Effects of light intensity on activity in four sympatric anuran tadpoles

Guo-Hua DING, Zhi-Hua LIN*, Li-Hua ZHAO, Xiao-Li FAN, Li WEI

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Effects of light intensity on activity in four sympatric anuran tadpoles

Guo-Hua DING, Zhi-Hua LIN, Li-Hua ZHAO, Xiao-Li FAN, Li WEI

College of Ecology, Lishui University, Lishui 323000, China

Though light conditions are known to affect the development and anti-predation strategies of several aquatic species, relatively little is known about how different species react to light, or how light can affect these species during different points in their life-cycle. In this study, we used four sympatric anuran tadpoles (,,and) as animal system to examine species-specific activities of the underdoing different light intensity treatments, so as to better understand how they respond to light. We exposed four different species of tadpoles to 1660 and 14 lux light intensity treatments and then measured several parameters including development stage, body length and tail length, and as well as their basic activities. The results of this observation and analysis showed that the activities of tadpoles were significantly greater inandthan inand; and were also significantly greater during times of high light intensity as compared to during low light intensity. Moreover, the observed relationship between species and light intensity was significant. The activities ofandtadpoles were greater in high light, while the activity oftadpoles was greater in low light intensity, whiletadpoles showed no differences in either low or high intensity light. Furthermore, the activities of,andtadpoles in terms of developmental stage, body size or tail length did not seem to differ with light intensity, but during early larval developmental period of,the activity of tadpoles was negatively correlated with development stage, but irrelevant to either body size or tail length in different light intensities. These results lead us to conclude the observed activities of the four sympatric anuran tadpoles are closely correlated with their specific anti-predation strategies.

Anura;Activity; Developmental stage; Light intensity; Tadpole

Activities of anuran tadpoles are known to be finely sculpted by aquatic predators (Relyea, 2005; Smith et al, 2008; Smith & Awan, 2009; Nelson et al, 2011a, b), as well as several additional factors such as group size (Spieler, 2005) or developmental stage (Golden et al, 2000), but one of the most intriguing is light conditions (Beiswenger, 1977; Griffiths et al, 1988; Warkentin, 1992; McClure et al, 2009). Light conditions during the day or night can induce both the spatial and temporal differences in the distribution and feeding behaviors of tadpoles (Branch, 1983). Tadpoles could likewise initiate orientation behaviors to match the photoperiodic changes they sense in surrounding environments (Justis & Taylor, 1976). Such phototactic behavior is common not only in adult anurans (Jaeger & Hailman, 1976), but also in their larvae, e.g., in tadpoles of(Ashby, 1969),(de Vlaming & Bury, 1970) and(Beiswenger, 1972)

Among the various parameters of light that can anuran affect tadpoles, light intensity is one of the most important ones in shaping behavior, growth and develop-ment. Indeed, light intensity is one of the key factors in differing anti-predation to counter both diurnal and nocturnal predators (Taylor, 1983; Holomuzki, 1986). High light intensity can, for example, increase the range of vision in tadpoles and enhance their ability in identifying predators, accordingly aiding in better anti-predation reaction and higher levels of activity. For example, in wood frog tadpoles (), light conditions do not influence the activity level in undisturbed tadpoles, but do affect the response to the shadow stimulus, with the greatest responses being during conditions of bright light (McClure et al, 2009). Better light conditions also cause environmental effects that can be beneficial, such as an increase water tempe-rature which benefits the growth and development of thermophilic tadpoles (Wright et al, 1988), and also induces tadpoles’ aggregation (Beiswe-nger, 1977).

Despite the previous observations, it is not entirely clear how species-specific some of these effects are, nor is it clear how different species respond to different light conditions. In this study we used tadpoles from four sympatric anuran amphibians—andall of which areendemic to China in Zhe-ji-ang, Jiangxi and Fujian province. All four species have breeding seasons from April to May, and their larvae are also sympatric (Fei et al, 2009). Using tadpoles from these four sympatric anurans as an animal system, we focused on investigating two key unanswered questions: (1) Are there species-specific behavior to light intensities in sympatric anuran tadpoles under different light intensities; and (2) Are there correlations between light intensity, activity and developmental stage, tail length and body length among these four tadpoles.

MATERIALS AND METHODS

Animal collection and rearing

From March to April in 2011, tadpoles ofandwere collected by net from the water bodies of suburban farmlands in Lishui, Zhejiang (N28°27', E119°54') and brought back to our laboratory at Lishui University. Tadpoles of each species were housed separately in four dedicated breeding tanks (60 cm×40 cm×40 cm, length×width×height) containing 20 cm of 24-hour-decholorinated tap-water. Aquatic plants, such as confervoid and algae were added to the tanks to imitate the natural environment of the tadpoles. Tadpoles were fed with abundant egg yolk and fish powder. Prior to experimentation, one week was allowed for tadpoles to become acquainted with the new tank environment.

