SMSG Blog

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.

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.

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.

Contributed by V. Laptikhovsky

Nothing vulgar about Octopus vulgaris!

The common octopus (Octopus vulgaris) received its unprepossessing name because it is supposedly common everywhere. It occurs in every ocean where the water is warm enough for exploration by wetsuited divers. The vulgar octopus hates cold waters and established itself worldwide between latitudes 50 of both North and South. However, the well established point of view that this species is a cosmopolitan citizen began to be shaken because, one by one, different populations were found to be different species.

The common octopus from Ascension Island was already supposed to be a distinct creature, and was once upon a time described as Octopus occidentalis. However, later on this name was considered to be a junior synonym of Octopus vulgaris though it is still likely to be a new species. The Ascension octopus could be thought of as underscribed … or more precisely, under-described.

Virtually nothing is known about the biology of this octopus, and because of this when Dr Jude Brown discovered a den during one of her dives surveys, we decided to have a closer look at the behaviour of this animal, and to include repeated return visits to the den when possible. Whenever we arrived at its shelter at each day we would find the creature sitting inside and watching us with its wise and wrinkled eyes of an elderly gentleman.

The den itself consisted of a few random volcanic pebbles scattered around and was a perfect disguise for a predatory ambusher. Leftovers of the day’s food betrayed the fact that this soft bodied animal used to leave its den at night, invisible to predators. With many hungry fish lurking around any camouflage would be useful.

During the first week of observation its daily prey consisted of one to three bivalve molluscs Americardia media, sometimes spiced by some meaty cowrie  Luria lurida. No fish, no crustaceans. Our octopus appears to have a very particular diet but we shall see whether it will change its food preferences during the forthcoming two weeks.

- Post supplied by Dr Vladimir Laptikhovsky.

Day Two

Most of the team have now arrived on Ascension and are getting straight into work. Yesterday (Monday 20th August) new arrivals were somewhat groggily taking their first dives just hours after touching down on Wideawake Airfield. First order of business was trialling the sampling methodology for transects along the seabed during which species counts would taken.

These early dives were done in English Bay, a sheltered beach with easy access and relatively calm waters so everyone could check out their gear and make sure all was in good working order. The weather was overcast with passing showers, but occasionally the sun broke through in full tropical strength.

Under the direction of Dr Simon Morley some team members also began to set up the lab in the offices of the Ascension Island Conservation Dept. complete with collapsible aquaria for climate change experiments. We still need to address the issue of transporting fresh seawater to the office everyday to keep the experiment running over the 3 weeks. Today, Simon and Dr Wetjens Dimmlich embarked on a scavenger hunt around the island following any rumour of containers which might hold water. Eventually, after scouring the local rubbish tip unsuccessfully they had to give up and now need to work on a new plan for getting water to the fish tanks.

This morning’s dives were on the wreck of the Derby, a steel hulled steam trawler which was used to transport guano from Boatswain Bird Island.  She was moored in English Bay but sank together with other small vessels during heavy rollers in January 1929. She lies off a reef in about 9m of water and involved more transect work and also photography of marine life near the wreck. This proved challenging in the swell, and was made more difficult by the countless urchins found in almost every crevice so any handholds or potential spots to kneel had to be carefully considered before committing yourself to it. In the meantime the subject generally decided not to wait around and had swum away. It’s likely that capturing images of the fish life is going to prove quite a lot more frustrating than anticipated.

While other team members were either diving or driving around Ascension in futile quests for water containers, Dr’s Pieter van West and Alexander Arkhipkin enjoyed a very memorable tour by Jolene and Natasha of Ascension Island Conservation. Shelly Beach is a location accessible only by 4wd vehicle and a half hour hike through a lava field. This used to be the site of a large sooty tern colony numbering in the hundreds of thousands of birds, but this was wiped out by feral cats. Rock pools are found here, separated from the sea by 100m of lava platform but replenished with seawater through underground fissures. These pools appear to contain ecosystems probably found nowhere else. Of particular interest are two unique species of shrimp (Procaris ascensionis and Typhlatya rogersi) living in these interconnected network of rock pools.

It may be that the shrimp are the most vulnerable species on the planet, found only in these pools and only on Ascension Island and are completely protected to the extent that we are only permitted to look but not touch. In addition to the shrimp Peter noticed other distinct species of algal and invertebrate life which may be worth further investigation. Unfortunately this visit to the rock pools was only a short one as the main object of the excursion was to collect oysters required for another study. A return visit to these pools will no doubt occur later in the trip with the underwater cameras to collect a full record of the species existing there.