Plankton on Parade

Saturday, 13th November 2010

The term ‘plankton’ comes from a Greek word meaning ‘wanderer’ or ‘made to wander’,an apt description for creatures which drift at the mercy of winds and currents. Some are large and easily seen – the Macroplankton, such as jellyfish and the Portuguese man-o’-war. Others are small – the Microplankton, which measure a millimetre or so across – or less – and are studied using a microscope. These are collected with a conical fine-mesh net swept or towed through the water and they include a great variety of plants and animals, in adult or larval stages. There is a third, even smaller, size category – the Nanoplankton, such as coccolithophores, which are just a few microns across and only seen with the aid of an electron microscope. Previously unsuspected, the existence of this group was deduced from studies of another marine creature, an early chordate which constructs a gelatinous ‘house’ in which to live and gathers its food by wafting water through ‘windows’ onto mucus-coated filters. It was observed that the openings in ‘grills’ over the windows were too small for microplankton to enter. Hence it was inferred that the water might contain even smaller creatures, in addition to ultrafine marine detritus which is also a food source.

Some planktonic creatures (a shorter term is planktont) can be regarded as ‘permanent’, in contrast to the ‘temporary’ group which are the larval stages of crustacea, molluscs, etc. Of those that live in surface waters, such as phytoplankton, diatoms and dinoflagellates, some are capable of making their own food, using carbon dioxide and releasing oxygen. Various mechanisms are used to enable planktonts to stay afloat without wasting energy. Some have constantly beating hairs (cilia) or flaps, e.g the pteropods. Others produce gas bubbles to give them positive buoyancy. An interesting example here is the Portuguese man-o’-war (Physalia), a large, colourful complex hydroid with a float bladder up to 12 inches long and 6 inches high beneath which is a mass of polyps having different functions, such as feeding tubes and reproductive organs, plus trailing tentacles up to 40 feet (12m) long armed with touch-sensitive stinging cells delivering a poison 75% as strong as a cobra’s venom. When prey, such as planktonic crustacea or small fish, is captured, the tentacles contract to draw it to the feeding tubes whose mouths expand sucker-like over the prey and secrete digestive juices. The float bladder can change buoyancy to counteract the weight of the prey. It also has the ability to deflate in storms and to immerse each side alternately in water to avoid drying. It is amazing to think that this colony with all its complex functions is derived from a single cell. Another example of positive buoyancy relates to a mollusc, Janthina, which floats at the surface attached to a raft of bubbles (gas enclosed by hardened saliva). When its eggs develop into larvae, these in turn develop their own bubble raft to float away.

Typical umbrella-shaped jellyfish can ‘swim’ upwards by rhythmic pulsations of the bell and then open it to descend slowly while capturing small planktonic animals using their stinging tentacles. In a rather similar fashion some planktonts maintain their position in the water column and minimise the tendency to sink by increasing their surface area relative to body mass, analogous to a sky diver spreading his limbs to slow his descent. This explains the fantastic bizarre to beautiful shapes of many small planktonic creatures embellished with spines, spikes, bumps and dimples. Some even have the ability to extend or reduce the length of spines and arms to adjust for differences in water density between warm and cold seasons and estuarine and marine environments. Planktonts typically feed on other planktonts, including nanoplankton, diatoms and algae, and on marine detritus, using a variety of mechanisms. Food particles can be drawn to the mouth by cilia, as in the case of dog whelk larvae, or wafted by fine spines, as in baby copepods. Others filter food from the water using a sieving system. The large white jellyfish Rhizostoma is a filter feeder with thousands of mouths on ‘arms’beneath the bell. By contrast the arrow worm, Sagitta, is a carnivore which actively seeks its prey, armed with two fans of spines at the head which capture the victim and pull it to rasping jaws.

Planktonic creatures can display some remarkable behaviour. Some are known to move systematically between higher and lower levels over a 24-hour period to ‘graze’ the water column. Those which are the larvae of crustacea, molluscs, etc. must adapt from a floating to a benthonic or sedentary existence. For example, a barnacle larva develops into a minute drifting two-shelled form which searches for a suitable settling place guided by a protein secreted by adult barnacles. It then anchors itself by its head antennae, secretes a cement, sheds its larval skin and develops limy plates, which can open to allow the feathery ‘feet’ to gather food. The close clustering of barnacles, which results from this larval behaviour, enables the extendable male organ to fertilise the eggs of neighbours (they are hermaphrodite). The planktonic larvae of the ribbon worm and of starfish do not develop into adult forms: they are simply carriers of the growing adult.

Although some plankton can have harmful effects on marine life, as witnessed by ‘red tides’ and ‘white tides’, this is far outweighed by their immense importance as a food source for creatures ranging from herring to the blue whale. Plankton populations fluctuate seasonally and with pollution, but these microscopic forms reproduce rapidly and overall their numbers appear to be stable.

The HMCG wishes to thank Dr Harris sincerely for his thoroughly interesting and enlightening talk and for reviewing this account.