Alvinocaridid shrimp are an emblematic faunal taxon at hydrothermal vents on the Mid-Atlantic Ridge. Unlike their physiological adaptations to cope with the fairly hostile physicochemical conditions in the hydrothermal environment, their sensory abilities have been poorly investigated. How the shrimp locate active vents and detect their habitat in the absence of sunlight is still enigmatic. They might use the hydrothermal fluid emissions as an orientation cue. Among the factors potentially exploited by the shrimp, the fluid chemicals and temperature are considered in the present study. Various approaches aim at getting insights into the sensory adaptations of vent shrimp, which could present particular chemo- and thermosensory abilities to detect their environment. The vent shrimp Mirocaris fortunata was used as the main model. The shallow-water, closely related species Palaemon elegans was used for comparison, to search for differences with the vent species that could eventually have an adaptive significance.
This work presents a detailed description of the olfactory system for the two species. Structural features were used as rough estimates of their olfactory abilities, including the number and dimensions of the olfactory aesthetasc sensilla located on the antennules, the number of the olfactory sensory neurons innervating the aesthetascs and the volume of the olfactory lobes in the brain. The olfactory system of M. fortunata is overall similar to the one of P. elegans, except that a dense bacterial coverage occurs on the antennal appendages of the vent species, which could have a functional role regarding their chemosensory abilities. At molecular level, the co-ionotropic receptor IR25a, involved in chemodetection, was identified in four vent species and in P. elegans. Differences in IR25a expression patterns within the chemosensory organs are discussed considering the lifestyle of each species. Preliminary results from a transcriptomic analysis are also presented, as it reveals in the vent species several chemo- and thermoreceptor candidates.
A new electroantennography method in water was developed to test the detection of hydrothermal fluid chemicals by the antennal appendages of the shrimp. Responses to sulfide, iron and manganese are discussed regarding the relevance of each chemical for short- or long-distance detection of the hydrothermal fluid. In addition, several attraction tests were conducted at atmospheric and in situ pressure to investigate the behavior of the vent and shallow-water species when exposed to a food odor, sulfide and warm temperatures (~20°C). The experiments with food and sulfide were not conclusive for the vent species. However, unlike P. elegans, M. fortunata exhibits a strong attraction behavior to warm temperature, which suggests an important role of thermodetection for the orientation of vent shrimp in their habitat.
Altogether, these results provide advances in the knowledge of the chemosensory biology of vent shrimp, and a substantial background for future researches on their chemo- and thermosensory abilities and sensory adaptations to the hydrothermal environment.
Key words: hydrothermal vents, Mirocaris fortunata, aesthetascs, chemodetection, thermodetection, sulfide, electroantennography, sensory biology