Whale Sharks, Land Crabs, and Crazy Ants: A Trophic Link

First, a small aside: the regional finals of the Siemens Competition in Math, Science, and Technology went well. Though the judges ultimately opted for a different individual project (James Meixong’s Inhibition of Bax/Bak activation by mitochondrial fusion: a novel mechanism to block programmed cell death), the private Q&A, poster session, and PowerPoint went very smoothly. My presentation, I gather, was eye-opening for many audience members, and it was great to have gone this far.

Shortly before I left for Georgia, I happened on an article by the BBC, Shark-cam captures ocean motion (Nov. 17). A team of researchers headed by Dr. Mike Meekan (Australian Institute of Marine Science), the BBC reports, struck “scientific gold” by collecting fecal material from a whale shark (Rhincodon typus), the largest fish in the sea. This act, what’s more, was caught on film and will see inclusion in a BBC Natural World documentary, Whale Shark.  Unimpressed? Consider this:

Short of splitting open multi-tonne fish to sort through their stomach contents, biologists face considerable challenges in determining what, exactly, these pelagic filter-feeders eat. The idea of examining R. typus fecal samples is not new, but digestive degradation often precludes morphological identification of prey species (Wilson & Newbound, 2001). More recently, researchers have used genetic analysis as a diagnostic tool for identifying prey species from feces, isolating a region of the nuclear large subunit (LSU) ribosomal RNA gene present in krill but not in other malacostracan crustaceans. This technique was pioneered with Adelie penguins, Pygoscelis adeliae, and pygmy blue whales Balaenoptera musculus brevicauda (Jarman et al., 2002), and was subsequently extended to whale sharks (Jarman & Wilson, 2004). Meekam’s team — though probably with a different gene region — seems to have done much the same.

So what’s so novel about Meekam’s work, you ask? Well, as Colman (1997) relates, migratory movement of whale sharks seems to correspond to peaks in prey density (like plankton blooms or mass-spawning events in fish). One such trackway brings significant numbers of whale sharks to the Indian Ocean’s Christmas Island during December and January — an aggregation that tellingly coincides with the seaward run of the world’s most famous land crab, Gecarcoidea natalis.  Meekam’s genetic study confirmed this hunch — the most massive of fish glut themselves on the young of these nearly-terrestrial crustaceans.

Commonly known as the Christmas Island red crab, G. natalis is the most abundant terrestrial Brachyuran on Christmas Island — population estimates range as high as 100 million individuals — and the dominant consumer of the forest floor, feeding on fallen leaves, seeds, and fruit as well as small animals (Green, 1997).  On this hearty diet, these burrow-dwelling, diurnal crabs can reach a carapace length of 120 mm and a weight of half a kilogram.  Impressive though these stats might be, it is their mode of reproduction that is the red crab’s claim to fame.  In a truly awe-inspiring series of overland migrations, G. natalensis march from the interior to the coast to mate and then release their eggs.  These eggs, as many as 100,000 per individual, are retained beneath the female’s abdomen like a fistful of caviar until they’re brought to the ocean’s edge “at night on the turn of the high tide between the last quarter of the moon and the new moon, sometimes from cliff faces” (Hicks, 1997). The larvae hatch on contact with seawater, spending 3 to 4 weeks as plankton before taking on a terrestrial body form and scuttling up from the beaches to the interior plateau.

I’ll let the photos speak for themselves:

But all is not well on Christmas Island.  Populations of the yellow crazy ant (Anoplolepis gracilipes), an invasive species likely introduced with timber, exploded in the 1990s; today, supercolonies occupy over 30% of Christmas Island’s rainforest (Abbott, 2006).  These voracious creatures have found in Christmas Island red crabs a fine solution for their caloric demand, effectively eliminating G. natalis from occupied areas and taking up residence in their burrows.  It is likely that several dozen millions of red crab have already perished, and as this keystone species continues to decline, massive disjunctions in seedling recruitment and leaf-litter processing are likely to ensue (O’Dowd et al., 2003).  In brief: invasional meltdown.

Above, yellow crazy ants swarm a crab (albeit what looks like a Sesarmid).

Could the ravages of A. gracilipes be felt as far away as whale shark bellies?  Perhaps so.

Resources:

Abbott, K. L. (2006). Spatial dynamics of supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on

Christmas Island, Indian Ocean. Diversity & Distributions, 12(1), 101-110.

Colman, J. G. (1997). A review of the biology and ecology of the whale shark. Journal of Fish Biology, 51(6), 1219-1234.

Green, P. T. (1997). Red Crabs in Rain Forest on Christmas Island, Indian Ocean: Activity Patterns, Density and Biomass. Journal of Tropical Ecology, 13(1), 17-38.

Hicks, J. (1985). The Breeding Behaviour and Migrations of the Terrestrial Crab Gecarcoidea natalis (Decapoda: Brachyura). Aust. J. Zool., 33(2), 127-142.

Jarman, S. N., Gales, N. J., Tierney, M., Gill, P. C., & Elliott, N. G. (2002). A DNA-based method for identification of krill species and its application to analysing the diet of marine vertebrate predators. Molecular Ecology, 11(12), 2679-2690.

Jarman, S. N., & Wilson, S. G. (2004). DNA-based species identification of krill consumed by whale sharks. Journal of Fish Biology, 65(2), 586-591.

O’Dowd, D. J., Green, P. T., & Lake, P. S. (2003). Invasional ‘meltdown’ on an oceanic island. Ecology Letters, 6(9),

812-817.

Wilson S., & Newbound D. R. (2001) Two whale shark faecal samples from Ningaloo Reef, Western Australia. Bulletin of Marine Science, 68, 361-362.