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THOMASH. CAREFOOTAND ELENA THOMASDepartment of Zoology, University of Bnt shColumbia, Vancouver, Canada-,-Abstract. The possibility that UV light has playeda role in the evolution of nocturnalism in sea hareswas tested with Aplysia dactylomela. Thls speclesinhabits shallow-water reglons of coral reefs worldwide. Such shallow habitats are subjected to strongsolar effects, as evidenced by noticable bleaching Incertain algal species. This study investigates pos6ible deleterious effects of UV light on food consumption, growth, spawn production, hatchingfEltes and percentages of eggs, and blood-glucosecroppings of dead coral. It is nocturnal in such regions, spending the nighttime hours mate-seeklng,copulating, and feeding. At or near dawn it searchesfor protective crevices or hides under rocks, whereit spends the daylight hours digesting food and laying eggs (Carefoot and Taylor 1988).There are several commonly accepted theonesregarding the function of nocturnalism which include avo danceof daytime competitors, avoidanceof predators, and m i n m i z a t oofn desiccation.posed to light with UV blocked out. Animals keptunder total light block had highest growth (but notsignificantly so) accompanied by lowest spawn production, and exhibited lowest blood-glucose levels.Rates of food consumption d dnot differ significantly between the three treatments, nor did hatching times of the larvae. The deleterious effects ofUVlight leading to reduced fecundity and Increasedexpenditure through stress could be plausible forces favouring selection of nocturnalism inthis sea hare species.ergy budgets In A. dactylomela (Carefoot and Taylor 1988) showed that a 12-h daytime quiescent period was energetically advantageous, lt dld notexplain why the species was active at night and notduring the day. In fact, such periodism of activityhas been observed in all Aplysia species studiedthus far (Susswein et a]. 1983; Susswein 1984; Carefoot 1989, I 99 I), and seems to be related to a needfor a quiescent penod to digest the large amountsof poor-quality food customarily eaten. A final idea,avo danceof light or, more specifically, avoidanceof potentially harmful ultraviolet wavelengths oflight through nocturnalism, is both applicable andtestable.From Jerlov's studies in the 1950's, we knowthat ultraviolet light in the UV-B (280-320 nm) andUV-A (320-400 nm) bands can penetrate qultedeeply Into seawater. Fleischmann (1989), for example, recorded 10% of surface UV radiation in the300-400 nm band at 25 m depth in Discovery Bay,IntroductionAplysla dactylomela is a comparatively large, actlve, seaweed-eating marine gastropod which, inareas of coral reefs, inhab tsshallow inshore (1-2m depth) regions of sand/coral rubble and out-82 1

Carefoot and ThomasJamaica. In the 1-2 m depth-range inhabited byAplysia dactylomela in Discovery Bay, over 60% ofthis band of UV light was present. Strong penetration of UV-B radiation into clear oceanic water hasbeen shown by Jerlov (1950) in the Sargasso Sea(attenuation of only 14%.m-I) and by Smith andBaker (1 979) in the Gulf of Mexico(l5-20%.m-'),and even in coastal waters can be substantial (Lenoble 1956; Wood 1987, 1989).Effects of solar UV radiation on the marine ecosystem are becoming increasingly well known, andinclude inhibition of phytoplankton productivityand motility (Lorenzen 1979; Biihlmann et al. 1987;lengths including UV (designated UVTRANSP), 2)e x p o s u r e t o a l l w a v e l e n g t h s e x c e p t uv(UVBLOCK), or 3) exposure to no direct light atall (TOTALBLOCK).The caging arrangements were as follows. The12 individuals in each group were randomly subdivided into six pairs, and each pair assigned to amesh basket (25 X 15 X 12 cm height, 0.6 cmplastic mesh). The six baskets per treatment weresuspended from the undersurface of PLEXIGLAS@acrylic plastic sheets (0.6 cm thick, 100 X 40 cm)of compositions which allowed different light wavelengths to be transmitted. Hinged panels of thesea anemones (Lesser and Shick 1989).The question was raised as to whether UV lightmight be affectingAplysia dactylomela when we observed that the seaweed Acan thophora spicifera hadThe light transmission characteristics forscreening material were, for