Multimodal signals: ultraviolet reflectance and chemical cues in stomatopod agonistic encounters

Intrapopulation variability in coloration is associated with reproductive season in the crayfish Faxonius virilis

Animal coloration has a wide range of biological functions and may be subject to different, sometimes conflicting, selective pressures. In crustaceans, the evolution of coloration is relatively unstudied, despite the broad range of colors and color patterns, which includes variability at multiple levels. Freshwater crayfish are known to show color variability within species and populations, as well as intra-individual variability, but the function, if any, of crayfish coloration is largely unknown. Here, I report on an experiment to understand patterns of color variability in the crayfish Faxonius virilis and show that variation is strongly correlated to ontogenetic changes from a summer non-reproductive form to a fall reproductive form. Crayfish showed comparatively little inter- and intra-individual color variation in their non-reproductive form, but substantial variation at both levels in the reproductive form. Transition to the reproductive form was associated with the development of greener or bluer coloration localized to the chelae on a subset of individuals, but these changes showed no clear correlation with sex or body size. Future investigations should focus on determining whether differences in color between individuals in the mating season are associated with any physiological or behavioral differences, or with differential susceptibility to predation.

Crustacean conundrums: a review of opsin diversity and evolution

Knowledge of crustacean vision is lacking compared to the more well-studied vertebrates and insects. While crustacean visual systems are typically conserved morphologically, the molecular components (i.e. opsins) remain understudied. This review aims to characterize opsin diversity across crustacean lineages for an integrated view of visual system evolution. Using publicly available data from 95 species, we identified opsin sequences and classified them by clade. Our analysis produced 485 putative visual opsins and 141 non-visual opsins. The visual opsins were separated into six clades: long wavelength sensitive (LWS), middle wavelength sensitive (MWS) 1 and 2, short wavelength or ultraviolet sensitive (SWS/UVS) and a clade of thecostracan opsins, with multiple LWS and MWS opsin copies observed. The SWS/UVS opsins were relatively conserved in most species. The crustacean classes Cephalocarida, Remipedia and Hexanauplia exhibited reduced visual opsin diversity compared to others, with the malacostracan decapods having the highest opsin diversity. Non-visual opsins were identified from all investigated classes except Cephalocarida. Additionally, a novel clade of non-visual crustacean-specific, R-type opsins (Rc) was discovered. This review aims to provide a framework for future research on crustacean vision, with an emphasis on the need for more work in spectral characterization and molecular analysis. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Evolutionary loss of a signalling colour is linked to increased response to conspecific chemicals

Behavioural responses to communicative signals combine input from multiple sensory modalities and signal compensation theory predicts that evolutionary shifts in one sensory modality could impact the response to signals in other sensory modalities. Here, we conducted two types of field experiments with 11 species spread across the lizard genus Sceloporus to test the hypothesis that the loss of visual signal elements affects behavioural responses to a chemical signal (conspecific scents) or to a predominantly visual signal (a conspecific lizard), both of which are used in intraspecific communication. We found that three species that have independently lost a visual signal trait, a colourful belly patch, responded to conspecific scents with increased chemosensory behaviour compared to a chemical control, while species with the belly patch did not. However, most species, with and without the belly patch, responded to live conspecifics with increased visual displays of similar magnitude. While aggressive responses to visual stimuli are taxonomically widespread in Sceloporus, our results suggest that increased chemosensory response behaviour is linked to colour patch loss. Thus, interactions across sensory modalities could constrain the evolution of complex signalling phenotypes, thereby influencing signal diversity.

Does the addition of a new signalling trait enhance receiver responses in diurnal geckos?

