Teetering on the Brink

Life on a active volcano is anything but boring; food can be bountiful but the hazards are legion. NW Rota presents some unusual challenges for the animals that colonize the vents. The sulphur-rich system supports abundant chemosynthetic production by bacteria, often forming extensive mats around areas where hydrothermal fluids emerge. For a larva settling from the water, these sites must seem a great choice for a new home. Over the past years, we have not encountered many species at NW Rota but inhabitants are often abundant: shrimp, limpets, barnacles and crabs. Lately, however, the new homestead appears to have a few problems!

The red laser dots on this sulphur crust are 10 cm apart. The small specks are hundreds of juvenile shrimp recently recruited to the volcano.

NW Rota has undergone a major reorganization as the eruptive cone grew and a large piece of the summit collapsed downslope – taking with it prime habitat. We have not located many sites of vigorous venting remaining. Last year, limpets had spread across the summit and a barnacle (a new species) had established several colonies. However, most of those animals rode the collapse into the depths – we have found only one small refuge of barnacles (so far). The limpets are hanging on – lots of egg cases where there are a few adults left and a recent recruitment of small limpets. This animal is also a new species – so far we have not seen it on any other Mariana Arc volcano although an allied species inhabits the northern Arc.

Last year, we observed the white vent crab at every site on NW Rota. It is an adaptable animal that appears to graze on bacteria but will also grab shrimp and smaller individuals of its own species. We have yet to see more than 10 individuals this year. The most impressive of the NW Rota species is a shrimp that we have seen in large numbers every visit to the volcano. Opaepele loihi seems to be a volcano specialist – it is now known from four sites of recent/ongoing eruptions. It is well adapted to grazing on bacterial filaments around vents – both hydrothermal and volcanic. We were surprised to find huge numbers of this animal on the volcano – they are very small so must have settled quite recently.

A small refugium for barnacles. The pink scaleworm is a predator and the small white dots are limpet egg cases.

Survival is linked to how well these animals can maintain their populations in this highly unstable habitat. Mostly, this feat is accomplished through reproduction. The neritid limpets foster the first part of development of their young in egg sacs but larvae hatch into a swimming phase around the seamount. Part of our objective here is to search for the larvae of shrimp with plankton net hauls from the ship; we hope to find new recruits ready to settle onto this island of food – perhaps only to disappear in the next eruptive event.

Verena Tunnicliffe

Professor, University of Victoria

Director, VENUS Subsea Observatory