UV radiation evokes negative phototaxis and covering behavior in the sea urchin Strongylocentrotus droebachiensis

Abstract

Intertidal and subtidal Strongylocentrotus droebachiensis (Müller) often hide among rocks or cover themselves with debris, including macroalgae, mussel shells, and pebbles. Similar reactions in other species of sea urchins have been interpreted as a response to bright sunlight. This study examined the response of S. droebachiensis specifically to ultraviolet radiation (UVR). In laboratory studies using artificial irradiation, S. droebachiensis exposed to UVR (290 to 400 nm) and photosynthetically active radiation (PAR, 400 to 700 nm) sought shade and covered themselves significantly more frequently than those exposed only to PAR. In outdoor aquaria, individuals were exposed to ambient solar radiation that was filtered to create 4 treatments (dark, PAR, PAR + UVA, or PAR + UVA + UVB) and observed for 6 h as total solar irradiance changed with time of day. Sea urchins covered themselves with significantly more material when exposed to PAR + UVA + UVB than in all other treatments, and in response to total irradiance. The amount of covering by sea urchins exposed to PAR + UVA (320 to 400 nm) varied over the course of the day, but were typically less than those exposed to UVB (295 to 320 nm). These sea urchins covered themselves more than those exposed to PAR alone or held in the dark. Sea urchins exposed to PAR alone did not differ in the amount of covering from those held in the dark, regardless of time of day. The amount of covering correlated significantly with UVB and UVA irradiance independently, but not PAR irradiance. This study does not rule out that multiple cues may cause the covering response, but it demonstrates that S. droebachiensis seeks shelter and covers itself in response to UVR, primarily UVB wavelengths or a combination of UVA and UVB, presumably to avoid UV-induced damage. KEY WORDS: Sea urchins · Covering behavior · Ultraviolet radiationResale or republication not permitted without written consent of the publisher

Sea urchin tube feet are photosensory organs that express a rhabdomeric-like opsin and PAX6

“…While we have no direct evidence that this opsin is functional, it could be conferring an important capability to detect and transmit differences in light on varying time scales, from seconds and minutes used in the behavioural responses of echonoderms (e.g. [4]) to longer-term changes in irradiance occurring on the scale of months, which are important for processes such as seasonal gametogenesis [3]. Additional work is required to determine the various phototransduction pathways involving these opsins, any additional structures or tissues, where they are expressed, and the temporal scales over which they provide information to modify urchin behaviour and physiology.…”

“…The photoperiod control of gametogenesis requires photosensory capabilities on the surface of the sea urchin to detect and respond to the low irradiances of light in their underwater habitat, which is dominated by the green portion of the spectrum [3]. Sea urchins are clearly capable of sensing changes in the underwater light field of their environment and using this information to modulate both their behaviour [4][5][6] and physiology [3]. How sea urchins detect these changes has been unclear, because they lack image-forming eyes and light-sensitive eyespots.…”

“…Several anatomical features of the sea urchin that might serve as photoreceptors include the spines, pedicellariae and tube feet (i.e. podia), and sea urchins are known to respond to both the visible and ultraviolet components of solar radiation [1,4,6,7], suggesting that their photosensory capability is on the surface of the test because the transmission of visible and ultraviolet radiation through the sea urchin test is less than 1 per cent of the available ambient underwater irradiance [3]. Additionally, recent work on the calcitic microlenses found in light-sensitive brittlestars [8] suggests that the ossicles within their radially arranged arms and underlying tissues may be involved in photoreception, and sea urchins also have ossicles in the distal portion of their tube feet [9].…”

Spatial vision in the purple sea urchinStrongylocentrotus purpuratus(Echinoidea)

“…Light-mediated behaviors are ubiquitous in this phylum, including simple phototaxis, covering reactions, UV avoidance, homing, polarization sensitivity, color changes, shelter seeking, diurnal migrations and movement towards small dark objects (Millot, 1954;Millot, 1955;Thornton, 1956;Yoshida, 1966;Millot and Yoshida, 1960;Scheibling, 1980;Hendler, 1984;Johnsen, 1994;Johnsen and Kier, 1999;Adams, 2001;Blevins and Johnsen, 2004). However, the underlying architecture and abilities of this system have remained mysterious.…”

Covering behaviour of echinoids in an Arctic fjord

“…Others have ascribed masking in shallow water echinoids to a response to light incidence or UV radiation (e.g. Adams 2001;Verling et al 2002). Further studies have argued that covers are used due to multiple factors (Dumont et al 2007), including protection from desiccation or as ballast in turbulent waters.…”