Krill Can Shrink As an Ecological Adaptation to Temporarily Unfavourable Environments

Paper 094 Disc Ecology Letters, (1999) 2 : 338±343 REPORT Krill can shrink as an ecological adaptation to temporarily unfavourable environments Abstract Baldo Marinovic1 and Marc Because environments vary with both predictable patterns and with unpredictable but Mangel1,2 recurring events, ecologists have long been interested in the ecological adaptations 1Institute of Marine Sciences, that organisms use to survive periods in which the environment may be exceptionally A316 Earth & Marine Sciences, harsh. In the north-east Pacific, one example of this is periodic warming episodes. University of California, Santa Here, we demonstrate for the first time that krill (Euphausia pacifica Hansen), which is Cruz, CA 95064. a centrally important species in coastal-upwelling systems, can survive periods of E-mail: abnormally high temperatures by shrinkage between molts, even if food is plentiful. [email protected] In addition, we demonstrate that there is a high amount of individual variation in 2Department of Environmental growth rates of krill. Krill are centrally important within pelagic foodwebs both Studies, University of California, Santa Cruz, CA 95064 worldwide and within the north-east Pacific, thus we explore the potential ecological consequences of such shrinkage for both krill and their predators. Keywords Community structure, El NinÄ o, krill, life history strategy, shrinkage Ecology Letters (1999)2 : 338±343 Ahed Bhed Ched those that are recurring but relatively unpredictable. There INTRODUCTION Dhed is widespread consensus among ecologists that popula- Ref marker Krill (euphausiids)form a key trophic link between tions respond to climactic events that may alter north-east Fig marker primary producers and higher level consumers in oceanic Pacific marine ecosystems on interannual (i.e. El NinÄ o)and Table mar- food webs worldwide. Indeed, most marine species interdecadal (i.e. regime shifts)time scales (Wooster 1983; ker (including humans)are only one or two trophic levels Roemmich & McGowan 1995; Brodeur et al. 1996; Ref end away from krill. That is, they are either prey of krill, McGowan et al. 1998). Such events affect temperature, Ref start predators of krill or predators of krill predators. Euphausia salinity, and patterns of nutrient input and can have pacifica Hansen is widely distributed throughout the profound and extended impacts throughout food webs north-east Pacific and is often the dominant (both in terms (Chelton et al. 1982; Barber & Chavez 1983; Roemmich & of numerical abundance and biomass)krill species within McGowan 1995; McGowan et al. 1998). Here we demon- both oceanic and coastal-upwelling ecosystems (Brinton strate that E. pacifica can shrink as a result of acclimation to 1962). Along the west coast of North America it can be acute interannual temperature increase associated with El found from Alaska to as far south as 258N, where mean NinÄ o events. Shrinkage can provide a mechanism for temperature in the top 250 m of the water column can be dealing with unpredictable warming events and addition- 158C and maximum surface temperatures can exceed 208C ally may have important implications for both krill (Brinton 1976). Recruitment, growth and reproductive population dynamics and fishery/predator interactions. activity within populations of E. pacifica are known to correlate with seasonal patterns of primary productivity (Smiles & Pearcy 1971; Brinton 1976; Ross et al. 1982; METHODS Bollens et al. 1992; Tanasichuk 1998). Experimental design Because krill play a central role in pelagic food webs, their demography and life history can have important Euphausia pacifica individuals were collected in late June implications for community structure and function. Life and early July 1997 from Monterey Bay, CA (36848.06 N, history strategies are means that organisms use to achieve 121859.15 W)using BONGO nets fitted with 333 mm successful reproduction in uncertain environments. Con- mesh. After capture, krill were maintained at ambient sequently they must address components that may be water temperature (approximately 14±158C)in 20 litre relatively predictable on a variety of time scales as well as plastic buckets (maximum density of eight individuals per #1999 Blackwell Science Ltd/CNRS Paper 094 Disc Krill shrink in response to ocean warming 339 litre)and transported to the laboratory. One hundred 35), and end (day 43±56) of the experiment. Mean values juveniles (total body lengths ranging between 8 and 12 were initially tested for normality and homogeneity of mm)were transferred individually into 2 litre plastic variance and then the appropriate multiple comparison buckets filled with 1 litre of filtered (0.2 mm)seawater. test was used to test for significant differences between the Cultures were maintained at ambient ocean temperature various periods of the experiment. and in constant darkness within a flowing seawater bath covered with a black plastic tent. Individuals were fed RESULTS daily on a diet of the