Experimental design

Totally, 20 tadpoles with intact tails of each species were randomly selected from the four tanks and housed in 80 small plastic containers (30 cm×30 cm×10 cm, length×width×height) containing 3 cm of water. These containers were placed in a psychrometric room with air temperature adjusted to 30±0.5 °C, and the light cycle (light period: 0700h?1900h) mimicked by fluorescence lights (28 W, 169 lux). During the trials, lighting conditi-ons were created by turning off the fluorescence light and to create different intensities, light from an artificial camera (1 300 W, 1 660 lux) was used to mimic high light-i-ng and a calico covered desk lamp (15 W, 14 lux) to mimic low lighting.Trials were conducted from 1100h. After a one-hour acclimation period, six one-minute obse-r-vations were conducted every 20 minutes to deter-mine the swimming activities of tadpoles. Only swim-ming activities lasting 10 seconds and longer were consi-dered as valid, otherwise they were discarded as being irrelevant.

Following the light intensity trials, high and low lighting were turned off and the fluorescence light was switched back on. Tadpoles werefor 3 hours, and then water was changed. Before next day’s trials, tadpoles were under fasting conditions for 18 hours. Tadpoles movement was evaluated based on its valid swimming activities during the 6 observations (Indivi-dual’s activity=valid swimming activities/times of obse-rv-ation×100%). Each lighting treatment combination was duplicated twice, yielded a total timeframe of four days. At the end of the experiment, tadpoles were examined under a Nikon XTS30 lens and staged according to the staging tables developed by Gosner (1960). Body length (from snout to vent) and tail length (from base to tip of the caudal fin) were determined using ImageJ 1.44 (±0.01 mm).

Data analyses

All statistical analyses were conducted using Statistica 6.0 (Tulsa, OK, USA). All variables were tested for normality and homogeneity before further analysis. As the lighting conditions had no effects on the activity of duplicate measurement of the tadpoles from each species (paired-sample-test; all>0.58), the data from each duplicated experiment were pooled for analysis. Multiple regression, paired-sample-test, ANOVA and repeated-measures ANOVA were used to evaluate relevant data. Variables with significant differe-nces were analyzed by Tukey’scomparisons. Throughout this study, all values are expressed as mean±, witha=0.05 being significant.

RESULTS

The descriptive statistics of the developmental stage, body length and tail length of the foursympatric anuran tadpoles are shown in Table 1. Taking species as factor, ANOVA analysis was used to evaluate the linear regression residuals for the developmental stage, body length and tail length of the foursympatric anuran tadpoles, which showed significant interspecies differen-ces in both the developmental stage and morph-ological traits (all<0.0001). Under the differing light treatments, the developmental stage was more advanced intadpoles than in the other three tadpoles (Tukey’s test; all<0.001; Figure 1). In general, there was also different observed mean values in terms of body length, ranging from the smallest,, to,and finallytadpoles being the largest (Tukey’s test; all<0.001; Figure 1). Similarly,andtadpoles had much longer tails than those ofandtadpoles (Tukey’s test; all<0.001; Figure 1).

Table 1 Descriptive statistics of developmental stage, body length and tail length in the four sympatric anuran tadpoles

Note: data presented as means±

Figure 1 Meanregression residuals of tail length against body length in four sympatric anuran tadpoles

Taking species as between-subject factor and light intensity as within-subject factor, the activities of the four sympatric anuran tadpoles were evaluated by repeated-measures ANOVA, which indicated significant interspecies differences (=34.25,<0.0001). The activities ofandtadpoles were significantly greater than those ofandtadpoles (Tukey’s test; both<0.001)The activities of tadpoles under different light intensity treatments were also significantly different (=52.23,<0.0001), being greater during high light intensity than in low light intensity (Tukey’s test;<0.001). Moreover, the activity was also affected by the interaction between species and light intensity (Tukey’s test;<0.001) (Figure 2). Intraspecificanalyses showed the activities of,andtadpoles were significantly affected by light intensity (pair-sampletest, all<0.04), though not fortadpoles (pair-sampletest,=0.95,=19,=0.356).andtadpoles were likewise more active under high light intensity, whiletadpoles seemed more activelow light intensity (Figure 2) andtadpolesacted no differently under either.

