Sunday, 2 January 2011

Not just for Christmas

For the last few months we have had one of the most unusual invertebrates I have seen on display anywhere on view in Bug World, a group of Christmas Island Blue Crabs, Discoplax hirtipes. Related to the more famous (and slightly larger) Red Crab, Geocarcoidea natalis, it is one of nearly 20 species of terrestrial crab (plus 160 marine species) found on or around Christmas Island.

Although in the UK the only terrestrial crustaceans we have are the various woodlice, several groups of crustaceans are at least capable of surviving on land, at least for short periods. Shore crabs can move across rocks to get to a new pool, and freshwater crayfish can do the same. Land crabs have taken this one stage further, and spend their entire adult lives on land, usually spending much of the time in burrows which they leave to feed, and then return to later.

Land crabs come in all sizes, but our crabs are adult and have a carapace about 10cm across, with a leg span of nearly 20cm, with formidable claws. D.hirtipes occurs on other islands than Christmas Island, but elsewhere the usual colour scheme is a purple shell with red legs, whereas the Christmas Island form is usually a pale china blue for both carapace and legs, Whether this means that local conditions or diet are the cause of the colour difference, or whether the Christmas Island Blue crab is a distinct species, is one of the many things that are not known.

With such a large number of species in the same place, there must be some way the different species partition the habitat, but how this works has not been studied to any great extent. D.hirtipes is found near fresh water seeps in rainforest, but other species seem to range more widely. Some species are diurnal, including the Blue Crab, others may be active at any time or only at night.

Diet is another possible variable, but crabs as a group seem to be omnivorous feeders, taking whatever is easiest. This makes them easy to feed, but there is one wrinkle – at least some species of terrestrial hermit crab ensure they eat a balanced diet by actively avoiding starting to feed on food that smells the same as their last meal, so it is vital to ring the changes in captivity. In the wild they are important predators of germinating seedlings, and have as a result a great influence on seedling recruitment of forest trees. They will also east slow moving live prey, such as the widely introduced African Giant Snail Achatina spp, and where land crabs occur with ground nesting birds they will prey on eggs and chicks as well (the extinct Dodo of Mauritius had a notorious bite, partly to help protect its nest against the swarming land crabs, though they may have eaten them as well).

Although the adults live on land (and in fact drown if submersed), most species of land crab produce marine larvae. These float in the current as they grow and eventually transform into recognisable miniature crabs. If they are lucky (and most years they are not), they are near land when this happens and they can emerge on land and make the perilous journey into the forest to find a place they can dig their own burrow, avoiding desiccation and predators (including larger crabs of their own and other species) on the way.

Unfortunately, they now have a new and deadly predator to avoid, the Yellow Crazy Ant, Anoplolepis gracilipes, which arrived on the island, probably from Malaysia, about 80 years ago, and in the last decade or so has suddenly begun to expand its range, forming supercolonies containing millions of ants. On every island they have reached, the ants have proved deadly to the native fauna, as they have a vicious formic acid sting and can overpower most small animals. Not only crabs are affected – the Christmas Island Pipistrelle (a small bat) is now on the edge of extinction, probably as a result of being attacked in the tree roosts it uses during the day. The supercolonies can also expand very rapidly, with up to 3m per day being recorded.

Clearly it is vital they be removed, but how to do this is not at all clear. The best bet in the long run may be some form of biological control, but ensuring this does not do more harm than good is a very time consuming process, and time is running out. In 2009 the Australian Government National Parks Department, which administers Christmas Island, used experimental aerial drops of bait poisoned with Fipronil at very low concentrations targeted on the colonies, and this resulted in a 99% reduction in colony size, which is a big step in the right direction.

To get back to our exhibit, how did we obtain them? In fact they are ex research animals from Bristol University, part of a study on the endocrinology of land crabs. For those interested, some of the scientific papers involved can be found here: So far at least, they are doing reasonably well in their enclosure, which is a 1.5m by 1.5m floor area, with a deep layer of coir compost to enable them to burrow, and a shallow tray of fresh water they can submerge in to keep their gills moist so they can maintain the correct humidity in their burrows. We have several in the same enclosure, and they seem to tolerate each other well.

Probably because of their smaller size, the only terrestrial crustaceans most people will have seen in other zoos, and indeed the pet trade, is one of the several species of land hermit crab Coenobita. The hermit crabs are a quite distinct group from the true crabs, but they have invaded land independently. Although widely kept, their mortality rate is far too high for comfort, with most individuals living only one or two years in captivity. How long the lifespan is in the wild is not known – they are hard to age – but given that many marine species (lobsters for example) can probably live to be at least 80, a short captive life is a sure sign of poorly understood husbandry requirements. Mortality is usually associated with problems during molting, which may be due to inadequate humidity or temperature during the molt or perhaps inadequate diet in the run up to the process. Given that in most cases we are not certain what the correct diet even should be, especially which are the key nutrients, much work still needs to be done. I am glad to be able to say that we have managed to get some of our Blue Crabs through the molt successfully already.

That concludes the first post of 2011. For more on Christmas Island and its environment, see the Australian Government website here:

Happy New Year to you all. Next week: Mantids!

No comments:

Post a Comment