The rostral micro-tooth morphology of blue marlin, Makaira nigricans.

Billfish rostra potentially have several functions; however, their role in feeding is unequivocal in some species. Recent work linked morphological variation in rostral micro-teeth to differences in feeding behavior in two billfish species, the striped marlin (Kajikia audax) and the sailfish (Istiophorus platypterus). Here, we present the rostral micro-tooth morphology for a third billfish species, the blue marlin (Makaira nigricans), for which the use of the rostrum in feeding behavior is still undocumented from systematic observations in the wild. We measured the micro-teeth on rostrum tips of blue marlin, striped marlin, and sailfish using a micro-computed tomography approach and compared the tooth morphology among the three species. This was done after an analysis of video-recorded hunting behavior of striped marlin and sailfish revealed that both species strike prey predominantly with the first third of the rostrum, which provided the justification to focus our analysis on the rostrum tips. In blue marlin, intact micro-teeth were longer compared to striped marlin but not to sailfish. Blue marlin had a higher fraction of broken teeth than both striped marlin and sailfish, and broken teeth were distributed more evenly on the rostrum. Micro-tooth regrowth was equally low in both marlin species but higher in sailfish. Based on the differences and similarities in the micro-tooth morphology between the billfish species, we discuss potential feeding-related rostrum use in blue marlin. We put forward the hypothesis that blue marlin might use their rostra in high-speed dashes as observed in striped marlin, rather than in the high-precision rostral strikes described for sailfish, possibly focusing on larger prey organisms.

Kinematics and behaviour in fish escape responses: guidelines for conducting, analysing and reporting experiments.

Work carried out since the late 1970s has provided key insights into the comparative biomechanics, kinematics, behaviour and neurobiology of fish escape responses. An escape response is an ecologically important behaviour used by fishes to evade predation and aggression via rapid swimming movements. With environmental change expected to affect the physiology and biomechanics of aquatic ectotherms, there is a growing interest in understanding how environmental stressors affect the swimming performance and behaviour of fishes during escape responses, particularly in the context of predator-prey interactions. As the study of fish swimming continues to expand, there have been repeated calls to standardise experiments and reporting practices to facilitate integrative and comparative studies. Here, we provide a set of practical guidelines for conducting, analysing and reporting experiments on escape responses in fish, including a reporting checklist to assist authors undertaking these experiments. These resources will facilitate executing and reporting escape response experiments in a rigorous and transparent fashion, helping to advance the study of fish swimming in an era of rapid environmental change.

Predator-informed looming stimulus experiments reveal how large filter feeding whales capture highly maneuverable forage fish.

The unique engulfment filtration strategy of microphagous rorqual whales has evolved relatively recently (<5 Ma) and exploits extreme predator/prey size ratios to overcome the maneuverability advantages of swarms of small prey, such as krill. Forage fish, in contrast, have been engaged in evolutionary arms races with their predators for more than 100 million years and have performance capabilities that suggest they should easily evade whale-sized predators, yet they are regularly hunted by some species of rorqual whales. To explore this phenomenon, we determined, in a laboratory setting, when individual anchovies initiated escape from virtually approaching whales, then used these results along with in situ humpback whale attack data to model how predator speed and engulfment timing affected capture rates. Anchovies were found to respond to approaching visual looming stimuli at expansion rates that give ample chance to escape from a sea lion-sized predator, but humpback whales could capture as much as 30-60% of a school at once because the increase in their apparent (visual) size does not cross their prey's response threshold until after rapid jaw expansion. Humpback whales are, thus, incentivized to delay engulfment until they are very close to a prey school, even if this results in higher hydrodynamic drag. This potential exaptation of a microphagous filter feeding strategy for fish foraging enables humpback whales to achieve 7× the energetic efficiency (per lunge) of krill foraging, allowing for flexible foraging strategies that may underlie their ecological success in fluctuating oceanic conditions.

Escaping from multiple visual threats: modulation of escape responses in Pacific staghorn sculpin (Leptocottus armatus).

