An Introduction to the Invertebrates, Second Edition - tiera.ru

246 INVERTEBRATE CHORDATA AND HEMICHORDATAHydrothermal ventsHydrothermal deep-sea vents, mostly chimneys called ‘black smokers’, werediscovered in 1977. They are formed by underwater volcanos making a new seafloor out of hot rocks rich in minerals, and superheated water presses up tothe surface and spurts out, making vents with openings about10 cm in diameter.The hot fluid is often very acidic and has a high concentration of H 2 S, methaneand metals, especially iron. The temperature is usually about 65 °C, but 390 °Chas been recorded. These vents have been found to be common in the Pacific,Atlantic,Indian and Arctic oceans.Amazingly, the edges of these smokers are inhabited by a large varietyof invertebrates: none of these is a completely new kind of animal but most areunfamiliar species of recognisable groups. Over 500 new species have alreadybeen found and 12 new families; 90% of the species are endemic (unique tothis habitat). They include gastropods (one species covers its foot in scalesmade from iron salts), bivalves, shrimps gathering near the vents, crabs feedingon bacteria, nemertines with symbiotic sulphur bacteria in their epidermal cellsand above all polychaete annelids, especially the vestimentiferan‘Pogonophora’(now called Siboglinidae). Some 200 000 of these were found together2500 metres down in a black smoker in the Pacific, off Costa Rica, all red orwhite in colour. Riftia is the most striking and characteristic vestimentiferan,sticking up at least 2 m; the tube grows in length by 85 cm in a year, makingthese the fastest-growing invertebrates known. Riftia produces yolky eggsthat float away and hatch into yolky larvae; there is never a mouth or a gut atany age.Animals around vents are zoned horizontally, according to their tolerationof high temperatures. The Alvinellidae, a new polychaete family relatedto the Terebellidae (Figure 9.6c), are common on the sides of black smokers,being able to survive sustained temperatures of 50 °C at the pressure prevailing:high pressure assists survival at high temperatures. All the vent animals dependon chemosynthetic bacteria, by filtering them, farming them and grazing,or by importing them as symbionts. They all have many special adaptations,of membrane and DNA structure and blood and enzyme function, enablingthem to survive at high temperatures.Cold seepsCold seeps also occur at similar depths, with surprisingly similar fauna, alsodependent on bacteria metabolising sulphur and methane. Here temperaturesare low, life is long and growth is slow (a bivalve takes 170 250 years to reach3minlength).The animals in hydrothermal vents and cold seeps are not, as at firstthought, ‘living fossils’, sheltered survivors from earlier epochs. Molecularevidence suggests that most of these animals arose relatively recently.Continuing exploration of these habitats is revealing more and more remarkableinvertebrates.