UVTRANSP: allvisible radiation above 275 nm transmittedinhabited by this animal. With the view, then, thatnocturnalism in sea hares may have evolved in re-UVBLOCK:UV Or shOrt-wavelength450 n m transmitted and, foreterious effects of natural levels of UV light in A.dactylornela. We predicted that exposure to UVlight would be stressful to the sea hares, resultingin higher blood-glucose levels, lower food consumption, and lessened body growth, spawn production, and spawn vitality.Ltd.). Light (400-700 nm) penetration toJunel 5 Cm depth was measured at I2O0On1991 in full sun as 88% under UVTRANSP, 87%under UVBLOCK and 27%bottomand scatter) underBody weight was recorded for each animal atDay 0, and thereafter at 5-d intervals, over the 25d duration of the experiment. Spawn weights weremeasured daily and selected pieces cultured individually to determine hatching times and hatchingpercentages (expressed as percentage of capsulesthat opened and released larvae). T o correct for different body weights in each treatment, and becausespawn from a given animal in a basket could notMaterials and MethodsSea hares of 150-400 g live wt were collected fromshallow coral-rock areas near the Discovery BayMarine Laboratory, Jamaica, in May 1991. Theywere sorted according to size and divided into threegroups of 12 individuals, each group of 230 g mean

Nocturnalism in Sea Hares-Fig. 1- Change in weight of A P Y Sdacrylomela; subjectedto differences in type and amount of light. Each point r e presents the mean o f 12 animals. UVTRANSP: exposure toall light wavelengths including UV; UVBLOCK: exposureto all wavelengths except UV; TOTALBLOCK: no exposure to direct light.Fig. 2. Cumulative food consumption by Aplysia dactylomela in the UV-experiment. Data points represent mean ofN 6. Treatment designation same as in Fig. I .pressed as equivalent dry weight eaten per live kganimal (after drying of selected pieces at 70" C toconstant weight). Blood samples were drawn fromselected animals (N 6) in each treatment at Day0, and at 8-d ifitervals thereafter, and analysed forglucose concentration (following the method inCarefoot 1991). Sampling was done at 1000 h oneach designated day, a time when animals weremetabolically quiescent (Carefoot 1991). Wheretimeeffects were involved, data were analysed byRepeated Measures ANOVA''by Tukey'sCom arisonsTests (TMCT). Other data,such as between-treatments comparisons of hatching times and percentages (after arcsine transformation), were analysed by ordinary ANOVA andFig. 1 shows weight changes in field animals over25 d. TOTALBLOCK animals grew more thanthose in the other treatments, but differences werenot significant (F, ,,,, 0.50, p 0.61, ANOVA).Time-effects were highly significant ( F , , , 3 1.85,p 0.001), with the data segregating into two statistically homogenous subsets: Days 0-5 and theremainder ( p 0.05, TMCT). The flat portions ofthe growth curves are likely explained by all extrabut weenergy being converted intohave no explanation for the Days 5-1 0 rise, as therewas no obvious change in amounts of spawn beingproduced during this time (see later).No significant difference was found in amountof food consumed among treatments (Fig. 2; F,,,segregating into three homogenous subsets, the firstrepresenting Days 0-5 (62 dry g alga eaten-live kgbody wt-I 5d-1); the second, Days 5-10 (83); andthe third, Days 10-25 (107-109; p 0.05, TMCT).These differences likely resulted from an initial several day delay before we were able to give the animals enough food to match their appetites. Oncead libitum feeding conditions were reached, foodconsumption rates became more even. Higher ratesof food consumption should have been reflected inhigher growth and/or spawn production but thiswas not evident (see Figs. 1 and 3). During the lastfive days of the experiment, each pair of sea hareswas eating an amount of fresh food equivalent to22% of their live body wt per day.-510152025Fig. 3. Cumulative spawnby Aplysja dactylomela in the Uv-experiment. Data points represent mean o fN 6. Treatments same as in Fig. I .Food, consisting mainly of the red alga Acanthophora spicifera, but including some Centroceras ciavuiatum, was provided daily. Uneaten remnantswere collected daily and their weights subtracted