Just imagine a world where familiar minibeasts were not so small; where you could really appreciate their marvellous micro-mechanics and delicious details without needing a magnifying lens.
You might think that a woodlouse the size of a cat is in the realm of Honey, I Shrunk the Kids science fiction, but it does in fact exist – not under a flowerpot in your garden, but in the cold reaches of the world’s oceans.
The giant isopod (Bathynomus giganteus) isn’t exactly a woodlouse, but there are many parallels. They both have a hard exoskeleton, 14 walking legs, and gills, and share a similar lifestyle, which is mostly a relatively sedate and simple affair.
The largest of about 20 species in the genus Bathynomus, the giant isopod lives at depths of around 2km beneath the ocean’s surface where it’s dark and cold. Here, it trundles around the ‘abyssal plain’ – vast deserts of soft silt – along with a collection of other deep-sea specialists, and makes a living in much the same way as its more familiar terrestrial cousins: by cleaning up.
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What do giant isopods eat?
Investigations of the gut contents of those unfortunate enough to have been dragged from the depths, along with footage from underwater remotely operated vehicles (ROVs), tell us that giant isopods are primarily ‘detritivores’.
It mostly feeds on dead animal material that has fallen from above and has been recorded scavenging on anything from whales to cephalopods. That said, it also probably actively predates anything it can overpower, such as worms and molluscs. There is even surprising footage of them latching onto the heads of small sharks!
How do giant isopods find food?
The giant isopod finds its food in the dark with the help of highly sensitive antennae, in much the same way that terrestrial woodlice locate detritus under a log or stone (where it is equally dingy). It has two pairs: a long pair that ‘feels’ as they constantly sweep the substrate in front of them, and a short pair that are held aloft detecting tell-tale chemical ‘tastes’ in the water.
How big is a giant isopod?
With a common name comprising the word ‘giant’, it comes as no surprise that this isopod can grow to a surprisingly substantial size. The record is in the region of 50cm long, with unsubstantiated claims of individuals even bigger.
Why are giant isopods so big?
Giant isopods exhibit abyssal or deep-sea gigantism: the tendency of deep-dwelling animals to be much larger than their shallow-water counterparts, from sea spiders to squid. This phenomenon isn’t well understood, mainly because it is difficult to carry out studies at these depths, but it is thought that being big confers several advantages.
One theory suggests that abyssal gigantism is due to the more significant amounts of dissolved oxygen available to animals at depth. In contrast, the size of terrestrial invertebrates is thought to be limited by the lower atmospheric oxygen levels, combined with their limited ability to absorb oxygen into their bodies as they don’t possess lungs. That’s why we don’t have beetles the size of basketballs.
The other theory is that the abyssal plains are vast, and animals that live here must be able to survive for long periods without encountering food (one giant isopod kept in an aquarium went four years without eating). A large body means you can effectively ‘cash in’ – stuff yourself when the opportunity arises and store those resources. When individuals do eat, they often consume so much that they can’t move, which leads to another possible explanation for deep-sea giants: the bigger you are, the fewer predators will be able to tackle you.
Main image: Getty
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