Adaptations to Heat avoidance

The original studies of Rimicaris exoculata concluded that this species was completely without eyes (Van Dover et al. 1989). It can be seen in images that the eyestalks are absent but instead, curiously large organs are present on the dorsal surface beneath a transparent carapace (shell) (Fig. 7).

Photoreceptors of Rimicaris exoculata
Fig. 7 Rimicaris exoculata (a) highlighting the position (arrow) where eye stalks are on species of the same genus and a dorsal view (b) of the two large visual organs (dotted area). Credit: Van Dover et al. (1989).

The chemical rich fluid emanating from the vents led to the proposition that Rimicaris exoculata navigated itself around the hydrothermal vent by responding to chemical cues (Schmidt,et al., 2008), however, many studies have postulated that the large organs on the dorsal surface are highly adapted photoreceptors that are able to detect low levels of light (Van Dover et al. 1989; White et al., 2002; Schmidt,et al., 2008). The ‘eyes’ or photoreceptors are not fully understood but high levels of the receptor pigment, rhodopsin, was identified (Pelli and Chamberlain 1989). This pigment has been suggested to allow the shrimp to detect the faint light radiated from the superheated hydrothermal fluid, known as black body radiation (Pelli and Chamberlain 1989; Chamberlain, 2000). This light source is not visible to humans, not even to the well adapted eyes of the submersible operators (Pelli and Chamberlain 1989). This is because the wavelength of the light emitted from the hot vent fluid is just below the visibility threshold for humans (Pelli and Chamberlain 1989). Light has been observed at vent sites with the aid of technology. Spectral imaging cameras have captured pictures of the illuminated vents from sites on the MAR (Fig. 8) (White et al., 2002).

Spectral imaging camera pictures of vent fluid
Fig. 8. Spectral imaging camera pictures of vent fluid from (a) Snake Pit and (b) Lucky Strike. Each tile was taken with different filter wavelengths. Credit: White et al. (2002).

The capability of the dorsal eyes of R. exoculata to detect the dim light given off by vent fluid has been calculated and it has been suggested that the shrimp will successfully detect the superheated fluid 99.9% of the time (Pelli and Chamberlain 1989). The sensitivity of the dorsal ‘eyes’ has also been indicated by their susceptibility to damage under the powerful lights of the investigating submersibles (Schmidt,et al., 2008). This may be a reason why physiological responses of the shrimp to light have not been recorded (Chamberlain, 2000). The dorsal ‘eye’ is largely accepted as being an efficient organ at detecting the dim vent fluid light (Nye et al., 2011), although others have suggested that the shrimps ability to detect chemical cues deems the need for visual detection unnecessary (White et al., 2002). However, without the support of behavioural studies of the responses of shrimp to vent fluid light, calculations and postulations alone cannot confirm the shrimp’s ability to see (Pelli and Chamberlain 1989; White et al., 2002).

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