Polarization vision in terrestrial hermit crabs

Polarization vision is used by a wide range of animals for navigating, orienting, and detecting objects or areas of interest. Shallow marine and semi-terrestrial crustaceans are particularly well known for their abilities to detect predator-like or conspecific-like objects based on their polarization properties. On land, some terrestrial invertebrates use polarization vision for detecting suitable habitats, oviposition sites or conspecifics, but examples of threat detection in the polarization domain are less well known. To test whether this also applies to crustaceans that have evolved to occupy terrestrial habitats, we determined the sensitivity of two species of land and one species of marine hermit crab to predator-like visual stimuli varying in the degree of polarization. All three species showed an ability to detect these cues based on polarization contrasts alone. One terrestrial species, Coenobita rugosus, showed an increased sensitivity to objects with a higher degree of polarization than the background. This is the inverse of most animals studied to date, suggesting that the ecological drivers for polarization vision may be different in the terrestrial environment.

Enclosure Background Preferences Differ between Sexes and Color Morphs in the Gouldian Finch

Most wild animals camouflage well into their environment, providing protection from predators, whereas captive animals often contrast with their background. This can cause stress for the animal, which may perceive it as being exposed. Theory suggests that prey is more difficult to detect in front of complex backgrounds; hence, animals should prefer complex over simple backgrounds. We tested this in the polymorphic Gouldian finch by providing a complex background pattern in one half of the flight cage and a simple background pattern in the other half for 10 days (phase 1). Patterns were then swapped and presented for another week (phase 2). Groups of four birds consisting of either pure black-headed or red-headed or mixed head color (two black-headed and two red-headed) pairings were tested. Gouldian finches spent significantly more time in front of the simple background in phase 1 but not in phase 2. Specifically, females preferred the simple background in phase 1 significantly more than males. Moreover, red-headed birds consistently perched in front of the simple background, whereas black-headed birds used both backgrounds, particularly in phase 2. Results indicate that background preferences differ between sexes and morphs, which should be considered when designing backgrounds. Moreover, natural habitat preferences need consideration.

Context-dependent use of visual cues in the shell selection behaviour of the hermit crab Pagurus bernhardus

Animals avoid predator attack in different ways; some carry defensive structures to reduce predation, with the classic example being hermit crabs and their use of a mollusc shell as a portable refugium. During shell selection, various shell characteristics are investigated by the crab to determine their suitability. Here we consider the role of visual cues. Previous research suggests that some hermit crabs are more likely to initially choose a conspicuous shell but also to move to backgrounds against which they are less conspicuous, suggesting a short-term/long-term trade-off. Across experiments in which we manipulated shell and background colour, we show initially that Pagurus bernhardus prefer black shells over white but this preference was lost in the absence of visual cues. We then show that the strength of preference was dependent on background colour. We repeated this last experiment with red and yellow shells against red or yellow backgrounds to investigate whether this preference extended to chromatic hues. A preference for darker (red) shells was expressed, but preference alteration with background was not observed. P. bernhardus therefore discriminate between shells in terms of shell and background colour, and discrimination may be rooted in a preference for darker shaded shells.

Polarization contrasts and their effect on the gaze stabilization of crustaceans

Many animals go to great lengths to stabilize their eyes relative to the visual scene and do so to enhance the localization of moving objects and to functionally partition the visual system relative to the outside world. An important cue that is used to control these stabilization movements is contrast within the visual surround. Previous studies on insects, spiders and fish have shown that gaze stabilization is achromatic (‘colour blind’), meaning that chromatic contrast alone (in the absence of apparent intensity contrasts) does not contribute to gaze stabilization. Following the assumption that polarization vision is analogous in many ways to colour vision, the present study shows that five different crustacean species do not use the polarization of light alone for gaze stabilization, despite being able to use this modality for detecting predator-like objects. This work therefore suggests that the gaze stabilization in many crustaceans cannot be elicited by the polarization of light alone.

Summary: Five different crustacean species do not use the polarization of light alone for gaze stabilization, despite being able to use this modality for detecting predator-like objects.