Sami Merilaita

Crypsis through disruptive coloration in an isopod

The white–spotted colour morph of the marine isopod Idotea baltica appears cryptic on the brown alga Fucus vesiculosus with its white–coloured epizoites Electra crustulenta and Balanus improvisus. This study shows that the crypsis of this coloration is achieved through disruptive coloration rather than through background matching. Crypsis through background matching requires that the sizes and the shapes of the pattern elements should closely resemble those of the visual background. Comparisons between the white spots of the isopods and those of their natural background contradicted this prediction. Disruptive coloration, which aims to obscure the true form of the animal by partly blending with the background and distracting the attention of the viewer from the contour of the animal to unessential patterns, presupposes more marginal elements than expected by the pattern element distribution in the background, and also highly variable and complex elements. Comparison between the observed spot distribution and simulated individuals with randomly distributed spots showed that the spots in these isopods do indeed touch the body outline more often than expected. Furthermore, the spots were highly variable and complex.

Fearful symmetry: pattern size and asymmetry affects aposematic signal efficacy

Aposematic animals use anti-predator defence mechanisms such as distastefulness coupled with distinctive odours, sounds, or colour visual signals to predation from domestic chicks we show that the protective value of such visual warning displays is enhanced by increasing size of the signal pattern elements and decreased by pattern asymmetry. These results provide the first experimental evidence that predation may select for individual symmetry of visual warning displays, and concur with earlier demonstrations that asymmetric signals are more difficult to detect, learn, and remember, compared to symmetric signals. Collectively, our findings suggest that prey species possessing warning coloration should be subjected to selection for large and symmetric pattern elements.

Evolution of sex differences in microhabitat choice and colour polymorphism in Idotea baltica

We studied microhabitat choice of colour morphs, causes of sex differences in microhabitat use and colour polymorphism in Idotea balticaa marine isopod living mainly on the brown alga Fucus vesiculosusThe colour morphs differ in frequencies between the sexes and appear to be cryptic on the visually heterogeneous FucusIn this study, no colour-morph-dependent preference for visually matching microhabitats was found. However, in all three experiments conducted, females were found more often on the lower parts of the Fucus than males. The microhabitat choice of the sexes was directed by some character of Fucus itself, not by preferred height within the plant. However, the sexes did not choose differently between upper and lower parts of Fucus as food. The food choice and substrate choice correlated in males but not in females, implying that microhabitat and feeding preferences are more tightly associated in males. We propose that the stronger preference for the less exposed lower parts of Fucus as microhabitat and the lack of correlation between microhabitat and substrate choice in females can be explained in terms of a greater investment in anti-predator protection in females than in males. Thus, the sexual difference in microhabitat choice would ultimately result from different strategies maximizing reproductive success in males and females. We suggest that the sexual differences in coloration and colour morph frequency in I. baltica are explained as an adaptation to sex differences in patterns of habitat use.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

Habitat heterogeneity, predation and gene flow: colour polymorphism in the isopod, Idotea baltica

The colour polymorphic isopod Idotea baltica inhabits the brown alga Fucus vesiculosus which is often colonised by the white epizoite Electra crustulenta (Bryozoa). In an experiment the predation risk for the different colour morphs of I. baltica was highly dependent on background colouration. Morph frequencies and Electra density varied substantially among 10 collecting sites but correlated poorly with each other, suggesting that local selection for cryptic colouration may be counteracted by gene flow. Indeed, estimates based on four polymorphic allozymes suggested rate of gene flow to be high. These results support the hypothesis that locally varying selection for cryptic colouration counteracted by gene flow contributes to the maintenance of colour polymorphism in I. baltica. The visual differences between the microhabitats and the differential microhabitat use between males and females seem to result in different patterns of selection on males and females for cryptic colouration. Also this is likely to play an important role for the polymorphism.

Female Resistance and Precopulatory Guarding in the Isopod Idotea Baltica (Pallas)

In a laboratory experiment in I. baltica the precopulatory guarding was preceded by a period of struggles between the sexes as males continuously tried to initiate the precopulatory guarding and females resisted their guarding attempts. This struggling lasted for a few days, during which the females escaped from the males on the average 1.3 times per hour. While the females resisted, the males usually responded by kicking back. Once the precopula started, on the average 43 h before the completion of the female parturial ecdysis, the female resistance stopped. If the guarding male was replaced by another male, the female accepted the new male without resistance or resisted only weakly. Larger males were able to perform longer precopulas, and furthermore, when males were hunger stressed they performed shorter precopulas than control males. The female resistance and the existence of struggles imply a conflict between the sexes over whether or not to start the precopulatory phase. This conflict may occur either because of different optimum precopula duration of the sexes or because of the unwillingness of the females to pair with whatever male. By resisting, females may, at least to some extent, control the duration of the guarding, and the resistance may lead to selection among male candidates. Thus the female resistance, although for so far largely neglected, may have potential importance in the mate choice and sexual selection of aquatic crustaceans with precopulatory guarding.

