. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. PHOTOBIOLOGY OF ANTHOPLEURA 693. Figure 6. Localized retraction of tentacles and contraction of radial muscles in the oral disc by immersed Anthopleura xanthogrammica in response to bright sunlight (550 /uEinsteins-m~^-s~'). Note that the shaded portion of the anemone remains expanded, as does the entirely shaded anemone at left. Also note the gravel attached to the verrucae of the body column. In addition to these enzymatic defenses, A. elegantissima shows behavioral re- sponses that reduce exposure to intense sunUght. Duri
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. PHOTOBIOLOGY OF ANTHOPLEURA 693. Figure 6. Localized retraction of tentacles and contraction of radial muscles in the oral disc by immersed Anthopleura xanthogrammica in response to bright sunlight (550 /uEinsteins-m~^-s~'). Note that the shaded portion of the anemone remains expanded, as does the entirely shaded anemone at left. Also note the gravel attached to the verrucae of the body column. In addition to these enzymatic defenses, A. elegantissima shows behavioral re- sponses that reduce exposure to intense sunUght. During intervals of high irradiance, this anemone retracts its alga-containing tentacles and contracts its marginal sphincter which constricts the oral disc (Pearse, 1974; Shick and Dykens, 1984). This behavior, which shades the majority of the zooxanthellae thereby reducing the production of oxygen, occurs in response to elevated levels of oxygen and to sunlight exposure per se (Shick and Dykens, 1984). It is a finely modulated behavior in that immersed anemones partially exposed to sunlight retract only those tentacles and contract that area of the oral disc that are directly illuminated, behaviors perhaps mediated by local electrical conduction systems (Marks, 1976; McFarlane, 1984) or by the effects of light on the muscles themselves (North and Pantin, 1958), while the shaded portion of the anemone remains expanded (shown in the congeneric A. xanthogrammica in Fig. 6). Reduced photosynthesis (P^net) during periods of peak irradiance, due to con- traction in A. elegantissima, is also seen in symbiotic corals (Porter, 1980) and tridacnid clams (Mangum and Johansen, 1982) and will reduce estimates of the contribution made by zooxanthellae to animal respiration (CZAR). The impact of reduced P^net on CZAR is only partially offset by concurrent reductions in animal respiration while contracted (Shick and Dykens, 1984). Both of these factors reinforce the need for long-term monitoring
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