Functional morphology of embryonic development in the Port Jackson shark Heterodontus portusjacksoni (Meyer)

The oviparous Port Jackson shark Heterodontus portusjacksoni embryo has a long incubation of 10–11 months during which it undergoes major morphological changes. Initially the egg capsule is sealed from the external environment by mucous plugs in either end of the capsule. Four months into incubation, the egg capsule opens to the surrounding sea water. Fifteen stages of development are defined for this species, the first 10 occur within the sealed capsule, the remaining five after capsule opening to hatching. The functional significance of major definitive characters such as circulation within the yolk membrane and embryo, rhythmic lateral movement of the embryo, external gill filaments, heart activity, internal yolk supplies, egg jelly and the significance of the opening of the egg capsule are described. The egg jelly in the sealed capsule functions to mechanically protect the embryo during early development, however, it eventually creates a hypoxic environment to the embryo as the available oxygen is used up. This generates several physiological challenges to the developing embryo. It is able to overcome these problems by morphological changes such as increasing the effective surface area for gaseous exchange with the development of external gill filaments, fins and extensive circulation in both the embryo and attached external yolk sac. These adaptations become limiting as the embryo grows and respiratory needs outweigh the available oxygen. At this time, the mucous plugs dissolve and the capsule becomes open to the external environment.     

版纳鱼螈胚胎和幼体鳃的退化观察

为了解版纳鱼螈(Ichthyophis bannanicus)胚胎和幼体鳃的退化特征,同窝卵中8枚在胚胎成熟阶段早期被随机取出剖产,随后余下的9枚卵在胚胎成熟阶段晚期时孵出或剖产,观察幼体或胚胎鳃的变化。成熟阶段早期的胚胎,初时其鳃色鲜红,鳃枝充盈,鳃丝与鳃轴近乎垂直;随后鳃色发白,鳃枝疲软,鳃丝卷曲,鳃丝与鳃轴夹角变小。成熟阶段晚期的胚胎和刚孵出的幼体,初时其鳃色纯白,鳃枝疲软,鳃丝卷曲,鳃丝与鳃轴夹角较小;随后鳃逐渐脱落。观察发现,版纳鱼螈胚胎或幼体鳃的退化模式不是重吸收,也不是重吸收加脱落,而仅为脱落。鳃退化过程经历外鳃供血减少-供血停止-鳃枝组织死亡和脱落3个阶段,每个阶段都有明显的外部形态特征。胚胎进入成熟阶段后,越早孵化,鳃枝留存越多,反之,鳃枝留存越少。发白后的鳃枝随机性脱落。     

Innervation pattern of the gill arches and gills of the carp (Cyprinus carpio)

The innervation pattern of the respiratory gill arches of the carp (Cyprinus carpio) is described. The gill region is innervated by the branchial branches of the glossopharyngeal and vagal nerves. Each branchial nerve divides at the level of or just distal to the epibranchial ganglion into: 1) a pretrematic branch, 2) a dorsal pharyngeal branch, and 3) a posttrematic branch. The dorsal pharyngeal branch innervates the palatal organ in the roof of the buccal cavity. The pretrematic and posttrematic branches innervate the posterior and anterior halves, respectively, of the gill arches bordering a gill slit. Each branch splits into an internal and an external part. The internal bundle innervates the buccal side of the gill arch, including the gill rakers. The external bundle terminates in the gill filaments. The epibranchial motor branch, a small nerve bundle containing only motor fibers, circumvents the ganglion and anastomoses distally with the posttrematic branch. The detailed course and branching patterns of these branches are described.     

Environmental and developmental effects on external gill loss in the red-eyed tree frog, Agalychnis callidryas

I examined the effects of development, hatching, and oxygen availability on external gill loss in red-eyed tree frogs, Agalychnis callidryas. Under natural conditions, the arboreal embryos maintained large external gills until hatching, which occurred from 5-8 d after oviposition. At hatching, when tadpoles entered the water, external gills began to regress. In older hatchlings this process was extremely rapid. Gill circulation was lost on average within 16 min and sometimes within 5 min. Gills often regressed completely in under 2 h. Younger hatchlings reduced gill circulation, shortened and adducted their gills, then resumed normal circulation for some time after hatching; half had completely lost external gills within 24 h. Experimentally increasing the area of egg surface exposed to the air induced loss of external gills in unhatched embryos. Older hatchlings in hypoxic water without access to air maintained their external gills. This suggests that loss of external gills is a response to increased oxygen availability, rather than a response to hatching per se. Extended maintenance of external gills by large, late-hatching embryos may facilitate continued rapid development in closely packed eggs.     