Fish perform rapid escape responses to avoid sudden predatory attacks. During escape responses, fish bend their bodies into a C-shape and quickly turn away from the predator and accelerate. The escape trajectory is determined by the initial turn (stage 1) and a contralateral bend (stage 2). Previous studies have used a single threat or model predator as a stimulus. In nature, however, multiple predators may attack from different directions simultaneously or in close succession. It is unknown whether fish are able to change the course of their escape response when startled by multiple stimuli at various time intervals. Pacific staghorn sculpin (Leptocottus armatus) were startled with a left and right visual stimulus in close succession. By varying the timing of the second stimulus, we were able to determine when and how a second stimulus could affect the escape response direction. Four treatments were used: a single visual stimulus (control); or two stimuli coming from opposite sides separated by a 0 ms (simultaneous treatment), 33 ms or 83 ms time interval. The 33 ms and 83 ms time intervals were chosen to occur either side of a predicted 60 ms visual escape latency (i.e. during stage 1). The 0 ms and 33 ms treatments influenced both the escape trajectory and the stage 1 turning angle, compared with a single stimulation, whereas the 83 ms treatment had no effect on the escape trajectory. We conclude that Pacific staghorn sculpin can modulate their escape trajectory only between stimulation and the onset of the response, but the escape trajectory cannot be modulated after the body motion has started.

Escape response kinematics in two species of tropical shark: short escape latencies and high turning performance.

Accelerative manoeuvres, such as fast-starts, are crucial for fish to avoid predation. Escape responses are fast-starts that include fundamental survival traits for prey that experience high predation pressure. However, no previous study has assessed escape performance in neonate tropical sharks. We quantitatively evaluated vulnerability traits of neonate tropical sharks by testing predictions on their fast-start escape performance. We predicted (1) high manoeuvrability, given their high flexibility, but (2) low propulsive locomotion owing to the drag costs associated with pectoral fin extension during escape responses. Further, based on previous work on dogfish, Squalus suckleyi, we predicted (3) long reaction times (as latencies longer than teleosts, >20 ms). We used two-dimensional, high-speed videography analysis of mechano-acoustically stimulated neonate blacktip reef shark, Carcharhinus melanopterus (n=12), and sicklefin lemon shark, Negaprion acutidens (n=8). Both species performed a characteristic C-start double-bend response (i.e. two body bends), but single-bend responses were only observed in N. acutidens. As predicted, neonate sharks showed high manoeuvrability with high turning rates and tight turning radii (3-11% of body length) but low propulsive performance (i.e. speed, acceleration and velocity) when compared with similar-sized teleosts and S. suckleyi. Contrary to expectations, escape latencies were <20 ms in both species, suggesting that the neurophysiological system of sharks when reacting to a predatory attack may not be limited to long response times. These results provide a quantitative assessment of survival traits in neonate tropical sharks that will be crucial for future studies that consider the vulnerability of these sharks to predation.

Lacunae rostralis: A new structure on the rostrum of sailfish Istiophorus platypterus.

Recent comparative studies of billfishes (Istiophoridae and Xiphiidae) have provided evidence of differences in the form and function of the rostra (bill) among species. Here, we report the discovery of a new structure, lacuna rostralis, on the rostra of sailfish Istiophorus platypterus, which is absent on the rostra of swordfish Xiphias gladius, striped marlin Kajikia audax and blue marlin Makaira nigricans. The lacunae rostralis are small cavities that contain teeth. They were found on the ventral rostrum surface of all I. platypterus specimens examined and dorsally in half of them. Ventrally, the lacunae rostralis were most prominent in the mid-section of the rostrum. Dorsally, they occurred closer to the tip. The density of lacunae rostralis increased towards the rostrum tip but, because they are smaller in size, the percentage of rostrum coverage decreased. The teeth located within the lacunae rostralis were found to be different in size, location and orientation from the previously identified micro-teeth of billfish. We propose two potential functions of the lacunae rostralis that both relate to the use of the bill in feeding: mechanoreception of prey before tapping it with the bill and more efficient prey handling via the creation of suction, or physical grip.

Age and sex influence social interactions, but not associations, within a killer whale pod.

Social structure is a fundamental aspect of animal populations. In order to understand the function and evolution of animal societies, it is important to quantify how individual attributes, such as age and sex, shape social relationships. Detecting these influences in wild populations under natural conditions can be challenging, especially when social interactions are difficult to observe and broad-scale measures of association are used as a proxy. In this study, we use unoccupied aerial systems to observe association, synchronous surfacing, and physical contact within a pod of southern resident killer whales (Orcinus orca). We show that interactions do not occur randomly between associated individuals, and that interaction types are not interchangeable. While age and sex did not detectably influence association network structure, both interaction networks showed significant social homophily by age and sex, and centrality within the contact network was higher among females and young individuals. These results suggest killer whales exhibit interesting parallels in social bond formation and social life histories with primates and other terrestrial social mammals, and demonstrate how important patterns can be missed when using associations as a proxy for interactions in animal social network studies.

Latency of mechanically stimulated escape responses in the Pacific spiny dogfish, Squalus suckleyi.

Fast escape responses to a predator threat are fundamental to the survival of mobile marine organisms. However, elasmobranchs are often underrepresented in such studies. Here, we measured the escape latency (time interval between the stimulus and first visible reaction) of mechanically induced escape responses in the Pacific spiny dogfish, Squalus suckleyi, and in two teleosts from the same region, the great sculpin, Myoxocephalus polyacanthocephalus, and the pile perch, Rhacochilus vacca We found that the dogfish had a longer minimum latency (66.7 ms) compared with that for the great sculpin (20.8 ms) and pile perch (16.7 ms). Furthermore, the dogfish had a longer latency than that of 48 different teleosts identified from 35 different studies. We suggest such long latencies in dogfish may be due to the absence of Mauthner cells, the giant neurons that control fast escape responses in fishes.

Escape responses of fish: a review of the diversity in motor control, kinematics and behaviour.

The study of fish escape responses has provided important insights into the accelerative motions and fast response times of these animals. In addition, the accessibility of the underlying neural circuits has made the escape response a fundamental model in neurobiology. Fish escape responses were originally viewed as highly stereotypic all-or-none behaviours. However, research on a wide variety of species has shown considerable taxon-specific and context-dependent variability in the kinematics and neural control of escape. In addition, escape-like motions have been reported: these resemble escape responses kinematically, but occur in situations that do not involve a response to a threatening stimulus. This Review focuses on the diversity of escape responses in fish by discussing recent work on: (1) the types of escape responses as defined by kinematic analysis (these include C- and S-starts, and single- versus double-bend responses); (2) the diversity of neuromuscular control; (3) the variability of escape responses in terms of behaviour and kinematics within the context of predator-prey interactions; and (4) the main escape-like motions observed in various species. Here, we aim to integrate recent knowledge on escape responses and highlight rich areas for research. Rapidly developing approaches for studying the kinematics of swimming motion both in the lab and within the natural environment provide new avenues for research on these critical and common behaviours.

Are all bony fishes oxygen regulators? Evidence for oxygen regulation in a putative oxygen conformer, the swamp eel Synbranchus marmoratus.

This study investigated the oxygen consumption of the putative oxygen conformer marbled swamp eel Synbranchus marmoratus during progressive hypoxia. Earlier studies have not reached an agreement on whether S. marmoratus is a conformer or a regulator. Our results support the view that S. marmoratus is an oxygen regulator, like most bony fishes.

Turbulent flow reduces oxygen consumption in the labriform swimming shiner perch, Cymatogaster aggregata.

Fish swimming energetics are often measured in laboratory environments which attempt to minimize turbulence, though turbulent flows are common in the natural environment. To test whether the swimming energetics and kinematics of shiner perch, Cymatogaster aggregata (a labriform swimmer), were affected by turbulence, two flow conditions were constructed in a swim-tunnel respirometer. A low-turbulence flow was created using a common swim-tunnel respirometry setup with a flow straightener and fine-mesh grid to minimize velocity fluctuations. A high-turbulence flow condition was created by allowing large velocity fluctuations to persist without a flow straightener or fine grid. The two conditions were tested with particle image velocimetry to confirm significantly different turbulence properties throughout a range of mean flow speeds. Oxygen consumption rate of the swimming fish increased with swimming speed and pectoral fin beat frequency in both flow conditions. Higher turbulence also caused a greater positional variability in swimming individuals (versus low-turbulence flow) at medium and high speeds. Surprisingly, fish used less oxygen in high-turbulence compared with low-turbulence flow at medium and high swimming speeds. Simultaneous measurements of swimming kinematics indicated that these reductions in oxygen consumption could not be explained by specific known flow-adaptive behaviours such as Kármán gaiting or entraining. Therefore, fish in high-turbulence flow may take advantage of the high variability in turbulent energy through time. These results suggest that swimming behaviour and energetics measured in the lab in straightened flow, typical of standard swimming respirometers, might differ from that of more turbulent, semi-natural flow conditions.

