Taking the plunge: California Grunion embryos emerge rapidly with environmentally cued hatching

Effects of Light and Water Agitation on Hatching Processes in False Clownfish Amphiprion ocellaris

False clownfish (Amphiprion ocellaris) employ a hatching strategy regulated by environmental cues, wherein parents provide water flow to encourage embryos to hatch after sunset on the hatching day. Despite previous studies demonstrating the necessity of complete darkness and water agitation for hatching, the regulatory mechanisms underlying these environmental cues remain elusive. This study aimed to investigate how darkness and water agitation affect the secretion of hatching enzymes and the hatching movements of embryos in false clownfish. Assessment of chorion digestion and live imaging of Ca2+ in hatching glands using GCaMP6s, a Ca2+ indicator, revealed that darkness stimulation triggers the secretion of hatching enzymes by increasing Ca2+ levels in hatching gland cells. On the other hand, water agitation primarily stimulated hatching movements in embryos, which led to the rupture of their egg envelopes. These results suggest that changes in light environments following sunset induce embryos to secrete hatching enzymes and that water agitation provided by parents stimulates hatching movements. These responses to environmental cues, light and water agitation, contribute to the rapid and synchronous hatching in false clownfish.

Egg-Clutch Biomechanics Affect Escape-Hatching Behavior and Performance

Arboreal embryos of phyllomedusine treefrogs hatch prematurely to escape snake predation, cued by vibrations in their egg clutches during attacks. However, escape success varies between species, from ∼77% in Agalychnis callidryas to just ∼9% in A. spurrelli at 1 day premature. Both species begin responding to snake attacks at similar developmental stages, when vestibular mechanosensory function begins, suggesting that sensory ability does not limit the hatching response in A. spurrelli. Agalychnis callidryas clutches are thick and gelatinous, while A. spurrelli clutches are thinner and stiffer. We hypothesized that this structural difference alters the egg motion excited by attacks. Since vibrations excited by snakes must propagate through clutches to reach embryos, we hypothesized that the species difference in attack-induced hatching may reflect effects of clutch biomechanics on the cues available to embryos. Mechanics predicts that thinner, stiffer structures have higher free vibration frequencies, greater spatial attenuation, and faster vibration damping than thicker, more flexible structures. We assessed clutch biomechanics by embedding small accelerometers in clutches of both species and recording vibrations during standardized excitation tests at two distances from the accelerometer. Analyses of recorded vibrations showed that A. spurrelli clutches have higher free vibration frequencies and greater vibration damping than A. callidryas clutches. Higher frequencies elicit less hatching in A. callidryas, and greater damping could reduce the amount of vibration embryos can perceive. To directly test if clutch structure affects escape success in snake attacks, we transplanted A. spurrelli eggs into A. callidryas clutches and compared their escape rates with untransplanted, age-matched conspecific controls. We also performed reciprocal transplantation of eggs between pairs of A. callidryas clutches as a method control. Transplanting A. spurrelli embryos into A. callidryas clutches nearly tripled their escape success (44%) compared to conspecific controls (15%), whereas transplanting A. callidryas embryos into different A. callidryas clutches only increased escape success by 10%. At hatching competence, A. callidryas eggs are no longer jelly-encapsulated, while A. spurrelli eggs retain their jelly coat. Therefore, we compared the hatching response and latency of A. spurrelli in de-jellied eggs and their control, jelly-encapsulated siblings using manual egg-jiggling to simulate predation cues. Embryos in de-jellied eggs were more likely to hatch and hatched faster than control siblings. Together, our results suggest that the properties of parentally produced egg-clutch structures, including their vibration biomechanics, constrain the information available to A. spurrelli embryos and contribute to interspecific differences in hatching responses to predator attacks.