Oxygen, gills, and embryo behavior: mechanisms of adaptive plasticity in hatching

Many species alter the timing of hatching in response to egg or larval predators, pathogens, or physical risks. This plasticity depends on separation between the onset of hatching competence and physiological limits to embryonic development. I present a framework based on heterokairy to categorize developmental mechanisms and identify traits contributing to and limiting hatching plasticity, then apply it to a case of predator-induced hatching. Red-eyed treefrogs have arboreal eggs, and tadpoles fall into ponds upon hatching. Egg and tadpole predators select for earlier and later hatching, respectively. Embryos hatch up to 30% early in predator attacks, and later if undisturbed. They maintain large external gills throughout the plastic hatching period, delaying gill regression while development otherwise continues. Rapid gill regression occurs upon hatching. Prolonged embryonic development depends on external gills; inducing gill regression causes hatching. External hypoxia retards development, kills eggs, and induces hatching. Nonetheless, embryos develop synchronously and without hatching prematurely across a broad range of perivitelline PO₂, from 0.5–12.5 kPa. Embryos exploit spatial variation of PO₂ within eggs by positioning gills against patches of air-exposed surface. Respiratory plasticity and oxygen-sensitive behavior appear critical for the hatching plasticity that balances a predation risk trade-off across life stages.     

Filter feeding,deviations from bilateral symmetry,developmental noise,and heterochrony of hemichordate and cephalochordate gills

We measured gill slit fluctuating asymmetry (FA), a measure of developmental noise, in adults of three invertebrate deuterostomes with different feeding modes: the cephalochordate Branchiostoma floridae (an obligate filter feeder), the enteropneusts Protoglossus graveolens (a facultative filter feeder/deposit feeder) and Saccoglossus bromophenolosus (a deposit feeder). FA was substantially and significantly low in B. floridae and P. graveolens and high in S. bromophenolosus. Our results suggest that the gills of species that have experienced a relaxation of the filter feeding trait exhibit elevated FA. We found that the timing of development of the secondary collagenous gill bars, compared to the primary gill bars, was highly variable in P. graveolens but not the other two species, demonstrating an independence of gill FA from gill bar heterochrony. We also discovered the occasional ectopic expression of a second set of paired gills posterior to the first set of gills in the enteropneusts and that these were more common in S. bromophenolosus. Moreover, our finding that gill slits in enteropneusts exhibit bilateral symmetry suggests that the left‐sidedness of larval cephalochordate gills, and the directional asymmetry of Cambrian stylophoran echinoderm fossil gills, evolved independently from a bilaterally symmetrical ancestor.     

Ultrastructure and development of the gills in<Emphasis Type="Italic"> Rana dalmatina</Emphasis> (Amphibia,Anura)

The appearance and the modification of the gill apparatus in Rana dalmatina tadpoles have been described in the different phases of larval development. The morphology and ultrastructure have been studied using light microscopy and both scanning and transmission electron microscopy. The organization of the gills during the initial phases of development (external gills or transient gills) brings to mind the characteristics of Urodela larvae in which the gills appear to consist of three tufts of filaments supported by the gill arches III, IV and V. The cellular composition of the transient gills appears to be extremely simple and the presence of specialized cells is not noted. Basal cells, pavement cells and ciliated cells form the thin mono- or bilayered epithelium. In the persistent gills (or internal gills) of the R. dalmatina tadpole (Ortons larval type 4) the gill arches carry four rows of gill tufts branching out to the ventral region. Meanwhile, from the dorsal portion of the arch the gill filters present an axial portion from which there is much branching out, which confers a characteristic appearance on this part of the gills. The cellular composition of the gill tufts and of the filters is different: in the gill tufts basal cells, pavement cells, ciliated cells, cubic cells and mitochondria-rich cells (MRCs) have been recognized, while in the gill filters the last cellular type does not appear. The MRC has highly variable forms and dimensions and is characterized by the presence of numerous mitochondria in the cytoplasm. Often the MRCs manifest themselves grouped together, in groups of three or more. The pavement cells and the cubic cells demonstrate identical ultrastructural characteristics and have an external surface area characterized by the presence of short superficial microridges and numerous vacuoles in the apical cytoplasm.     

