Saturday, February 20, 2010
Sunday Spinelessness - Animals that don't move
There aren't many universal laws in biology. Snails proved Dollo wrong, retorviruses did for Crick's Central Dogma of Molecular Biology and every lesson on Mendel's Laws of Inheritance includes a section of the exceptions to those rules. Biology's disregard of human laws notwithstanding, you might think, at least as far as macro-organisms are concerned, you could safely generalise that animals move and plants stay still. But once you consider the ocean even that generalisation can't be supported; corals, bryozoans, sea squirts, anemones and sponges are all animals that spend their adult life in one spot.
While I was in Masterton for christmas my girlfriend went to Vanuatu (no, you're right, that doesn't quite seem fair...) with a waterproof camera so I've stolen a few of her photos of coral to illustrate todays sunday spinelessness.
It's easy to see why early naturalist thought corals were plants but what you are looking at in those photos is not a single organism, rather it's a colony of tiny genetically identical animals. Corals are members of the phylum Cnidaria (the 'c' is silent) which includes corals, anemones and a diverse bunch of animals we call jellyfish. Such a diverse collection of animals are united under the name Cnidaria because they all employ the impressive nematocyst, a barbed harpoon like cell, to catch and deliver toxin to their prey. Cnidarians have two distinct life stages - a swimming "medusa" (adult jellyfish being the classic example) and a sessile polyp (like the sea anemones familiar to rock pool fossickers the world over). Individual coral colonies (termed "heads") are made entirely of polyps which reproduce asexually depositing a calcium carbonate base as they grow - the exact pattern in which polyps bud from their parents determines the shape a coral head takes.
Many tropical corals supplement their diets by forging a symbiotic relationship with swimming algae (arguably plants that move...) called zooxanthella, the algae get carbon dioxide from the coral's respiration while the coral gets energy from the algae. This relationship is of huge importance in the tropics because it allows corals to grow in those region's warm, nutrient poor waters. Without coral reefs, made from thousands of years of calcification from corals, tropical waters would be nowhere near as biodiverse as they are now and people whose love of animals only goes as far as that peculiar phylum Chordata should care about that: