Something Old, Something New

When we first met the shrimp of NW Rota in 2004 we wondered what makes them so special that they can thrive in such a volatile environment. Last year both species, Opaepele loihi and an undescribed Alvinocaris species, had expanded their distributions and seemed to be doing quite well for themselves. Hydrothermal vents with bacterial mats were plentiful and there were plenty of shrimp around to take advantage of the favourable conditions. There , were a large number of adults of both species and where one species was found, the other was almost surely there. Alas all good things must come to an end, or in the case of NW Rota a new beginning.

Figure 1- A steep cliff face covered with Opaepele loihi shrimp.

The major landslide that occurred at Rota has given us some excellent clues as to how a major disturbance affects a community dominated by mobile vent organisms and how new habitats on an erupting volcano are colonized by shrimp. There are few remaining habitats from last year and the newly disturbed parts of the volcano have already been colonized by vast numbers of new recruits. The extent of this colonization in less than a year is astounding with thousands of juvenile shrimp in areas newly exposed by the slide.
Even more surprising is that this new crop of shrimp appears to contain just a single shrimp species, Opaepele loihi. This “volcano specialist” is well adapted to living on erupting submarine volcanoes, found at 4 thus far, but we still know nothing about how it survives the harsh conditions or how it finds and colonizes new habitats. By comparing the new populations at NW Rota with populations from previous years and by collecting shrimp larvae in plankton hauls around the seamount we hope to uncover how these shrimp recruit to NW Rota. The Alvinocaris shrimp are notably absent in the slide-affected region yet they remain a part of the vent community in unaffected habitats.

Figure 2- An adult Alvinocaris shrimp stands proud among the smaller “loihi” shrimp.

Both species of shrimp feed by grazing on bacterial mat throughout some or all of their post-larval life. The “loihi” shrimp have specialized claws like garden shears for clipping bacterial mat which allows them to do so very efficiently while the Alvinocaris shrimp also graze mat but transition into scavengers and predators as they get larger. One of our goals this year is to compare bacterial mat samples from areas with and without shrimp. By finding out what they like to eat and comparing this with where they are and the fluid chemistry at different vents we can better understand what makes a suitable habitat for these amazing shrimp.

John Sherrin
University of Victoria

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