Chronic Exposure to Environmentally Relevant Concentrations of Imidacloprid Impact Survival and Ecologically Relevant Behaviors of Fathead Minnow Larvae

Imidacloprid (IM) has emerged as a contaminant of concern in several areas within the United States due to its frequent detection in aquatic ecosystems and its pseudo-persistence, which pose potential risks to nontarget species. We evaluated the sublethal toxicity of IM to fathead minnow larvae following chronic exposure beginning just after fertilization. Our in silico analysis and in vivo bioassays suggest that IM has a low binding affinity for the vertebrate nicotinate acetylcholine receptor (nAChR), as expected. However, chronic exposure to ≥0.16 µg IM/L reduced survival by 10%, and exposure to ≥18 µg IM/L reduced survival by approximately 20%-40%. Surviving fish exposed to ≥0.16 µg IM/L showed reduced growth, altered embryonic motor activity, and premature hatching. Furthermore, a significant proportion of fish exposed to ≥0.16 µg IM/L were slower to respond to vibrational stimuli and slower to swim away, indicating that chronic exposure to IM has the potential to impair the ability of larvae to escape predation. The adverse health effects we observed indicate that chronic exposure to environmentally relevant concentrations of IM may elicit sublethal responses that culminate in a significant increase in mortality during early life stages, ultimately translating to reduced recruitment in wild fish populations. Environ Toxicol Chem 2023;42:2184-2192. © 2023 SETAC.

Embryonic exposure to predation risk and hatch time variation in fathead minnows

Organisms are exposed to a wealth of chemical information during their development. Some of these chemical cues indicate present or future dangers, such as the presence of predators that feed on either the developing embryos or their nearby parents. Organisms may use this information to modify their morphology or life-history, including hatching timing, or may retain information about risk until it gains relevance. Previous research has shown predation-induced alterations in hatching among embryonic minnows that were exposed to mechanical-injury-released alarm cues from conspecific embryos. Here, we test whether minnows likewise hatch early in response to alarm cues from injured adult conspecifics. We know that embryonic minnows can detect adult alarm cues and use them to facilitate learned recognition of predators; however, it is unknown whether these adult alarm cues will also induce a change in hatching time. Early hatching may allow animals to rapidly disperse away from potential predators, but late hatching may allow animals to grow and develop structures that allow them to effectively escape when they do hatch. Here, we found here that unlike embryonic fathead minnows (Pimephales promelas) exposed to embryonic cues, embryonic minnows exposed to adult alarm cues do not exhibit early hatching. The ability of embryos to recognize adult alarm cues as a future threat, but not a current one, demonstrates sophisticated ontogenetic specificity in the hatching response of embryonic minnows.

Ranking the ecological effects of coastal armoring on mobile macroinvertebrates across intertidal zones on sandy beaches

Coastal armoring is widely applied to coastal ecosystems, such as sandy beaches, in response to shoreline erosion and threats to infrastructure. Use of armoring is expected to increase due to coastal population growth and effects of climate change. An increased understanding of armoring effects on those ecosystems and the services they provide is needed for impact assessments and the design of these structures. We investigated the following hypotheses: 1) impacts of coastal armoring on beach macroinvertebrates increase from lower to upper intertidal zones and 2) location of an armoring structure on beach profiles affects the number of intertidal zones, using comparative surveys of armored and unarmored beach sections in Chile and California. The effects of armoring were greater for upper intertidal (talitrid amphipods) and mid-intertidal species (cirolanid isopods) than for lower shore fauna (hippid crabs). Our surveys of sections of armoring structures located higher and lower on the beach profile (with and without interactions with waves and tides), showed loss of upper zone talitrid amphipods and mid-zone isopods and a reduction of lower zone hippid crabs in sections where the structures were lower on the beach profile and interacted with waves, compared to non-interacting sections. Our results support the hypothesis that impacts of armoring on intertidal macroinvertebrates increase from the lower to the upper intertidal zones of sandy beaches and also suggest that the relative position of an armoring structure on the beach profile, determines the number of intertidal zones it affects. Our findings also imply that by altering the position of existing armoring structures on the shore profile and increasing the amount of interaction with waves and tides, sea level rise and regional factors, such as coseismic coastal subsidence, can be expected to exacerbate the impacts of these widely used coastal defense structures on sandy beach ecosystems.

The Effect of an Anthropogenic Magnetic Field on the Early Developmental Stages of Fishes-A Review

The number of sources of anthropogenic magnetic and electromagnetic fields generated by various underwater facilities, industrial equipment, and transferring devices in aquatic environment is increasing. These have an effect on an array of fish life processes, but especially the early developmental stages. The magnitude of these effects depends on field strength and time of exposure and is species-specific. We review studies on the effect of magnetic fields on the course of embryogenesis, with special reference to survival, the size of the embryos, embryonic motor function, changes in pigment cells, respiration hatching, and directional reactions. We also describe the effect of magnetic fields on sperm motility and egg activation. Magnetic fields can exert positive effects, as in the case of the considerable extension of sperm capability of activation, or have a negative influence in the form of a disturbance in heart rate or developmental instability in inner ear organs.

