Marine Biology -2001) 138: 717±730 Ó Springer-Verlag 2001 S. Johnsen á E. A. Widder Ultraviolet absorption in transparent zooplankton and its implications for depth distribution and visual predation Received: 14 April 2000 / Accepted: 16 November 2000 Abstract The use of transparency as camou¯age in the and minimum attainable depth were modeled using epipelagic realm is complicated by the presence of ul- contrast theory and the physics of light attenuation. traviolet radiation, because the presence of UV-protec- Because UV absorption was generally signi®cantly tive pigments decreases UV transparency and may reveal greater in the UVB than in the UVA spectrum -where transparent zooplankton to predators and prey with UV UV vision occurs), and because the highest UV ab- vision. DuringJuly 1999, September 1999, and June sorption was often found in less transparent individuals, 2000, transparency measurements -from 280 to 500 nm) its modeled eects on visibility were slight compared to were made on livingspecimens of 15 epipelagic-col- its eects on minimum attainable depth. lection depth: 0±20 m, average: 11 1 m) and 19 mesopelagic -collection depth: 150±790 m, average: 370 40 m) species of transparent zooplankton from Oceanographer Canyon and Wilkinson Basin in the Introduction Northwest Atlantic Ocean. In addition, measurements of downwellingirradiance -from 330 to 500 nm) versus Two strikingcharacteristics of many oceanic zooplank- depth were made. The tissues from epipelagic zoo- ton are their delicacy and their high degree of exposure plankton had lower UV transparency than those from to visual predators and prey. Transparency is one of the mesopelagic zooplankton, while the average visible most common adaptations to this environment and ap- transparency -at 480 nm) of the two groups was not pears to have evolved many times in parallel. All major signi®cantly dierent. Percent transparency was posi- pelagic phyla have transparent representatives, and tively correlated with wavelength over most of the many taxonomic groups -cnidarians, ctenophores, measured range, with a rapid decrease below a certain chaetognaths, polychaetes, salps, doliolids, cranchid cuto wavelength. In mesopelagic tissues, the cuto squid, pseudothecosomate pteropods, heteropods, hy- wavelength was generally 300 nm. In epipelagic tissues, periid amphipods) are dominated by transparent forms the cuto wavelength was between 300 and 400 nm. -McFall-Ngai 1990; Hamner 1996; Johnsen and Widder Twelve out of 19 epipelagic tissues had transparencies at 1998; Johnsen 2000). Many species achieve nearly 320 nm that were half or less than their 480 nm trans- complete invisibility, and it has longbeen assumed that parency values, versus only 4 out of 21 mesopelagic transparency functions as camou¯age -Hobson and tissues. The eects of UV absorption on UV visibility Chess 1978; McFall-Ngai 1990). Although transparent, mostly gelatinous zooplank- ton are poorly represented in trawls, research usingblue- Communicated by J. P. Grassle, New Brunswick water divingtechniques and submersibles has shown that they are diverse, abundant, and play critical roles as S. Johnsen -&) herbivores, predators and prey of zooplankton and Biology Department, MS #33, ichthyoplankton, and conveyers of organic mass to Woods Hole Oceanographic Institution, Woods Hole, MA 02543-1049, USA deeper waters -e.g. Hamner et al. 1975; Alldredge and Madin 1982; Alldredge 1984; Caron et al. 1989; Lalli e-mail: [email protected] Tel.: +1-508-2893603; Fax: +1-508-4572134 and Gilmer 1989; Pages et al. 1996; Madin et al. 1997; Purcell 1997). All pelagic phyla contain numerous E. A. Widder Marine Science Division, transparent species that either prey on, or are preyed Harbor Branch Oceanographic Institution, upon by species with well-developed visual systems -re- Fort Pierce, Florida, USA viewed by Johnsen and Widder 1998). In addition, many 718 of these species -e.g. cnidarians, ctenophores, and 1995; Losey et al. 1999). Indeed, amongfreshwater te- chaetognaths) prey upon copepods and larval crusta- leosts UV vision appears to be fairly widespread -re- ceans -Baier and Purcell 1997; Harbison et al. 1978), viewed by Jacobs 1992, Goldsmith 1994, and Shashar which react defensively to shadows -Forward 1976; 1995; Carleton et al. 2000). Several researchers have Buskey et al. 1986) and thus may react to opaque or hypothesized that UV vision is primarily used to im- translucent predators passingoverhead. prove detection of planktonic prey -Loew et al. 1993; Since the majority of transparent zooplankton are Cronin et al. 1994; McFarland and Loew 1994), and more delicate and less agile than their visually orienting Browman et al. -1994) have shown that the presence of predators or prey, their success in