European bullheads are found in the genus cottus - members of which known as 'sculpins' - and black gobies are in the genus gobius - with all members of which being known as 'gobies'. These two groups of fish diverged millions of years ago, and their last common ancestor was thought to have looked quite different, while still being a benthic fish. The bullhead's closest living relative in Britain are other sculpins, particularly scorpion fish, whereas other goby species, such as giant, rock and steven's gobies are the closest British relatives of black gobies today.. So, how did these fish come to looking and acting so similar? Well, it all comes down to their habitat - while one inhabits freshwater and the other dwells in saltwater, they both live nearly exclusively on the bed of the water body on gravelly or muddy substrates. Their large heads - the factor that gave the bullhead it's very name - evolved to be able to have a large mouth for lunging at and eating prey; while their upward facing eyes are adapted to spotting predators while resting on the sea or river floor. These charismatic fish species are an example if two different fish that are perfectly adapted to their habitat and conditions, and can end up looking and acting incredibly similar.
Benthic - a species that spends most or all of it's time living in the benthic zone, an are at the very bottom of a water body such as a stream or ocean, including the substrate surface and a small area of water.
Detritus - decaying organic matter such as algae, plant materials or dead organisms.
Fry - the stage of a fish's lifespan when it has recently hatched and is not fully grown or sexually mature.
Genus - genetic groups of closely related animals or other organisms.
Last common ancestor - the last species that two or more animals or genera share as an ancestor and of which all species referred to diverged from into different species or groups
Personally, I think bullheads and black gobies are a fascinating case of convergent evolution, showing how ecologically distinct species can evolve very similar adaptations under similar circumstances. Both species are very intriguing in their own ways, from the bullhead's courtship 'songs' to the black goby's mollusc-dominated diet. I have great memories of flipping rocks in hope of finding a miller's thumb looking back up at me on warm summer's days by the river, although most often I would only find a stone loach if I was lucky! Thank you for reading this edition of Pernature, and I hope you have learned something about benthic fish and the wonders of convergent evolution! Thanks again,
Isaacp Per[Nature]🌿
Comments
Post a Comment