Wednesday, 27 July 2011

Marine life

I've been at the beach for the last week (hence silence on the blog!) having fun with family and friends. Despite the fact I grey up about as far away from the sea as is possible in the UK (which, I admit, isn't really that far...), I love beaches and sealife. It's a lot further to the sea from most of the safari circuit than where I used to live, but many visitors to East Africa will combine a safari with a beach trip, so you might find youself down there from time to time. And if you don't, I thoroughly recommend it for a fun trip some time. Tourists don't usually think of the beach as a place where wildlife happens, but actually marine and coastal habitats are completely fascinating and I thougth I'd make use of the pictures I took down there to give a few hints about what you might talk about if you do find yourself on the long drive to the beach. So today I'm going to start with what I think is most amazing about sea life - it's extraordinary diversity and plain weirdness.

This mollusc has an unusual foot - and you can see its eyes too!
Now, you have probably been told that after kingdom, the main division of all life is the phylum (in fact, if you were told this, it wasn't quite right - botanists refer to plant divisions, leaving phyla to zoologists - that's the  plural, btw, it's from the Greek phylon meaning race or stock). And the Animal kingdom is divided into around 40 phyla (12 divisions for plants). Being such a basic division of animal life, animals in different phyla can be expected to be remarkably different - insects belong to the phylum Arthropoda, and it's fairly clear they different to us humans, being representatives of the phylum Cordata. Similarly, snails belong to Mollusca and are pretty different to earth worms, belonging to Annelida. You get the idea - these divisions are pretty fundamental. Now, of the 40 phyla out there, I wonder how many you could name? (I don't think I'd do too well either, to be honest...) But what I do know is that whilst there are no extant Onychophora (velvet worms) living in the oceans, all the other phyla are found there but only 10 are found on land (if we exclude internal parasites of other species). So three-quarters of the most fundamental of divisions of the animal kingdom are entirely aquatic, most of them restricted to marine environments. Which, of course, means life is pretty different in the sea. Why this should be is obvious, of course - the original animals that formed the ancestors of modern phyla lived a very, very long time ago (during a period known as the Cambrian explosion, about 500-580 Million year ago) when there was only life in the sea (there's a nice time line of life here).
Tide-line discoveries from Ushongo beach, July 2011

Cuttlefish shell, also a mollusc
Why does this interest me? Well, when you go to the beach you have a chance of finding some of those rather more bizarre life-forms that never made it onto land. The above picture shows a few of the more obvious things we found on the beach. As everyone knows, there are lots of mollusc shells (phylum Mollusca) - but look at the major divisions within this phylum too - only gastropods, the snail type, have made it onto land, but the sea is full of bivalves and other classes we never see too. In fact, nearly a quarter of all classified marine species are molluscs and there's every reason to believe that some of them (octopus - the're also molluscs and belong to the same group as cuttlefish in the picture beside) have evolved remarkable intelligence, capable of solving problems and allowing them to practice deception.
Note crab carapace, top right, sponge top left and corals.

Hermit crabs have almost made the terrestrial transition but still return to sea to breed
Then you'll see a crab exoskeleton. Crabs are members of the Arthropoda, just like insects and spiders, and some of them have taken to the land - though they must return to water to lay eggs. The subphylum they belong to Crustacea does have truly terrestrial representatives - the woodlice are an example - but most are still aquatic creatures and there's another example here too in the goose barnacles near the back in the middle. Look further and you'll start seeing som true aquatic specials. There's a couple of sorts of coral there, belonging to phylum Cnidaria which also includes the sea anemones and jelly fish. These, in the form of hydra, make it to fresh water, but no cnidarian with their usually soft bodies have made it onto land. And there's also a cnidarian look-alike in the sponge belonging to Porifera. Actually, this phylum probably includes animals from more than one group and excludes some of the descendents of the original sponge (all the rest of the animal kingdom, in fact!), so we'd call if polyphyletic and expect taxonomists to break it into other groups when they understand the relationships better. Poriferans are an extraordinary group of animals that, when I was doing my first degree, everyone was excited to think might be a whole different kingdom (or even several kingdoms). Now this seems less likely, but it seems very likely that all the rest of animal life did evolve from a sponge, rather a long time ago.
Note sea-urchin top centre and goose barnacles bottom right

What else is there? Ah, a sea urchin, belonging to phylum Echinodermata. Here, at last, we find a purely marine phylum - starfish, sea urchins, sea cucumbers and crinoids are all here, and none of them has even moved up the rivers. They all have five-sided symmetry as adults (or there abouts), but as larvae they start life, as us, with bilateral symmetry. Very odd, but very important in some places, as grazing by these species can completely alter the ecology of a coral reef or sea-weed forest.
Casts from some annelid worm (I expect) in roots of a mangrove

Annelid tentacle trails radiate from the animals home in the centre
A few other things didn't make it into the group photo, but are interesting too - the vast numbers of 'worms' of one sort or another (mostly Annelida, but maybe one or two other phyla are around too, I've certainly found others before on these beaches). They're responsible for these casts and the strange patterns around this hole - at high tide the inhabitant stick lots of tentacles out of the hole to filter food back to the mouth at the top of the hole. And these worms and other invertebrates can occur at fantasic densities, providing masses and masses of food in the intertidal zone, for lots of nice birds and beasts. Those will have to wait for another time now, though, whilst we remain amazed by the extraordinarily diverse numbers of ways marine organisims seem to have evolved to survive. Remarkably, I'll leave you with the surprising thought that despite this huge diversity of live-forms, there are far more species of animals on land. In fact, far, far more. And most of them are beetles, phylum Arthropoda. Why should it be that such extraordinary diversity has evolved on the land and not in the sea, where evolution has been going on for an awful lot longer? Odd...

No comments:

Post a Comment