Multimodal mechanosensing enables treefrog embryos to escape egg-predators

Mechanosensory-cued hatching (MCH) is widespread, diverse and important for survival in many animals. From flatworms and insects to frogs and turtles, embryos use mechanosensory cues and signals to inform hatching timing, yet mechanisms mediating mechanosensing in ovo are largely unknown. The arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, hatch prematurely to escape predation, cued by physical disturbance in snake attacks. When otoconial organs in the developing vestibular system become functional, this response strengthens, but its earlier occurrence indicates another sensor must contribute. Post-hatching, tadpoles use lateral line neuromasts to detect water motion. We ablated neuromast function with gentamicin to assess their role in A. callidryas' hatching response to disturbance. Prior to vestibular function, this nearly eliminated the hatching response to a complex simulated attack cue, egg jiggling, revealing that neuromasts mediate early MCH. Vestibular function onset increased hatching, independent of neuromast function, indicating young embryos use multiple mechanosensory systems. MCH increased developmentally. All older embryos hatched in response to egg jiggling, but neuromast function reduced response latency. In contrast, neuromast ablation had no effect on the timing or level of hatching in motion-only vibration playbacks. It appears only a subset of egg-disturbance cues stimulate neuromasts; thus, embryos in attacked clutches may receive unimodal or multimodal stimuli. Agalychnis callidryas embryos have more neuromasts than described for any other species at hatching, suggesting precocious sensory development may facilitate MCH. Our findings provide insight into the behavioral roles of two mechanosensory systems in ovo and open possibilities for exploring sensory perception across taxa in early life stages.

How do red-eyed treefrog embryos sense motion in predator attacks? Assessing the role of vestibular mechanoreception

The widespread ability to alter timing of hatching in response to environmental cues can serve as a defense against threats to eggs. Arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, can hatch up to 30% prematurely to escape predation. This escape-hatching response is cued by physical disturbance of eggs during attacks, including vibrations or motion, and thus depends critically on mechanosensory ability. Predator-induced hatching appears later in development than flooding-induced, hypoxia-cued hatching; thus, its onset is not constrained by the development of hatching ability. It may, instead, reflect the development of mechanosensor function. We hypothesize that vestibular mechanoreception mediates escape-hatching in snake attacks, and that the developmental period when hatching-competent embryos fail to flee from snakes reflects a sensory constraint. We assessed the ontogenetic congruence of escape-hatching responses and an indicator of vestibular function, the vestibulo-ocular reflex (VOR), in three ways. First, we measured VOR in two developmental series of embryos 3-7 days old to compare with the published ontogeny of escape success in attacks. Second, during the period of greatest variation in VOR and escape success, we compared hatching responses and VOR across sibships. Finally, in developmental series, we compared the response of individual embryos to a simulated attack cue with their VOR. The onset of VOR and hatching responses were largely concurrent at all three scales. Moreover, latency to hatch in simulated attacks decreased with increasing VOR. These results are consistent with a key role of the vestibular system in the escape-hatching response of A. callidryas embryos to attacks.

Basagran® induces developmental malformations and changes the bacterial community of zebrafish embryos

This study aimed to assess the effects of Basagran^® on zebrafish (Danio rerio) embryos. The embryos were exposed to Basagran^® at concentrations ranging from 120.0 to 480.6 mg/L, and the effects on embryo development (up to 96 h) and bacterial communities of 96 h-larvae were assessed. The embryo development response was time-dependent and concentration-dependent (106.35 < EC₅₀ < 421.58 mg/L). The sensitivity of embryo-related endpoints decreased as follows: blood clotting in the head and/or around the yolk sac > delay or anomaly in yolk sac absorption > change in swimming equilibrium > development of pericardial and/or yolk sac oedema > scoliosis. A PCR-DGGE analysis was used to evaluate changes in the structure, richness, evenness and diversity of bacterial communities after herbicide exposure. A herbicide-induced structural adjustment of bacterial community was observed. In this study, it was successfully demonstrated that Basagran^® affected zebrafish embryos and associated bacterial communities, showing time-dependent and concentration-dependent embryos' developmental response and structural changes in bacterial community. Thus, this work provides for the first time a complementary approach, which is useful to derive robust toxicity thresholds considering the embryo-microbiota system as a whole. The aquatic hazard assessment will be strengthened by combining current ecotoxicological tests with molecular microbiology tools.

