Phylogenetic meta-analysis reveals system-specific behavioural type-behavioural predictability correlations

The biological significance of behavioural predictability (environment-independent within-individual behavioural variation) became accepted recently as an important part of an individual's behavioural strategy besides behavioural type (individual mean behaviour). However, we do not know how behavioural type and predictability evolve. Here, we tested different evolutionary scenarios: (i) the two traits evolve independently (lack of correlations) and (ii) the two traits' evolution is constrained (abundant correlations) due to either (ii/a) proximate constraints (direction of correlations is similar) or (ii/b) local adaptations (direction of correlations is variable). We applied a set of phylogenetic meta-analyses based on 93 effect sizes across 44 vertebrate and invertebrate species, focusing on activity and risk-taking. The general correlation between behavioural type and predictability did not differ from zero. Effect sizes for correlations showed considerable heterogeneity, with both negative and positive correlations occurring. The overall absolute (unsigned) effect size was high (Zr = 0.58), and significantly exceeded the null expectation based on randomized data. Our results support the adaptive scenario: correlations between behavioural type and predictability are abundant in nature, but their direction is variable. We suggest that the evolution of these behavioural components might be constrained in a system-specific way.

An escape theory model for directionally moving prey and an experimental test in juvenile Chinook salmon

Prey evaluate risk and make decisions based on the balance between the costs of predation and those of engaging in antipredator behaviour. Economic escape theory has been valuable in understanding the responses of stationary prey under predation risk; however, current models are not applicable for directionally moving prey.

Here we present an extension of existing escape theory that predicts how much predation risk is perceived by directionally moving prey. Perceived risk is measured by the extent antipredator behaviour causes a change in travel speed (the distance to a destination divided by the total time to reach that destination). Cryptic or cautious antipredator behaviour slows travel speed, while prey may also speed up to reduce predator–prey overlap. Next, we applied the sensitization hypothesis to our model, which predicts that prey with more predator experience should engage in more antipredator behaviour, which leads to a larger change in travel speed under predation risk. We then compared the qualitative predictions of our model to the results of a behavioural assay with juvenile Chinook salmon Oncorhynchus tshawytscha that varied in their past predator experience.

We timed salmon swimming downstream through a mesh enclosure in the river with and without predator cues present to measure their reaction to a predator. Hatchery salmon had the least predator experience, followed by wild salmon captured upstream (wild‐upstream) and wild‐salmon captured downstream (wild‐downstream).

Both wild salmon groups slowed down in response to predator cues, whereas hatchery salmon did not change travel speed. The magnitude of reaction to predator cues by salmon group followed the gradient of previous predator experience, supporting the sensitization hypothesis.

Moving animals are conspicuous and vulnerable to predators. Here we provide a novel conceptual framework for understanding how directionally moving prey perceive risk and make antipredator decisions. Our study extends the scope of economic escape theory and improves general understanding of non‐lethal effects of predators on moving prey.

Radiolar Eyes of Serpulid Worms (Annelida, Serpulidae): Structures, Function, and Phototransduction

Fan worms, represented by sabellid and serpulid polychaetes, have an astonishing array of unusual eyes and photoreceptors located on their eponymous feeding appendages. Here we organize the previous descriptions of these eyes in serpulids and report new anatomical, molecular, and physiological data regarding their structure, function, and evolution and the likely identity of their phototransduction machinery. We report that, as in sabellids, serpulids display a broad diversity of radiolar eye arrangements and ocellar structures. Furthermore, the visual pigment expressed in the eyes of Spirobranchus corniculatus, a species of the charismatic Christmas tree worms, absorbs light maximally at 464 nm in wavelength. This visual pigment closely matches the spectrum of downwelling irradiance in shallow coral reef habitats and lends support to the hypothesis that these radiolar photoreceptors function as a silhouette-detecting "burglar alarm" that triggers a rapid withdrawal response when the worm is threatened by potential predators. Finally, we report on the transcriptomic sequencing results for the radiolar eyes of S. corniculatus, which express invertebrate c-type opsins in their ciliary radiolar photoreceptors, closely related to the opsin found in the radiolar eyes of the sabellid Acromegalomma interruptum. We explore the potential for a shared evolutionary lineage between the radiolar photoreceptors of serpulids and sabellids and consider these unique innovations in the broader context of metazoan eye evolution.