A shrimp found on the NW Rota-1 volcano is also known from the Loihi seamount located off the coast of Hawaii. Click image for larger view and image credit.
Biological Studies of the Mariana Arc
Verena Tunnicliffe
Canada Research Chair in Deep Oceans
Professor, University of Victoria, Canada
John Dower
Fisheries Oceanographer
University of Victoria, Canada
Kim Juniper
Biologist
University of Quebec, Montreal
Anna Metaxas
Assistant Professor, Department of Oceanography
Dalhousie University
Biological studies on the Mariana Arc during 2004 and 2005 expeditions reveal a multitude of unusual animals and phenomena. Research is focusing on some properties of the volcanic arc vent ecosystem and on the biology of several interesting species.
Biogeography
A major objective of the 2004 expedition was to discover what animals live at arc vents and how they relate to other hydrothermal systems. In 2004, 34 species were identified from the collections of which 14 are new to science. Most are currently under description. For example, several new species of snails were discovered from five volcanoes. Although hydrothermal vents are known to exist at 3600 meters depth (> 11,000 feet) at the Mariana Backarc spreading center, ~ 100+ kilometers (~50+ nautical miles) west of the Mariana Arc, few species occur in both places. Depth is a barrier over which the swimming larvae do not pass. Most species appear to occur only in the Izu-Bonin-Mariana Arc region. But there are surprises: a shrimp that was very abundant at NW Rota-1 and NW Eifuku is known at only one other location: off Hawai'i, over 6000 km (3250 nm) away, on another volcano. We will continue careful collections on SROF'06 to augment the distribution and species lists.
A limpet from NW Eifuku seamount gives a ride to two juveniles. The inset shows the two larvae that hatched from eggs on board ship. Click image for larger view and image credit.
Larval Dispersal
How do vent animals on seamounts get around? Plankton above vents at deep ocean ridges include i) dispersing larval stages of vent animals, and ii) a variety of planktonic invertebrates that may use suspended food in the hydrothermal plume. At the Mariana Arc, copepods were the most abundant near-bottom plankton above the vents. The greatest abundance and diversity of non-vent holoplankton was recorded above vent fields at E Diamante (330 meters depth / 1082 feet); interestingly, this seamount harbours lush photosynthetic communities at the summit (170 m / 558 ft). Larvae of vent invertebrates (e.g. snails, clams, worms) are present in low abundance mostly at heights < 5 m (16 ft) above vent fields.
Bottom samples from E Diamante and NW Eifuku held great abundances of snail juveniles and eggs. Several species lay egg cases on rocks around vents; veligers that hatch from these egg cases remain near the bottom, increasing the probability of their retention within the adult habitat. Such larvae with limited dispersal potential may enhance colonization on these seamounts where water tends to circulate for many days around the summits. Short-distance dispersal and localized colonization may explain the large variability observed among seamounts in the composition of adult assemblages.
On East Diamante seamount, a banded butterfly fish hovers over a white mat of bacterial filaments growing in venting fluids. Click image for larger view and image credit.
Mariana Arc Vent Food Webs
Organic matter produced by chemosynthetic microbes at hydrothermal vents creates a food source used by the vent fauna through the local food web. We cannot sit and watch what animals are eating, so researchers use chemical tracers in natural food. Food sources often have unique carbon isotope and lipid 'signatures' conserved in the tissues of animal consumers. The nitrogen isotope signature of animals provides information about their level in the food chain ("trophic level"). By combining information on food sources and trophic level, researchers can interpret vent food web structure, and how it is influenced by factors such as fluid chemistry and volcanic eruptions.
Analyses of NW Rota-1 samples, where eruptive activity was on-going, revealed a simple food web. The stable isotope analysis of two shrimp species indicates they co-exist by feeding on different food sources (bacteria versus worms) rather than competing for food. At East Diamante volcano - shallow enough to reach the photic zone - some animals benefit from food of both chemosynthetic and photosynthetic origin; vent animals receive considerable input from the photic zone. We did see several species of tropical fish from the upper ocean grazing on bacterial mats. We expect to resolve the relative input from these two food sources into the deep corals during SROF'06.
On Daikoku seamount, flatfish are very abundant on sediments that also harbour snails feeding on bacterial mats. Click image for larger view and image credit.
The Mariana Tonguefish
During the SROF 2004 cruise, dense aggregations of a small flatfish were seen on Kasuga-2 and Daikoku Seamounts - flatfish are not part of vent faunas elsewhere. The fish is a new species, currently under description (Tom Munroe, Smithsonian). On a second visit to the region with our Japanese colleagues, we collected specimens from Kasuga-2, Daikoku and Nikko Seamounts. The Nikko flatfish were about twice as large as the flatfish from either Kasuga-2 or Daikoku. Current molecular work (using the Barcode of Life Data System) will determine the relationship among these populations - and verify that they are the same species. We will undertake otolith analyses (tiny head bones that record daily growth) to understand growth rates and age structure. The observed size differences might reflect differences in food availability between the three seamounts, as Nikko Seamount appeared far more productive. However, the smaller fish may represent recent colonization events.
During SROF 2006, we will repeat collections to examine seasonal differences and we will search for new sites to support studies on population relationships over broad distances. Collection will also support stable isotope work to determine the food sources for these fish. Under the microscope, the sandy sediments from the fish habitat are full of filamentous, likely chemosynthetic, bacteria and tiny nematodes. Our current hypothesis is that the fish are probably feeding on both, and thus ultimately supported by chemosynthesis since the nematodes are also likely feeding on the bacterial filaments. Stable isotope and lipid analysis of samples from these "fish spas" are currently underway.
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