Animal signals in multiple modalities expands the opportunity for effective communication. Among diurnal geckos of the genus Cnemaspis, chemical signalling traits preceded the evolution of visual traits. Males of all species possess chemical secreting ventral glands, but only in some species, males also express yellow gular patches. This difference in the expression of unimodal or multimodal signalling traits between closely related species provided us with an opportunity to understand the use of multimodal signals for communication. We studied receiver responses in Cnemaspis indica, a sexually monochromatic species, and in C. littoralis, a species where males possess yellow gulars. We performed behavioural trials where individuals of each species were exposed to only chemical stimuli, only visual stimuli, or both chemical and visual stimuli simultaneously from male and female conspecifics. Our results show that only chemical stimuli were necessary and sufficient to elicit responses in males and females of C. indica as well as in females of C. littoralis. However, males of the dimorphic C. littoralis required the multimodal stimulus to elicit movement-based responses. Our results suggest that the evolution of colour traits in diurnal geckos is associated with a partial shift in some receiver responses toward multimodal communication, with no addition to the behavioural repertoire.

NeoR, a near-infrared absorbing rhodopsin

The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Here, we show that RGC1 and RGC2 function as light-activated cyclases only upon heterodimerization with RGC3 (NeoR). RGC1/2 utilize conventional green or blue-light-sensitive rhodopsins (λmax = 550 and 480 nm, respectively), with short-lived signaling states, responsible for light-activation of the enzyme. The bistable NeoR is photoswitchable between a near-infrared-sensitive (NIR, λmax = 690 nm) highly fluorescent state (QF = 0.2) and a UV-sensitive non-fluorescent state, thereby modulating the activity by NIR pre-illumination. No other rhodopsin has been reported so far to be functional as a heterooligomer, or as having such a long wavelength absorption or high fluorescence yield. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors.

The Power of Mantis Shrimp Strikes: Interdisciplinary Impacts of an Extreme Cascade of Energy Release

In the course of a single raptorial strike by a mantis shrimp (Stomatopoda), the stages of energy release span six to seven orders of magnitude of duration. To achieve their mechanical feats of striking at the outer limits of speeds, accelerations, and impacts among organisms, they use a mechanism that exemplifies a cascade of energy release-beginning with a slow and forceful, spring-loading muscle contraction that lasts for hundreds of milliseconds and ending with implosions of cavitation bubbles that occur in nanoseconds. Mantis shrimp use an elastic mechanism built of exoskeleton and controlled with a latching mechanism. Inspired by both their mechanical capabilities and evolutionary diversity, research on mantis shrimp strikes has provided interdisciplinary and fundamental insights to the fields of elastic mechanisms, fluid dynamics, evolutionary dynamics, contest dynamics, the physics of fast, small systems, and the rapidly-expanding field of bioinspired materials science. Even with these myriad connections, numerous discoveries await, especially in the arena of energy flow through materials actuating and controlling fast, impact fracture resistant systems.

Songs versus colours versus horns: what explains the diversity of sexually selected traits?

Papers on sexual selection often highlight the incredible diversity of sexually selected traits across animals. Yet, few studies have tried to explain why this diversity evolved. Animals use many different types of traits to attract mates and outcompete rivals, including colours, songs, and horns, but it remains unclear why, for example, some taxa have songs, others have colours, and others horns. Here, we first conduct a systematic survey of the basic diversity and distribution of different types of sexually selected signals and weapons across the animal Tree of Life. Based on this survey, we describe seven major patterns in trait diversity and distributions. We then discuss 10 unanswered questions raised by these patterns, and how they might be addressed. One major pattern is that most types of sexually selected signals and weapons are apparently absent from most animal phyla (88%), in contrast to the conventional wisdom that a diversity of sexually selected traits is present across animals. Furthermore, most trait diversity is clustered in Arthropoda and Chordata, but only within certain clades. Within these clades, many different types of traits have evolved, and many types appear to have evolved repeatedly. By contrast, other major arthropod and chordate clades appear to lack all or most trait types, and similar patterns are repeated at smaller phylogenetic scales (e.g. within insects). Although most research on sexual selection focuses on female choice, we find similar numbers of traits (among sampled species) are involved in male contests (44%) and female choice (55%). Overall, these patterns are largely unexplained and unexplored, as are many other fundamental questions about the evolution of these traits. We suggest that understanding the diversity of sexually selected traits may require a shift towards macroevolutionary studies at relatively deep timescales (e.g. tens to hundreds of millions of years ago).

