Saturday, 20 August 2011

Milly: Ugly Fish pt 8

Milly: Ugly Fish pt 1
Milly: Ugly Fish pt 2
Milly: Ugly Fish pt 3
Milly: Ugly Fish pt 4
Milly: Ugly Fish pt 5
Milly: Ugly Fish pt 6
Milly: Ugly Fish pt 7

Coming back to my post about pressure (pt 6), how are deep sea animals adapted to survive at depth?

As there are all sorts of animals currently inhabiting the deep, I'll concentrate on fish. Fish are the best anyway. Slightly biased.

Well the problem is, fish don't just have to cope with the crushing pressure (up to 800x greater than at the surface) but the deep is also very dark (beyond 1000m there is negligible surface light) and cold (2°C) making things such as moving around, finding food and reproducing much more challenging. Let's have a look at a few fish and see how they do it. Enter handsome fellow number one, the anglerfish.

Whipnose anglerfish, about the size of a football.
Note the long lure with the tiny esca on the end, capable of bioluminescing.
Image: Dianne Bray
A selection of anglers.
Image: Dr Theodore W. Pietsch and Christopher P. Kenaley

There are 11 families of deep sea anglerfish, some only containing one species, such as the lonely prickly seadevilCentrophryne spinulosa (Centrophrynidae family) others have many more such as the footballfishHimantolophidae with 19 species. They are a surprisingly diverse group of ugly fish. As you can see from the pictures above, these animals don't look particularly athletic. If you were to design an animal for a race underwater I doubt 'round' would be the shape you would go for. Nevertheless, these are very successful bathypelagic (1000 - 4000m) animals, but why? Anglerfish are sit and wait predators and with the aid of their bioluminescent lure, can draw in prey such as fish and cephalopods (squid etc) within gobbling distance. The bioluminescent light, which can be controlled producing flashes or sustained glowing is the product of many bioluminescent bacteria that colonise the lure (or esca). They don't even need to feed particularly often since they expend such little energy, evident when you touch one of these animals. Their bodies are both flabby and bony (an attractive combination) suggesting they do very little excercise. Lazy little anglerfish. Not only are they lazy but seriously greedy. An expandable stomach allows anglerfish to munch down prey twice their size. A human man could fulfil his daily allowance of calories with 850g of meat, less than the weight of a mature trout! Seriously rubbish in comparison. 

Sea devil (Melanocetus niger), about the size of a
golf ball, look at that mouth! Image: Milly Sharkey

A female angler with the tiny parasitic male attached.
Image: Dr Theodore W. Pietsch. University of Washington
One problem that could arise, being a voracious predator in the dark munching on anything that moves, is that you might accidentally eat your mate. Finding a mate in the dark expanse of the deep sea is likely to be a very rare event. Some anglers have evolved a bizarre strategy whereby the male does not develop fully, is unable to feed properly and has enhanced olfactory senses (smell). This spurs the male on to quickly find a female before he starves, detecting the pheromones she expels into the water. When they do meet, the male bites the female, releasing enzymes that break down the skin between them, causing the two to fuse. The lucky male is now parasitic and no longer requires digestive organs, brain or his own heart (I told you they were lazy), obtaining everything he needs from the female. In return, he provides her with sperm from his last remaining organ, his gonads, a perfect relationship some may say. Bit clingy, plus he's a bit screwed...sorry I'll stop now.

More later, I need to locate pre-trawl pudding.

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