Previously unsuspected dietary habits of hydrothermal vent fauna:The bacterivorous shrimp Rimicaris hybisae can be carnivorous

Emma Versteegh1*, Cindy Van Dover2, Max Coleman1,3

*emma.versteegh@jpl.nasa.gov1NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena CA, USA; 2Duke Marine Lab, Duke University, Beaufort NC, USA; NASA Astrobiology Institute

Research questions

• What causes the previously observed wide range of δ13C values in R. hybisae tissues?• Are the different δ13C values related to differences between dense and sparse shrimp aggregations?• What can δ15N and δ34S values tell about ecological or physiological differences between dense and sparse shrimp?

HypothesesWith the aim to investigate diet & trophic shifts of dense and sparse R. Hybisae, we tested the following hypotheses:• Dense and sparse Rimicaris differ in diet: varying δ13C values show real differences in food sources between individuals.• They are metabolically different.

Preliminary conclusionsDense and sparse R. hybisae use different food sources. Dense shrimp eat bacteria only, while sparse shrimp appear to eat crustacea, and possibly other animals.

The switch between diets might be related to the molting cycle, as the shrimp temporarily lose their episymbionts when molting.

Ongoing and future workMore δ13C and δ15N analyses of gut contents are under way to complete the picture on isotopic composition of food.

NASA/JPL/Ted Stryk

Mid-Cayman Rise hydrothermal vents

In 2009 two hydrothermal vent fields were discovered at the ultra-slow spreading Mid-Cayman Rise. These include the world’s deepest, the Piccard Vent Field at 4960m, which vents fluids at extremely high temperatures (398°C). The nearby Von Damm vent field (2309m deep) is an off-axis vent field, located on the upper slopes of an oceanic core complex, with diffuse venting at up to 226°C.

Life in the dark:the food web

The Von Damm vent field supports a complex food web, which includes bactivorous shrimp and carnivorous anemones. There is probably some influx of photosynthetically produced carbon as well.

Rimicaris hybisaeThe shrimp species Rimicaris hybisae is abundant at both known MCR vent fields and shows a high degree of spatial variability in population structure and reproductive features. In previous work it has been considered bacterivorous.

Dense and sparseWe observed that shrimp tended to be either in dense aggregations on active chimneys, or more sparsely distributed, peripheral in ambient or near-ambient temperatures.

δ13C and δ15N valuesStable isotopes of carbon (δ13C values) and nitrogen (δ15N values) have been proven to be useful tools in determining trophic positions and disentangling food webs. Generally an animal is enriched by about 1‰ in 13C and about 3.4‰ in 15N relative to its diet.

Large variations in R. hybisae tissue δ13C values have so far been unexplained, and it has been argued that δ13C values are not a good food web tracer in hydrothermal vent ecosystems.

ResultsGut contents of all shrimp from dense aggregations at the Von Damm field consisted of white, amorphous material that resembled bacteria. Sparsely distributed peripheral shrimp had stomachs filled with fragments of crustacean exoskeleton (5 out of 13), a mixture of bacteria-like material and crustacean exoskeleton (3 out of 13), or bacteria-like material only (5 out of 13).

The range of δ13C values of sparse R. hybisae partially overlaps with, but tends to be lower than for dense individuals. δ15N values all fall in a narrow range and are slightly elevated in some sparse shrimp in comparison with dense individuals. δ34S values of sparse shrimp are lower by up to 5‰ than dense shrimp. Carnivorous Lebbeus virentova shrimp are isotopically more similar to dense than to sparse R. hybisae.

Tail tissue isotopic compositions reflect those of the gut contents. No enrichment was found between gut content and tail tissue for δ13C and δ34S values. With respect to δ15N values, tail tissue was enriched by 3.3‰; the expected amount for 1 trophic level.These results suggest dense and sparse R. hybisae use different food sources.

MethodsSamples were collected during the E/V Nautilus Expedition on August 26 2013 at the Von Damm vent field. Individual shrimp were sampled with the remotely operated vehicle Hercules from dense and sparse assemblages, separated by ~1m. R. hybisae were dissected and gut contents identified.

Samples were analyzed using a combined analysis for δ13C and δ15N values and a separate setup for δ34S values on a Thermo MAT 253 isotope ratio mass spectrometer, connected to a Costech ECS 4010 elemental analyzer.

Abstract

Dense and sparse assemblages of the shrimp species Rimicaris hybisae were sampled at the Von Damm hydrothermal vent field (Mid-Cayman Rise).Although previously considered bacterivorous, examination of gut contents show differences in diet between dense and sparse shrimp. This is corroborated by stable isotope analyses of tail tissues and gut contents. The switch in diet might or might not be a life-history trait related to the molting cycle. www.nautiluslive.org

Location of the Mid-Cayman Spreading Center, in the middle of the Cayman Trough, south of Cuba.(Jack Cook, WHOI)

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8Life in the dark: Europa?Jupiter’s moon Europa has a metallic core and a further rocky composition like the terrestrial planets. It is covered by a deep ice-covered ocean (together ~100km).Due to Jupiter’s tidal heating, there is potential hydrothermal activity. Oxygen can be produced by breaking down of water molecules under the influence of Jupiter’s radiation field. Thus, Europa could support a chemosynthetically-based ecosystem hosting species similar to, but not the same as Rimicaris hybisae.Several other moons in the Solar system, e.g. Enceladus, are thought to have subsurface oceans and might harbor similar life-supporting conditions.

The ocean on Europa could potentially drive hydrothermal circula-tion. Image coutesy of NASA (Europa Jupiter System Mission Report, 2010)

NASA/JPL/Ted Stryk

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