Differences in signal contrast and camouflage among different colour variations of a stomatopod crustacean, Neogonodactylus oerstedii

Animal colouration is often a trade-off between background matching for camouflage from predators, and conspicuousness for communication with con- or heterospecifics. Stomatopods are marine crustaceans known to use colour signals during courtship and contests, while their overall body colouration may provide camouflage. However, we have little understanding of how stomatopods perceive these signals in their environment or whether overall body coloration does provide camouflage from predators. Neogonodactylus oerstedii assess meral spot colour during contests, and meral spot colour varies depending on local habitat. By calculating quantum catch for N. oerstedii's 12 photoreceptors associated with chromatic vision, we found that variation in meral spot total reflectance does not function to increase signal contrast in the local habitat. Neogonodactylus oerstedii also show between-habitat variation in dorsal body colouration. We used visual models to predict a trichromatic fish predator's perception of these colour variations. Our results suggest that sandy and green stomatopods are camouflaged from a typical fish predator in rubble fields and seagrass beds, respectively. To our knowledge, this is the first study to investigate signal contrast and camouflage in a stomatopod. These results provide new insight into the function and evolution of colouration in a species with a complex visual system.

Meral-Spot Reflectance Signals Weapon Performance in the Mantis Shrimp Neogonodactylus oerstedii (Stomatopoda)

During animal contests over resources, opponents often signal their fighting ability in an attempt to avoid escalating to physical attack. A reliable signal is beneficial to receivers because it allows them to avoid injuries from engaging in contests they are unlikely to win. However, a signaler could benefit from deceiving an opponent by signaling greater fighting ability or greater aggressive intent than the signaler possesses. Therefore, the reliability of agonistic signals has long intrigued researchers. We investigated whether a colored patch, the meral spot, signals weapon performance in the stomatopod Neogonodactylus oerstedii. During fights over possession of refuges, stomatopods can injure or even kill opponents with their ultrafast strike. We found that darker meral spots correlate with higher strike impulse, which reflects the total force integrated over time. Furthermore, we demonstrate that stomatopods that strike more often with both appendages have darker meral spots and that the first hit in a two-appendage strike has a greater mean strike impulse than that of a single-appendage strike. This indicates that stomatopods with darker meral spots tend to invest more energy in each strike. Our results provide evidence that stomatopods use total reflectance as an honest signal of weapon performance or aggressive intent. This improves our understanding of the evolution of agonistic signals.

Behavioural evidence for polychromatic ultraviolet sensitivity in mantis shrimp

Stomatopod crustaceans are renowned for their elaborate visual systems. Their eyes contain a plethora of photoreceptors specialized for chromatic and polarization detection, including several that are sensitive to varying wavelength ranges and angles of polarization within the ultraviolet (UV) range (less than 400 nm). Behavioural experiments have previously suggested that UV photoreception plays a role in stomatopod communication, but these experiments have only manipulated the entire UV range. Here, using a behavioural approach, we examine UV vision in the stomatopod Haptosquilla trispinosa Using binary trained choice assays as well as innate burrow-choice experiments, we assessed the ability of H. trispinosa to detect and respond to narrow-band LED stimuli peaking near 314 nm (UVB) versus 379 nm (UVA) in wavelength. We find that H. trispinosa can discriminate these stimuli and appears to display an aversive reaction to UVB light, suggesting segregated behavioural responses to stimuli within the UV range. Furthermore, we find that H. trispinosa can discriminate stimuli peaking near 379 nm versus 351 nm in wavelength, suggesting that their wavelength discrimination in the UV is comparable to their performance in the human-visible range.