A closer look at the epibiotic communities of a hydrothermal shrimp

A review of the paper: Guri, M., Durand, L., Cueff-Gauchard, V., Zbinden, M., Crassous, P., Shillito, B., Cambon-Bonavita, M., (2012), Acquisition of epibiotic bacteria along the life cycle of the hydrothermal shrimp Rimicaris exoculata, The ISME Journal, 6, 597-609

The hydrothermal shrimp, Rimicaris exoculata, is part of the megafauna at several mid-atlantic ridge (MAR) vent sites, and forms dense aggregations around the chimney walls. It is also known to have a symbiosis with a large diversity of chemosynthetic bacteria, which can be found in the gill chamber and the mouthparts. Compared with shore shrimps, Rimicaris exoculata has short moult cycle of only ten days, and a new epibiotic community is formed within two days of the shed. Little about the life cycle of this shrimp is known, except for the fact that they lay eggs and planktotrophic larvae are assumed but not known. Combined with these factors, it is also unknown when the epibionts colonise the larvae. The aim of this study was to analyse the diversity and the development of the epibionts in the gill chamber at different stages in the shrimp development and at different moult stages. The authors ask the question of 1) When is the first acquisition of epibionts? 2) Does the community differ between the early life stages and adult, or between moult stages?

They collected several specimens at 3 vent s along the MAR using a suction sampler of an ROV, with samples of eggs, hatched eggs, juveniles and adults at different moulting stages. Living individuals were immediately dissected and eggs frozen. The samples were sorted according to moulting stage and several techniques were used for analysis: 16S rRNA clone libraries, transmission and scanning electron microscopy and FISH.

Results from looking at the adult gill chamber showed that the development of the epibiots seemed to follow a chronological order along the moult cycle. The first to colonise was a mat of rod shaped bacteria, and these were soon replaced with long filamentous bacteria. FISH showed the predominance of Episilonproteobacteria, which was congruent with molecular studies. There was also a presence of Gammaproteobacteria, methanotrophic bacteria with a specific location, which showed that it was not opportunistic. Results from the eggs showed a presence of a mat of rod-shaped bacteria embedded in the layer of mucus surrounding the eggs but none were found within the egg. The larvae only contained single celled bacteria (no mats).

This was the first time that hatched eggs containing larvae were collected, which provides more information on the life cycles. However, a lack of year round data makes it difficult to conclude the full life cycle. The results showed a clear difference between the communities in the early stages (egg and hatched egg) and the latter stages. This shows that there is indeed a complex, stable episymbiosis occurring, a primitive form of an endosymbiosis.