Catching mullet on camera
their larvae have shown that adults swim to the edge of the continental shelf to spawn. They remain at the surface, so it seems, but for some as yet unknown reason, the mullet will only spawn when there is a great depth of water beneath them, a few hundreds or thousands of metres.
“There’s only been one observation of actual spawning,” says George. “That was in the Gulf of Mexico, in December, many years ago.” Not much more is known about their mating habits. “Why they go so far offshore to do it, I don’t know,” he says.
One thing we do know is that mullet differ from other Atlantic species, the European and American eels, which migrate thousands of miles to meet up to mate in the Sargasso Sea. Mullet don’t go so far. In Florida especially, the continental shelf is relatively narrow, and the mullet can easily reach deeper water. And they congregate at various spots along the coast, not all in one place like the eels. “There’s not one ‘X’ on a map where they go,” George adds.
The school finds a good place – how they decide where that is, remains yet another mullet mystery – and the spawning begins. “There’s an orgy of a spawning event, where they all do it at the same time,” he explains. The females lay their eggs, up to 2 million in one go, and the males release sperm, with fertilisation taking place in the sea. The fertilised eggs are rich in oils and they float, which may explain the mullet’s choice of offshore spawning sites. George thinks the eggs may get swept away on surface currents that run along the coast of Florida, helping to disperse the next generation of mullet to new areas.
As for the adult mullet, once they’ve spawned, they return to shore. They’re not like salmon, which migrate in the opposite direction from sea to freshwater, where they spawn just the once and then die. Individual mullet will continue their spawning migrations for many years. “We don’t have tagging studies that have followed individuals out and back in,”
George says, meaning no one knows for sure what route mullet take or if they go back to the exact same places inshore where they were before.
Back on the beach, the bait balls gradually shrink as more mullet schools break away and head into deeper waters to spawn. Then, usually by Halloween, the dark slicks have dispersed and the mullet run is over. The mullet swim through those narrow inlets and go back to eating mud, occasionally leaping in the air and waiting for another year to pass when they’ll be ready to run the gauntlet of the coast and dash out to sea once more.
is a writer, marine biologist and broadcaster. Her books include Eye of the Shoal.
FIND OUT MORE
Watch a clip of migrating mullet from Seven Worlds, One Planet: bbc.co.uk/programmes/p07w41ks
Taking
over empty shells for protection is a neat trick. However, for hermit crabs, the quest for the perfect home is a lifelong preoccupation full of compromise. Swapping shells is a risky, yet necessary, procedure for a constantly growing crab, and finding a good fit is more of an art than an exact science. This hasn’t stopped researchers observing how the European hermit crab surveys a prospective shell. A crab will use its stalked compound eyes to gauge a shell’s size, weight and even colour, and it also has a good feel, using its legs and antennae, to estimate the shell’s volume, shape, condition and manoeuvrability, and whether it is already occupied. Gathering and processing all this information is a remarkable cognitive feat. Recently scientists discovered that if hermit crabs ingest microplastics, their decision-making is impaired, disrupting this essential survival behaviour.
The theory of natural selection predicts that individuals should compete for resources rather than share, but that co-operation exists when it is mutually beneficial. Spiders are usually solitary by nature but, remarkably, a few are known to co-operate. For example, group-living ‘social’ spiders, such as South American Anelosimus and African Stegodyphus, share prey and webs. This strategy works because individuals are
Do spiders ever share?
closely related to one another – helping others ensures their own genes survive. So-called ‘colonial’ spiders, such as tropical Parawixia and Mediterranean Cyrtophora, also share communal webs for prey capture and protection – but in this case, individuals are not closely related. As a result, some of these spiders turn out to be free-loaders who cheat on the others, confirming the expectation that competition wins over sharing.
Yes.
Plenty of animals have regional variations in their calls or songs. In Britain, differences have been recorded in the vocalisations of a variety of small birds, including chaffinches, coal and great tits, and yellowhammers, whose songs can have a different pitch and tone depending on where they live. ‘Cockney’ mallard ducks in London apparently have a harsher quack than those in Cornwall, perhaps an adaptation to being heard above the city hubbub. Farmers
Do animals have accents?
in south-west England have also reported a different ‘West Country’ moo among herds of cows. Elsewhere, in the forests of southern Asia, singing gibbons have highly differentiated regional accents, and Africa’s rock hyraxes (guinea-pig-like relatives of elephants) have localised dialects, too. In the underwater world, different populations of sperm whales, orcas and humpbacks are known to incorporate unique phrases, or codas, into their songs.
Having
a larval stage that lives a different existence from the adult has been a brilliant success. It’s an ancient strategy, thought to have originated nearly 400 million years ago, before the appearance of ‘modern’ insects, such as butterflies. Insects, though, are so fragile that the fossil record is patchy – larvae are the least likely to be preserved. Believed to be a caterpillar fossil, Metabolarva bella, from the mid-Carboniferous period, is the oldest immature insect we know of. But what it might have transformed into is anyone’s guess, and precisely when the first caterpillars evolved is also speculation. But we do know that insect larvae were crawling on Earth long before there were butterflies. Richard Jones
The caterpillar or the butterfly – which came first?