Warming has a greater effect than elevated CO2 on predator-prey interactions in coral reef fish.

Ocean acidification and warming, driven by anthropogenic CO2 emissions, are considered to be among the greatest threats facing marine organisms. While each stressor in isolation has been studied extensively, there has been less focus on their combined effects, which could impact key ecological processes. We tested the independent and combined effects of short-term exposure to elevated CO2 and temperature on the predator-prey interactions of a common pair of coral reef fishes (Pomacentrus wardi and its predator, Pseudochromis fuscus). We found that predator success increased following independent exposure to high temperature and elevated CO2 Overall, high temperature had an overwhelming effect on the escape behaviour of the prey compared with the combined exposure to elevated CO2 and high temperature or the independent effect of elevated CO2 Exposure to high temperatures led to an increase in attack and predation rates. By contrast, we observed little influence of elevated CO2 on the behaviour of the predator, suggesting that the attack behaviour of P. fuscus was robust to this environmental change. This is the first study to address how the kinematics and swimming performance at the basis of predator-prey interactions may change in response to concurrent exposure to elevated CO2 and high temperatures and represents an important step to forecasting the responses of interacting species to climate change.

Effects of elevated carbon dioxide and temperature on locomotion and the repeatability of lateralization in a keystone marine mollusc.

Recent work has shown that the behaviour of marine organisms can be affected by elevated PCO2 , although little is known about the effect of multiple stressors. We therefore investigated the effect of elevated PCO2  and temperature on locomotion and behaviour during prey searching in the marine gastropod Concholepas concholepas, a predator characteristic of the southeastern Pacific coast. Movement duration, decision time, route finding and lateralization were measured using a T-maze tank with a prey positioned behind a barrier. Four treatments, representing present day and near-future scenarios of ocean acidification and warming were used in rearing the individuals for 6 months. Regardless of the treatment, no significant differences were found in relative and absolute lateralization before and after exposure for 6 months. However, relative lateralization was not repeatable for animals tested after 6 months at elevated PCO2  at both experimental temperatures, whereas it was repeatable in individuals kept at the present day level of PCO2 We suggest that these effects may be related to a behavioural malfunction caused by elevated PCO2 Movement duration, decision time and route finding were not repeatable. However, movement duration and decision time increased and route finding decreased in elevated PCO2  (at 15°C), suggesting that elevated PCO2  has negative effects on the locomotor and sensory performance of C. concholepas in the presence of a prey odour, thereby decreasing their ability to forage efficiently.

Proto-cooperation: group hunting sailfish improve hunting success by alternating attacks on grouping prey.

We present evidence of a novel form of group hunting. Individual sailfish (Istiophorus platypterus) alternate attacks with other group members on their schooling prey (Sardinella aurita). While only 24% of attacks result in prey capture, multiple prey are injured in 95% of attacks, resulting in an increase of injured fish in the school with the number of attacks. How quickly prey are captured is positively correlated with the level of injury of the school, suggesting that hunters can benefit from other conspecifics' attacks on the prey. To explore this, we built a mathematical model capturing the dynamics of the hunt. We show that group hunting provides major efficiency gains (prey caught per unit time) for individuals in groups of up to 70 members. We also demonstrate that a free riding strategy, where some individuals wait until the prey are sufficiently injured before attacking, is only beneficial if the cost of attacking is high, and only then when waiting times are short. Our findings provide evidence that cooperative benefits can be realized through the facilitative effects of individuals' hunting actions without spatial coordination of attacks. Such 'proto-cooperation' may be the pre-cursor to more complex group-hunting strategies.

Eye movements are coordinated with pectoral fin beats during locomotion in a marine teleost fish.

Animals must simultaneously engage multiple functional systems in order to navigate, feed and survive in complex environments. Nearly all vertebrates perform rapid gaze-shifting eye movements called saccades, but we know little about the behaviour of saccades during rhythmic locomotion. This study examined how saccades are coordinated with locomotor movements in a pectoral-fin-propelled teleost fish, Cymatogaster aggregata, the shiner surfperch. Individual fish were filmed swimming in a flow tank at 10 cm s(-1), and timing data were analysed using circular statistics. The results reveal that C. aggregata generates saccades non-uniformly throughout the pectoral fin cycle. Saccades primarily occur during fin abduction, when a large amount of thrust is produced, and rarely occur during the thrust-free refractory phase. Because vision is known to be impaired during saccades, we hypothesize that C. aggregata synchronizes saccades with periods of high acceleration in order to stabilize retinal images during low-acceleration phases, which are nearly saccade-free.