centric diatom Thalassiosira weisfloggii at a concentration between 1.0 and 9.0 6 103 cells/ml Mean temperatures differed significantly between the (determined through three replicate counts with a beginning (mean = 15.508C)middle (mean = 16.46 8C) haemocytometer). In order to compare food levels to in and end (mean = 18.188C)of the experiment ( ANOVA; situ values, cell concentration counts were converted to F = 27.51, P 5 0.001)(Fig. 1A).Phytoplankton concen- Chlorophyll A (Chl A)values by using the standard value tration remained high within culture vessels throughout for Chl A/cell ratio of 5.56 picograms for Thalassiosira the experiment with no significant differences during the weisfloggii (J. Goldman, personal communication). Using beginning, middle and end (Kruskal±Wallis Test; this approach, we calculated Chl A values of 9.8±93.2 mg H = 12.4, P 4 0.05)(Fig. 1B).Daily mortality remained Chl A/l within our experimental treatment, which is well relatively constant at approximately 2%±3%, with no above the average values reported for Spring/Summer significant differences between the beginning, middle, and blooms in Monterey Bay (5±6 mg Chl A/l±F. Chavez, end of the experiment (ANOVA; F = 2.54, P = 0.09). IP personal communication). In addition, as E. pacifica has remained relatively constant at 3±4 days throughout the been shown to have an omnivorous diet (Ohman 1984; experiment, though there was a slight, significant increase Dilling et al. 1998), cultures were supplemented every between the beginning (mean = 3.40 days)of the experi- other day with newly hatched Artemia nauplii. Tempera- ment and the middle (mean = 3.64)or end (mean = 3.54) ture, food (phytoplankton)concentration and individual of the experiment (Kruskal±Wallis Test; H = 14.013, P 5 mortality were recorded daily and containers were 0.001). In contrast, mean MI decreased significantly with examined for the presence of molts. All molts were increasing temperature (Fig. 1D)(Kruskal±Wallis Test; recovered for subsequent measurement and individuals H = 9.428, P 5 0.01), with positive values were transferred to fresh containers with new phyto- (mean = 1.84%)recorded during the initial period when plankton and Artemia. Dead individuals were replaced for temperatures were lowest, and negative values (i.e. a period of 1 week after the start of the experiment from shrinkage)associated with the increasingly warmer tem- spare cultures maintained at identical conditions. peratures during the middle (mean = ±0.45%)and end periods (mean = ±0.53%). We further examined this relationship by conducting a linear regression between Data analysis temperature and mean MI (defined as the mean MI of all Intermolt period (IP)(the number of days between krill during one intermolt period)and found a negative and successive ecdyses)was determined for all surviving krill significant correlation (R2 = 0.572, P 5 0.001)(Fig. 2). throughout the course of the experiment. Molt increment (MI)(the percentage increase in size between molts)was DISCUSSION calculated by measuring the percentage increase of mean uropod lengths (UL)between successive molts. If one Like most species of krill, E. pacifica undergoes diurnal uropod was damaged, then only the undamaged uropod migrations, rising to the surface to feed at night and was used to calculate percentage growth. Uropod lengths descending to depths as great as 500 m during the day; (tip of exopod to base of coxa)were determined by though in coastal regions, the bulk of euphuasiid photographing the abdominal portion of each ecdysis populations tends to remain above 250 m (Smiles & with a Pulnix digital camera mounted on a Wild dissecting Pearcy 1971; Brinton 1976; Youngbluth 1976; Bollens et scope and then measuring each resulting image to the al. 1992; Tanasichuk 1998). As a consequence of such nearest 0.01 mm utilizing NIH image analysis software. migrations, individuals in the field would be exposed to To verify the relationship between UL and total length temperatures similar to those in our experiment for only (TL), we measured UL and TL for 69 individuals and part of each 24 h cycle. In addition, we provided conducted a linear regression. We found that individuals with continuous access to abundant food, TL = 5.643UL±0.054 (r = 0.969; P 5 0.001). Tempera- which would not have been the case for natural ture, food concentration, daily mortality, IP and MI were populations during the concurrent period. Nevertheless, compared for the beginning (day 1±14), middle (day 22± two striking conclusions can be drawn from these results. #1999 Blackwell Science Ltd/CNRS Paper 094 Disc 340 B. Marinovic and M. Mangel Figure 2 Regression plots (predicted, + se)for temperature and mean molt increment (MMI)( + se)for E. pacifica cultured in the laboratory; MMI = ±0.969T + 16.705 (R2 = 0.572, P 5 0.001). First, the data unequivocally demonstrate that E. pacifica individuals are capable of shrinkage between molts (i.e. a decrease in MI)in response to prolonged exposure to elevated temperatures (Fig.

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