Costs of intersexual conflict in the isopod Idotea baltica

In sexual reproduction one sex can increase its reproductive success at the cost of the other, a situation known as intersexual conflict. In the marine isopod Idotea baltica, males guard females before copulation. The guarding phase is preceded by struggles as females resist males’ attempts to initiate guarding. We determined whether the struggle and/or mate‐guarding result in fitness costs in the form of decreasing fecundity and lower levels of the energy storage compounds, glycogen and lipids. Females that underwent the period of struggles with males had decreased glycogen levels compared with females maintained alone. No such cost was found for males. Females guarded by a male also had smaller eggs than females that were not guarded. Thus the intersexual conflict, imposed by the fitness maximization strategy of the males, gave rise to both a fecundity cost and an energetic cost for females. The fecundity cost confirms the existence of intersexual conflict in I. baltica. This cost is shared by males, suggesting that the intersexual conflict restrains the reproductive output of both sexes.

Different roles of feeding and protection in diel microhabitat choice of sexes in Idotea baltica

Abstractu2002In Idotea baltica, a marine isopod that lives and feeds on the brown alga Fucus vesiculosus, microhabitat choice differs between sexes so that males are found more often than females on the light-coloured and exposed apical parts of the alga. We investigated how the requirements of avoiding visual predators and feeding were related to microhabitat choice in relation to diurnal and life-cycle stage in males and females. Faced with a choice between an apical and a basal piece of the alga, females spent more time than males on the basal piece, but this difference was not due to food choice. Faced with a choice between a dark, concealing and a light, exposing background, the preference for a dark background was stronger at day than at night, and stronger in females than in males. This suggests that a sex difference in the importance of avoiding visual predators can explain the sex difference in microhabitat choice. Further, the preference for a dark background and night feeding both increased with age, suggesting that feeding is increasingly subordinated to the need to avoid visual predators. Our experiment found no effect of the presence of the opposite sex on microhabitat choice. Our results support the hypothesis that the sexes trade off feeding against predation risk differently, presumably because growth is more important to males than to females, which have more to gain by protection and therefore spend more time on the lower parts of the alga.

Grab my tail: evolution of dazzle stripes and colourful tails in lizards

Understanding the functions of animal coloration has been a long‐standing question in evolutionary biology. For example, the widespread occurrence of striking longitudinal stripes and colourful tails in lizards begs for an explanation. Experiments have suggested that colourful tails can deflect attacks towards the tail (the ‘deflection’ hypothesis), which is sacrificable in most lizards, thereby increasing the chance of escape. Studies also suggest that in moving lizards, longitudinal body stripes can redirect predators’ strikes towards the tail through the ‘motion dazzle’ effect. Despite these experimental studies, the ecological factors associated with the evolution of such striking colorations remain unexplored. Here, we investigated whether predictions from motion dazzle and attack deflection could explain the widespread occurrence of these striking marks using comparative methods and information on eco‐physiological variables (caudal autotomy, diel activity, microhabitat and body temperature) potentially linked to their functioning. We found both longitudinal stripes and colourful tails are associated with diurnal activity and with the ability to lose the tail. Compared to stripeless species, striped species are more likely to be ground‐dwelling and have higher body temperature, emphasizing the connection of stripes to mobility and rapid escape strategy. Colourful tails and stripes have evolved multiple times in a correlated fashion, suggesting that their functions may be linked. Overall, our results together with previous experimental studies support the notion that stripes and colourful tails in lizards may have protective functions based on deflective and motion dazzle effects.

Scary clowns: adaptive function of anemonefish coloration

Clownfishes, with their showy coloration, are well known for their symbiosis with sea anemones and for their hierarchical reproductive system, but the function of their coloration is unclear. We used a phylogeny of 27 clownfish species to test whether fish coloration (i) serves a protective function that involves their anemone hosts, or (ii) signals species identity in species with overlapping host ranges that can potentially share the same host. We tested for an association between fish colour pattern traits, host morphology and host toxicity and examined coloration in relation to host sharing and geographic proximity. Fish with fewer stripes occupied fewer anemone species, and hosts with shorter tentacles, than fish with multiple stripes. There was a negative relationship between anemone toxicity and tentacle length and these protective traits together were correlated with the evolution of stripes. Host sharing or range overlap was not associated with coloration divergence. We propose that ancestral anemonefishes had multiple stripes that served for hiding/camouflage among the hosts’ long tentacles, whereas increased specialization towards fewer and more toxic hosts (with shorter tentacles) led to the use of coloration as an aposematic signal. The intriguing notion that an aposematic signal could advertise the defence of another species may reflect the unique symbiotic relationship between anemonefishes and their hosts.

Size and contrast increase the divertive effect of eyespots

&NA; Recent studies have shown that some eyespots of prey divert the strikes of predators, increasing the likelihood of prey escape. However, little is known about what makes eyespots effective divertive (deflective) prey marks. The size of eyespots varies much both between and even within taxa. Yet, whether size is important for the divertive function of eyespots is unknown. Furthermore, eyespots have often been described as highly contrasting, but the effects of contrast on the divertive function of eyespots has never been tested experimentally. Using artificial prey and the three‐spined stickleback (Gasterosteus aculeatus) as a model for predator cognition and behavior, we tested the importance of size as well as internal contrast for the divertive effect of eyespots. We independently increased the internal contrast and size of eyespots and found that both increased the divertive effect. The effect of size was significant over all 4 subsequent prey presentations, whereas the effect of contrast decreased after the initial presentations. These results suggest that the size and contrast of divertive marks are probably shaped by selection imposed by predation. We also discuss the involvement of predation in the seasonal and ontogenic plasticity of eyespots found in some taxa.