predator/prey inter- UV light improves the search behavior of certain UV- actions with these species depends critically upon their sensitive zooplanktivorous ®sh. The presence of UV visibility and, in particular, their sighting distance -the sensitivity in planktivorous but not in non-planktivo- maximum distance at which they are detectable by an rous life stages of salmonids -reviewed by Tovee 1995), organism relying on visual cues) -Johnsen and Widder the correlation between UV vision and planktivory in 1998). Because underwater sighting distances are short, coral reef ®sh -McFarland et al., in preparation), and they are generally determined more by an object's the correlation between ocular UV transparency and contrast than by its size -Mertens 1970). An object's planktivory -Siebeck and Marshall 2001) all suggest that contrast depends on its distance from the viewer, its UV vision is often used to increase the contrast of transparency, and the transparency of the surrounding planktonic prey. water. Beyond the sighting distance, the object's contrast The second problem related to UV radiation is po- drops below what the viewer can detect. Because the tential radiation damage. Numerous studies have shown sighting distance depends on the optical properties of the that pelagic organisms are damaged by UV radiation in zooplankton and the surroundingwater, changesin ei- various ways, includingdeleterious eects on DNA, ther aect predator±prey relationships and possibly the proteins, tissue, activity, growth, reproduction, and species composition of the ecosystem -Zaret and Kerfoot chemical defenses -Damkaer et al. 1980, 1981; Worrest 1975; Greene 1983; reviewed by McFall-Ngai 1990). 1982; HaÈ der and Worrest 1991; Smith et al. 1992; Cro- The issue of ultraviolet -UV) transparency is partic- nin and Hay 1995; El-Sayed et al. 1996; Hay 1996). ularly intriguing. Recent research has shown that UV Dierential levels of these eects have been shown to radiation is more abundant in epipelagic ecosystems in¯uence biomass, sex ratios, and species compositions -de®ned in this paper as the depth range 0±20 m) than of both terrestrial and marine ecosystems -Bogenrieder previously supposed -Fleischmann 1989; Frank and and Klein 1982; Bothwell et al. 1994; Chalker-Scott Widder 1996; reviewed by El-Sayed et al. 1996 and 1995; WaÈ ngberg et al. 1996; Odmark et al. 1998). These Losey et al. 1999). This presents two problems for zoo- eects are primarily due to UVB radiation -280± plankton that have evolved a concealment strategy 320 nm), and in clear polar water have been observed to based on transparency. The ®rst involves UV vision depths of 20±25 m -reviewed by El-Sayed et al. 1996). At amongpotential predators and prey. UV vision has been lower latitudes, where surface UV irradiance is higher, demonstrated in many marine species, and it has been these eects are likely to be observed even deeper. conservatively estimated that there is sucient UV light One of the primary defensive mechanisms against UV for vision down to 200 m in clear ocean water -reviewed radiation damage is the use of UV-absorbing pigments by Shashar 1995 and Losey et al. 1999). Visual pigments -Dunlap et al. 1991; Douglas and Thorpe 1992; Thorpe with UV sensitivity -though not necessarily peaking in et al. 1993; Karentz 1994; Carroll and Shick 1996; the UV) have been found in the Atlantic halibut -Helvik Hannach and Sigelo 1998). However, because UV-pro- et al., in preparation), 22 -out of 41 examined) species of tective pigments must attenuate UV light to be eective, coral reef ®sh -McFarland and Loew 1994; McFarland their presence reduces an organism's transparency in the et al., in preparation), juvenile salmonids -Bowmaker UV, and thus increases its sighting distance for preda- and Kunz 1987; Coughlin and Hawryshyn 1994), and tors and prey with UV vision. This presents a potential decapod and stomatopod crustaceans -Cronin and dilemma for transparent epipelagic zooplankton: Frank 1996; Marshall and Oberwinkler 1999). In addi- protection or concealment. This con¯ict is particularly tion, Siebeck and Marshall -2001) examined the lenses dicult to resolve in clear, oceanic waters, where UV and corneas of 211 species of coral reef ®sh and found radiation levels are high and camou¯age is especially that 50% had high transparency at UV wavelengths. challenging. Reports of decreasing ozone levels at polar, Since the majority of the lenses and corneas in high UV temperate, and tropical latitudes -Solomon 1990; Sto- environments have high UV absorption -presumably to larski et al. 1992) create an additional complication, protect the retina from radiation damage) -Thorpe et al. because transparent zooplankton may face concomitant 1993), Siebeck and Marshall suggest that