Expedition blogs and news from the Shallow Marine Surveys Group

Studying Sally lightfoot

[caption id="attachment_409" align="aligncenter" width="584"] The Ascension Island crab, Grapsus adscensionis.
Image: W Dimmlich

Nicknamed “Sally lightfoot” by sailors, there are two crabs of the genus Grapsus which have chosen the tropical eastern parts of oceans as their habitat. One of them, G. grapsus, settled along the Pacific coasts of America and made the long jump to Galapagos where the local population impressed Charles Darwin with their agility and capacity to avoid sailor hands. Apart from their impressive acceleration from a standing start, another notable characteristic is that they perform a valuable service to marine iguanas by removing ticks from them, similar to the role played by cleaner shrimps with fish.

[caption id="attachment_416" align="aligncenter" width="584"] The crabs can be seen clustering on the lava platforms in all sizes.
Image: W Dimmlich

The second species, G. adscensionis, inhabits the east Atlantic and in the historical past also made a long jump - to Ascension and St. Helena islands. The crab inhabits the splash zone though at night they ramble the sand dunes of the island as some ten-legged hosts. Nothing is known about their biology on this spearhead of their westward expansion of the species range (do they target South America as much as Spanish conquistadors?). In respect to feeding habits, for example they were seen to be feeding on a scientist's sandwich as well as witnessed kidnapping a baby green turtle. It is not exactly what Grapsus do with reptiles on Galapagos… However, neither sandwiches or turtle hatchlings could hardly be considered as a staple throughout the year.

[caption id="attachment_408" align="aligncenter" width="584"] Steve Brown and Vlad Laptikhovksy attempting to catch the very speedy subjects of the investigation.
Image: W Dimmlich

Steve Brown challenged the project to study the reproductive biology of the species, particularly size at maturation and fecundity on Ascension Island. The idea is to compare this information with available data on continental populations of the species, as well as with those on G. grapsus from both the American continent and Galapagos (which are pretty much a "Pacific Ascension") and to reveal how this harsh islands’ environment impacted reproductive features. Because of our intensive diving schedule, the sampling usually occurs every day while tanks are being filled. It takes about two minutes for a crab to be caught, measured all across, sexed, cursed for its strong claws and scratchy pointed legs and released back to the shoreline. Some females however are deprived of their broods that would be further investigated in the lab.

[caption id="attachment_414" align="aligncenter" width="584"] Measuring carapace dimensions of G. adscensionis, while keeping careful hold of the claws!
Image: W Dimmlich

Contributed by V. Laptikhovsky

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