Marine mollusc predator-escape behaviour altered by near-future carbon dioxide levels.

Ocean acidification poses a range of threats to marine invertebrates; however, the potential effects of rising carbon dioxide (CO2) on marine invertebrate behaviour are largely unknown. Marine gastropod conch snails have a modified foot and operculum allowing them to leap backwards rapidly when faced with a predator, such as a venomous cone shell. Here, we show that projected near-future seawater CO2 levels (961 µatm) impair this escape behaviour during a predator-prey interaction. Elevated-CO2 halved the number of snails that jumped from the predator, increased their latency to jump and altered their escape trajectory. Physical ability to jump was not affected by elevated-CO2 indicating instead that decision-making was impaired. Antipredator behaviour was fully restored by treatment with gabazine, a GABA antagonist of some invertebrate nervous systems, indicating potential interference of neurotransmitter receptor function by elevated-CO2, as previously observed in marine fishes. Altered behaviour of marine invertebrates at projected future CO2 levels could have potentially far-reaching implications for marine ecosystems.

Parental effects improve escape performance of juvenile reef fish in a high-CO2 world.

Rising CO2 levels in the oceans are predicted to have serious consequences for many marine taxa. Recent studies suggest that non-genetic parental effects may reduce the impact of high CO2 on the growth, survival and routine metabolic rate of marine fishes, but whether the parental environment mitigates behavioural and sensory impairment associated with high CO2 remains unknown. Here, we tested the acute effects of elevated CO2 on the escape responses of juvenile fish and whether such effects were altered by exposure of parents to increased CO2 (transgenerational acclimation). Elevated CO2 negatively affected the reactivity and locomotor performance of juvenile fish, but parental exposure to high CO2 reduced the effects in some traits, indicating the potential for acclimation of behavioural impairment across generations. However, acclimation was not complete in some traits, and absent in others, suggesting that transgenerational acclimation does not completely compensate the effects of high CO2 on escape responses.

Severe hypoxia impairs lateralization in a marine teleost fish.

In intertidal environments, the recurring hypoxic condition at low tide is one of the main factors affecting fish behaviour, causing broad effects on ecological interactions. We assessed the effects of hypoxia on lateralization (e.g. the tendency to turn left or right), a behaviour related to brain functional asymmetry, which is thought to play a key role in several life history aspects of fish. Using staghorn sculpin (Leptocottus armatus), a benthic fish that typically inhabits the intertidal zone, we found that hypoxia affects behavioural lateralization at the population level. On average, staghorn sculpins showed a distinct preference for right turns under normoxic conditions (>90% oxygen saturation), but an equal probability of turning right or left after exposure to hypoxia for 2 h (20% oxygen saturation). The specific turning preference observed in the staghorn sculpin control population is likely to have an adaptive value, for example in predator-prey interactions by enhancing attack success or survival from predatory attacks. Therefore the alteration of lateralization expressed by staghorn sculpins under hypoxic conditions may have far-reaching implications for species ecology and trophic interactions. Moreover, our work raises the need to study this effect in other species, in which a hypoxia-driven disruption of lateralization could affect a wider range of behaviours, such as social interactions and schooling.

The interplay of group size and flow velocity modulates fish exploratory behaviour.

Social facilitation is a well-known phenomenon where the presence of organisms belonging to the same species enhances an individual organism's performance in a specific task. As far as fishes are concerned, most studies on social facilitation have been conducted in standing-water conditions. However, for riverine species, fish are most commonly located in moving waters, and the effects of hydrodynamics on social facilitation remain largely unknown. To bridge this knowledge gap, we designed and performed flume experiments where the behaviour of wild juvenile Italian riffle dace (Telestes muticellus) in varying group sizes and at different mean flow velocities, was studied. An artificial intelligence (AI) deep learning algorithm was developed and employed to track fish positions in time and subsequently assess their exploration, swimming activity, and space use. Results indicate that energy-saving strategies dictated space use in flowing waters regardless of group size. Instead, exploration and swimming activity increased by increasing group size, but the magnitude of this enhancement (which quantifies social facilitation) was modulated by flow velocity. These results have implications for how future research efforts should be designed to understand the social dynamics of riverine fish populations, which can no longer ignore the contribution of hydrodynamics.