From our archive
Inner space – 1370 million cubic kilometres of water form the oceans and seas of our blue planet. And in a single drop of sea water there is more life (as we know it) than we have found in all the vastness of outer space. Inner space teems with life forms as beautiful, fragile, bizarre and alien as any we could imagine from other worlds. Sea water is an ideal culture medium, nurturing microscopic, single-celled plants and animals, and supporting huge, floppy jellyfish, the giants of the plankton. The term ‘plankton’ literally means ‘wanderers’ – the drifting or weakly swimming, unattached life which moves at the mercy of ocean currents and winds.
A plankton platter
These barnacles are in a feeding frenzy, in a cloud of plankton which includes their own offspring. In coastal areas, huge numbers of larvae of sea bed animals, including barnacles, mussels, scallops, crustaceans, urchins, starfish, worms and fish, swell the numbers of animal plankton for a time in spring and summer. They grow rapidly on planktonic baby food, drifting for a short time before settling to colonise a new area of sea bed. And baby food it may be, but it’s highly nutritious – concentrated plankton also sustains the mighty basking shark, the biggest fish in British waters.
This alien-looking sea, with the consistency of orange semolina, is a localised ‘red tide’ off the north coast of Skye. It’s caused by vast numbers of dinoflagellates, tiny plants the size of pinheads which swim and generally behave more like animals. Its name, Noctiluca scintillans, means ‘shining night light’, a reference to its spectacular phosphorescence. In late summer, we can go down to the jetty at midnight and write our initials briefly in ghostly sparks with sticks in the sea – lets face it, it’s the only way we are likely to see our names in lights! But concentrated algal blooms are not all sweetness and light. The sheer volume of plant cells clogs the gills of fish, suffocating them, and some algal blooms are toxic. Filter feeders such as scallops and mussels accumulate these toxins, which appear to do them no harm, but can have a very nasty effect on humans eating them, causing amnesic, paralytic or diarrhoeic shellfish poisoning.
Inner space craft
From thimble-sized medusae to giants of over a metre across, jellyfish are living spaceships, cruising inner space in search of prey. There are many different kinds – medusae of sea firs, true jellyfish, colonial siphonophores and comb jellies – with different ways of swimming and feeding. The more toxic jellyfish have few enemies, but turtles can chomp their way through even the deadly Portuguese man-of-war without apparent harm.
Like irresistible tractor beams, the lines on this large medusa (Aequorea aequorea) lead to death for at least five sea gooseberries (comb jellies). All sea fir medusae are voracious predators. Aequorea is one of the biggest, growing up to 20cm across. Sea fir medusae are inner space colonisers, carrying larvae to settle new areas of sea bed.
Moons in orbit
Moon jellyfish (Aurelia aurita) were presumably named for their appearance, but the moon, which controls the tides, also has a great influence on them. Their weak pulsations keep them up in the food-rich surface layers, but are no match for tidal streams and wind-driven currents, which concentrate them into gyrating swarms. Small plankton sticks to conveyor belts of mucus that runs all over their bodies. This collects in blobs around the rim, and is periodically licked off. Maybe an alternative name could be the ‘runny-nose jellyfish’.
The cauliflower jelly (Rhizostoma octopus), one of the biggest and most solid jellyfish in British waters, doesn’t sting its prey to death, but filter feeds on tiny plankton. Thousands of beating hairs draw a feeding current through hundreds of tiny mouths on its huge lips (the ‘cauliflower’ part).
Our largest jellyfish, the lion’s mane (Cyanea capillata), can grow up to 2m across in Arctic waters. It grows to 50cm across around Britain, which is quite big enough for me – tentacles stretched out like a fishing net several metres long pack a vicious sting. The lion’s mane eats moon jellies – the pink reproductive rings of a moon jelly are just visible inside the frilly orange lips of this lion’s mane.
Inter dimensional warrior
The beautiful but deadly Portuguese man-of-war (Physalia physalis), occasionally blown on to our southwestern shores, is trapped in two dimensions at the sea surface – its gas-filled float acts as a sail in the air, while deadly tentacles hang in the ocean below. The float is occasionally deliberately capsized to keep it moist, and to allow the animal to ‘tack’ to change direction. The shape of the float gives the Portuguese man-of-war its historic name – seamen of old thought it resembled a caravel, a type of fast sailing ship with a broad bow and high narrow poop, which Columbus used on his explorations, and was later also used as a man-of-war.
Resistance is futile
The Portuguese man-of-war is a colonial animal (siphonophore). Like drones in a Borg collective, its many different parts (polyps) have different functions – catching, feeding, reproduction – but work together to keep the whole colony alive. The elastic strings of beautiful, translucent blue beads are batteries of powerful toxins, which paralyse planktonic crustaceans and small fish and reel them in to dozens of trumpet-shaped feeding polyps. Their mouths clamp on and expand grotesquely to cover the hapless prey.
This 6cm battleship is a comb jelly (Bolinopsis infundibulum), searching inner space for prey. Eight comb-like rows of hairs beat in waves, breaking sunlight into beautiful rainbow flashes. ‘Comb-drive’ gives it a curious random trundling motion, but is enough to ensure it occasionally bumps into another comb jelly, which it promptly engulfs with its huge mouth.
A tiny amphipod (Hyperia galba), less than 1cm long, peeps out from inside a moon jellyfish. Perfectly capable of swimming on its own – if the jellyfish gets stranded, for instance – it’s safe inside the jelly from the inner space jungle of toxic tentacles and feeding fish. The amphipod holds onto its pulsating taxi with feet like a climber’s spikes. It probably shares the jellyfish’s food, or eats its tiny larvae, which can be seen in the photo as tiny ovals and yellowish clusters below the amphipod. It can change colour rapidly from red-brown to pure translucent white, to match its background.
Small fish often hide among the tentacles of the more venomous larger jellyfish, such as this compass jellyfish (Chrysaora hysoscella). The little fish are either very clever at avoiding the stinging bits, or are immune to the toxin. One theory is that the fish are able to coat themselves with jellyfish mucus, so that the jellyfish doesn’t recognise them as fish, and therefore doesn’t trigger its sting. The fish gain protection from other predators, and in return may act as a lure to tempt larger fish into the jellyfish tentacles.