Fish embryos on land: terrestrial embryo deposition lowers oxygen uptake without altering growth or survival in the amphibious fish Kryptolebias marmoratus

Few teleost fishes incubate embryos out of water, but the oxygen-rich terrestrial environment could provide advantages for early growth and development. We tested the hypothesis that embryonic oxygen uptake is limited in aquatic environments relative to air using the self-fertilizing amphibious mangrove rivulus, Kryptolebias marmoratus, which typically inhabits hypoxic, water-filled crab burrows. We found that adult mangrove rivulus released twice as many embryos in terrestrial versus aquatic environments and that air-reared embryos had accelerated developmental rates. Surprisingly, air-reared embryos consumed 44% less oxygen and possessed larger yolk reserves, but attained the same mass, length and chorion thickness. Water-reared embryos moved their opercula ∼2.5 more times per minute compared with air-reared embryos at 7 days post-release, which probably contributed to the higher rates of oxygen uptake and yolk utilization we observed. Genetically identical air- and water-reared embryos from the same parent were raised to maturity, but the embryonic environment did not affect growth, reproduction or emersion ability in adults. Therefore, although aspects of early development were plastic, these early differences were not sustained into adulthood. Kryptolebias marmoratus embryos hatched out of water when exposed to aerial hypoxia. We conclude that exposure to a terrestrial environment reduces the energetic costs of development partly by reducing the necessity of embryonic movements to dispel stagnant boundary layers. Terrestrial incubation of young would be especially beneficial to amphibious fishes that occupy aquatic habitats of poor water quality, assuming low terrestrial predation and desiccation risks.

How embryos escape from danger: the mechanism of rapid, plastic hatching in red-eyed treefrogs

Environmentally cued hatching allows embryos to escape dangers and exploit new opportunities. Such adaptive responses require a flexibly regulated hatching mechanism sufficiently fast to meet relevant challenges. Anurans show widespread, diverse cued hatching responses, but their described hatching mechanisms are slow, and regulation of timing is unknown. Arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, escape from snake attacks and other threats by very rapid premature hatching. We used videography, manipulation of hatching embryos and electron microscopy to investigate their hatching mechanism. High-speed video revealed three stages of the hatching process: pre-rupture shaking and gaping, vitelline membrane rupture near the snout, and muscular thrashing to exit through the hole. Hatching took 6.5–49 s. We hypothesized membrane rupture to be enzymatic, with hatching enzyme released from the snout during shaking. To test this, we displaced hatching embryos to move their snout from its location during shaking. The membrane ruptured at the original snout position and embryos became trapped in collapsed capsules; they either moved repeatedly to relocate the hole or shook again and made a second hole to exit. Electron microscopy revealed that hatching glands are densely concentrated on the snout and absent elsewhere. They are full of vesicles in embryos and release most of their contents rapidly at hatching. Agalychnis callidryas' hatching mechanism contrasts with the slow process described in anurans to date and exemplifies one way in which embryos can achieve rapid, flexibly timed hatching to escape from acute threats. Other amphibians with cued hatching may also have novel hatching mechanisms.

Annual Killifish Transcriptomics and Candidate Genes for Metazoan Diapause

Dormancy has evolved in all major metazoan lineages. It is critical for survival when environmental stresses are not conducive to growth, maturation, or reproduction. Embryonic diapause is a form of dormancy where development is reversibly delayed and metabolism is depressed. We report the diapause transcriptome of the annual killifish Nematolebias whitei, and compare gene expression between diapause embryos and free-living larvae to identify a candidate set of 945 differentially expressed "diapause" genes for this species. Similarity of transcriptional patterns among N. whitei and other diapausing animals is striking for a small set of genes associated with stress resistance, circadian rhythm, and metabolism, while other genes show discordant patterns. Although convergent evolution of diapause may require shared molecular mechanisms for fundamental processes, similar physiological phenotypes also may arise through modification of alternative pathways. Annual killifishes are a tractable model system for comparative transcriptomic studies on the evolution of diapause.