Figure 2 Mean activity of four sympatric anuran tadpoles under different light intensity treatments

***:<0.0001; *:<0.05.

Taking the developmental stage, body length and tail length as independent variables, and taking activity as dependent variable, multiple regression showed that under different light intensity treatments, the activities of,andtadpoles were not associated with their developmental stage, body length nor tail length (all>0.216), but the activity oftadpoles was negatively correlated with its developmental stage, and not connected with its body length and tail length (all>0.137).

DISCUSSION

The results of our study showed that,tadpoles are more active under high light intensity treatments, while their activities were reduced 35% and 32% under low light intensity treatments, respectively (Figure 2).andtadpoles were previously shown to be inactive at night, but when a light is shined on them they become active (Eterovick & Sazima, 1999; Griffiths et al, 1988). Similarly, the activity oftadpoles increase as light increases, and decreases on overcast days (Beiswenger, 1977), so much so that typical daytime behaviors could be induced in inactive tadpoles by creating artificial lighting conditions during nighttime (Branch, 1983). These studies not only suggest the specific activity rhymes of Bufonidae tadpoles, but also indicate that their activity patterns are likely influenced by light conditions. Conversely, intadpoles activity under high light intensity treatment was found to be some 4% lower than under low light intensity (Figure 2), which was in accordance with the results previously reported forandtadpoles in caliginous environment, such as during night or evening (McIntyre et al, 2004; Smith et al, 2007). Compared with Bufonidae and Ranidaetadpoles, different light intensity treatments showed no significant effects on the activities oftadpoles (Figure 2), but with no other species to test against, it is not possible to determine if this phenomenon is universal intadpoles still needs further study and more data.

Under different light intensity treatments, the activity oftadpoles in their early developmental period was all negatively correlated with developmental stage (Figure 3), which manifested in impacts of developmental stage on individual’s activity (Golden et al, 2000). Huey (1980) reported that the swimming ability of tadpoles before Gosner stage 40 were significantly positively correlated with their developmental stage and Anholt et al (2000) found that tadpoles with low swimming ability moved more frequently to increase preying rate. As such, the high observed activity level oftadpoles in their early developmental period could function as compensation to their low swimming abilities. A similar phenomenon was found intadpoles (Golden et al, 2000). But in our study, this specific correlation between activity and developmental stage was not found in the other three anuran tadpoles (all>0.05), though this may be due to the small sample size.

Figure 3 Relationships between activity and developmental stage intadpoles under different light intensity treatments

In this study, the mean values for activities ofandtadpoles were signif-icantly greater than those ofandtadpoles (Figure 2), which were correlated with their specific anti-predation strategies. There may be some factors that help explain this difference. Bufonidae tadpoles can release toxic chemicals to decrease their palatability as prey, so some aquatic predators would accordingly choose to avoid Bufonidae tadpoles after they have tasted them once (Laurila, 1998; álvarez & Nicieza, 2009; Nelson et al, 2011a, b; Wei et al, 2013). Likewise, by moving more frequently, Bufonidae tadp-oles can accelerate the release of those toxic chemicals and alert other individuals, thus decreasing their chances of being preyed upon (Hews, 1988; Van Buskirk, 2001). In our study, a high active level was found inandtadpoles (Figure 2). Paired alongside their ability to release toxic chemicals such activity could effectively increase both their individual and species defense against predators. Compared to the Bufonidae tadpoles, Ranidae tadpoles are usually pleasant food for aquatic predators (Van Buskirk & McCollum, 2000; Laurila et al, 2002; Wilson et al, 2005), and accordingly they decrease their active levels to lower risks of encountering predators and increase their survival rates (Lefcort, 1996; Anholt et al, 2005). Similarly, in this study, tail morphological traits ofandtadpoles were much more apparent than those of the other two Bufonidaetadpoles (Figure 1). This makes sense—longer tails endow individuals with greater burst speed (Lardner, 2000; Van Buskirk & McCollum, 2000; Dayton et al, 2005; Arendt, 2010). Presumably then, the decrease in activity and increase in burst speed may offer effective anti-predation strategies of the two Ranidaetadpoles.

In conclusion, our study suggests three things. First, thatandtadpoles are more active under high lighting conditions, and the activity oftadpoles decrease as light intensity increase, whereas, light intensity had no effects on the activity oftadpoles. Second, that during the early developmental period, the activity oftadpoles is negatively correlated with their developmental stage. And finally, that the specific activity patterns of these four sympatric anuran tadpoles under different light intensities are closely correlated with their anti-predation strategies.

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08October2013; Accepted: 27November2013

This work was supported by the National Natural Science Foundation of China (30970435, 31270443), the Zhejiang Provincial National Science Foundation of China (C030403),the Open Research Fund program of Laboratory of Lishui University (2014-26-10) and the Scientific Research Foundation of Ph.D. in Lishui University (QD1301).

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