Internal and external modifications to the shrimp’s morphology are the key to the success of their highly specialized mode of life (Gebruk et al., 1993; Petersen et al., 2010). They have developed an expanded gill chamber and modified mouthparts to house the chemosynthetic epibionts that they use as their main supply of nutrition (Hugler et al., 2011). The cells forming these modified body parts have expanded via hypertrophy to allow them to become atypically enlarged (Gebruk et al., 1993). Both the inside wall of the shrimp’s enlarged gill chamber, known as the branchiostegites, and the specially adapted parts of the maxillipeds (the scaphognathites and expodites) that form part of the mouth, are covered in a dense matting of setae (Fig. 4; Fig. 5) (Gebruk et al., 1993; Petersen et al., 2010).

Internal morphology of the gill chamber showing modified mouth parts.
Fig. 4. Internal morphology of Rimicaris exoculata. A. Mouthparts within the gill chamber, (Sc) scaphognathite and (Ex) expedite, scale bar represents 5 mm. B. Removed scaphognathite, scale bar represents 5 mm. C. Fuzzy white bacteria can be seen on the close up of scaphognathite setae. D. Scanning electron image of filamentous bacteria. Inset: wide filament (white arrow) and a narrow filament (yellow arrow). Scale bar represent 10 µm. Credit: Petersen et al. (2010).

These morphological adaptions combine to drastically increase the surface area to which the filamentous bacteria symbionts can attach. These modifications are exceptional among this family of shrimp (Bresiliidae) (Gebruk et al., 1993). Rimicaris exoculata is now listed as an extreme deep-sea model organism, for its symbiosis with the chemosynthetic epibionts is unique among the crustaceans (Ponsard et al., 2012).

Setae covered by epibiotic filamentous bacteria.
Fig. 5. Setae covered by epibiotic filamentous bacteria. Scale bar represents 100 µm. Credit: Gebruk et al. (1993)

Deep sea shrimp aggregate in great numbers and move in and out of the turbulent black smoker plumes (Gebruk et al., 1993). Crystals of sulphide that are present in the vent fluid settle on the dense mats of bacteria covered setae. Shrimp have also been seen clinging to the walls of vent chimneys where turbulence is reduced. In these areas shrimp use their enlarged antennae to create a flow of water over the bacteria associated with the cavities in the shrimp’s carapace (Gebruk et al., 1993). The antennae have adapted so that vertical beating creates this process of ventilation. Rimicaris exoculata are able to graze on the bacteria that they harbour, the bacteria can be scraped from the surface of the setae with its comb like modified mouth parts (Fig. 6) and are then ingested (Gebruk et al., 1993). Some studies have suggested that R. exoculata also possess a community of bacteria within the gut and that they may graze on other free living microbes associated with the hydrothermal vents to supplement their diet (Hugler et al., 2011). Although the epibiotic bacteria are considered a food source for R. exoculata, the exact method they adopt to gain energy from this bacterial source has been unclear and a subject of much debate (Gebruk et al., 1993; Hugler et al., 2011; Ponsard et al., 2012). Within the last year progress has been made towards understanding the feeding mechanisms of this species (Petersen, 2013; Ponsard et al., 2013; Jan et al., 2014).

Setal brush and brushing action of the modified mouthparts.
Fig. 6. Setal brush and brushing action of the modified mouthparts (maxillipeds). Arrows show the range of movement. The position of the mouth is indicated by the ‘m’. Credit: Van Dover et al. (1988).


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