Consequences of induced hatching plasticity depend on predator community

Many prey species face trade-offs in the timing of life history switch points like hatching and metamorphosis. Costs associated with transitioning early depend on the biotic and abiotic conditions found in the subsequent life stage. The red-eyed treefrog, Agalychnis callidryas, faces risks from predators in multiple, successive life stages, and can hatch early in response to mortality threats at the egg stage. Here we tested how the consequences of life history plasticity, specifically early hatching in response to terrestrial egg predators, depend on the assemblage of aquatic larval predators. We predicted that diverse predator assemblages would impose lower total predation pressure than the most effective single predator species and might thereby reduce the costs of hatching early. We then conducted a mesocosm experiment where we crossed hatchling phenotype (early vs. normal hatching) with five larval-predator environments (no predators, either waterbugs, dragonflies, or mosquitofish singly, or all three predator species together). The consequences of hatching early varied across predator treatments, and tended to disappear through time in some predation treatments, notably the waterbug and diverse predator assemblages. We demonstrate that the fitness costs of life history plasticity in an early life stage depend critically on the predator community composition in the next stage.

Theme and variations: amphibious air-breathing intertidal fishes

Over 70 species of intertidal fishes from 12 families breathe air while emerging from water. Amphibious intertidal fishes generally have no specialized air-breathing organ but rely on vascularized mucosae and cutaneous surfaces in air to exchange both oxygen and carbon dioxide. They differ from air-breathing freshwater fishes in morphology, physiology, ecology and behaviour. Air breathing and terrestrial activity are present to varying degrees in intertidal fish species, correlated with the tidal height of their habitat. The gradient of amphibious lifestyle includes passive remainers that stay in the intertidal zone as tides ebb, active emergers that deliberately leave water in response to poor aquatic conditions and highly mobile amphibious skipper fishes that may spend more time out of water than in it. Normal terrestrial activity is usually aerobic and metabolic rates in air and water are similar. Anaerobic metabolism may be employed during forced exercise or when exposed to aquatic hypoxia. Adaptations for amphibious life include reductions in gill surface area, increased reliance on the skin for respiration and ion exchange, high affinity of haemoglobin for oxygen and adjustments to ventilation and metabolism while in air. Intertidal fishes remain close to water and do not travel far terrestrially, and are unlikely to migrate or colonize new habitats at present, although in the past this may have happened. Many fish species spawn in the intertidal zone, including some that do not breathe air, as eggs and embryos that develop in the intertidal zone benefit from tidal air emergence. With air breathing, amphibious intertidal fishes survive in a variable habitat with minimal adjustments to existing structures. Closely related species in different microhabitats provide unique opportunities for comparative studies.

A rapid transcriptome response is associated with desiccation resistance in aerially-exposed killifish embryos

Delayed hatching is a form of dormancy evolved in some amphibian and fish embryos to cope with environmental conditions transiently hostile to the survival of hatchlings or larvae. While diapause and cryptobiosis have been extensively studied in several animals, very little is known concerning the molecular mechanisms involved in the sensing and response of fish embryos to environmental cues. Embryos of the euryhaline killifish Fundulus heteroclitus advance dvelopment when exposed to air but hatching is suspended until flooding with seawater. Here, we investigated how transcriptome regulation underpins this adaptive response by examining changes in gene expression profiles of aerially incubated killifish embryos at ∼100% relative humidity, compared to embryos continuously flooded in water. The results confirm that mid-gastrula embryos are able to stimulate development in response to aerial incubation, which is accompanied by the differential expression of at least 806 distinct genes during a 24 h period. Most of these genes (∼70%) appear to be differentially expressed within 3 h of aerial exposure, suggesting a broad and rapid transcriptomic response. This response seems to include an early sensing phase, which overlaps with a tissue remodeling and activation of embryonic development phase involving many regulatory and metabolic pathways. Interestingly, we found fast (0.5–1 h) transcriptional differences in representatives of classical “stress” proteins, such as some molecular chaperones, members of signalling pathways typically involved in the transduction of sensor signals to stress response genes, and oxidative stress-related proteins, similar to that described in other animals undergoing dormancy, diapause or desiccation. To our knowledge, these data represent the first transcriptional profiling of molecular processes associated with desiccation resistance during delayed hatching in non-mammalian vertebrates. The exceptional transcriptomic plasticity observed in killifish embryos provides an important insight as to how the embryos are able to rapidly adapt to non-lethal desiccation conditions.