Salinity tolerance and structure of external and internal gills in tadpoles of the crab-eating frog,Rana cancrivora

Summary Salinity tolerance and histology of gills were studied in Rana cancrivora larvae. The tadpoles at the external gill stages (W stages 21–22) were able to survive in media containing up to 40% seawater, but died in water of higher salinity. Their external gills appear to have no critical role in adaptation to seawater. However, advanced tadpoles with internal gills (T-K stages I–XVIII) were able to tolerate 50% or higher seawater. In the internal gills, there are numerous mitochondriarich cells (MR cells) scattered on the ventral and lateral epithelia of the gill arches and the gill tufts in both freshwater-and seawater-acclimated tadpoles. In freshwater-acclimated tadpoles there are three types of MR cell: (1) microplicated, (2) microvillous, and (3) apically vacuolated. In tadpoles acclimated to dilute seawater, the ratio of type-1 to type-2 cells is lower, although all three types of MR cell are present. In 60%-seawater-acclimated tadpoles, a few MR cells with a lumen and concave cytoplasm at the apical membrane (type 4) are present. The changes in MR cell morphology under ambient conditions of low or high salinity may reflect alterations in the physiological roles of the gills with regard to transport of ions.     

Analysis of cutaneous and internal gill gas exchange morphology in early larval amphibians, Pseudophryne bibronii and Crinia georgiana

This study uses stereological techniques to examine body, internal gill and cardiovascular morphology of two larval amphibians, Pseudophryne bibronii and Crinia georgiana, to evaluate the roles of diffusive and convective gas exchange. Gosner stage 27 specimens were prepared for light microscopy and six parallel sections of equal distance taken through the body as well as a further six through the heart and internal gills. Body, internal gill and heart volume as well as body and internal gill surface areas were determined. The harmonic mean distance across the internal gills was also measured and used to estimate oxygen diffusive conductance, DO?. The species were of similar body size and surface area, but the heart and internal gills were larger in P. bibronii, which may represent precursors for greater growth of the species beyond stage 27. The much larger surface area of the skin compared to the internal gills in both species suggests it is the main site for gas exchange, with the gills supplementing oxygen uptake. The sparse cutaneous capillary network suggests diffusion is the main oxygen transport mechanism across the skin and directly into deeper tissues. A numerical model that simplifies larval shape, and has an internal (axial vessels) and external oxygen source, confirms that diffusion is able to maintain tissue oxygen with limited convective input.     

Reproductive biology of the slender smoothhound,Gollum attenuatus,collected from New Zealand waters

Synopsis The reproductive biology of 385 male and 373 female slender smoothhounds,Gollum attenuatus, collected from New Zealand waters was examined. Size at maturity for both sexes was about 700 mm TL. Litter size was usually two. The sex ratio of embryos was 1:1. The right ovary ovulated 50–100 ova, 4–8 mm in diameter, and 30–80 ova were enclosed in each egg capsule. Only one embryo developed from the many ova in each egg capsule, the other undeveloped ova were ingested and passed to an external yolk sac which formed the yolk supply for the developing embryo.     

Structural organization of the gills in pipefish (Teleostei,Syngnathidae)

Summary The morphology and structural features of the gills of the two Western Baltic pipefish Nerophis ophidion and Syngnathus rostellatus were investigated using scanning electron microscopy. The general anatomy of the gills complies with the general pattern in fish. Several adaptations though, show the highly specialized nature of pipefish gills. The filaments are extremely short, few in number and carry only a few lamellae due to the limited space in the branchial cavity. The lamellae have a widely projecting form yet still have a small area in comparison to other fish. Gill irrigation is performed by a specialized pumping mechanism which forces respiratory water through the small but densely packed gill sieve. Although both species live in the same habitat and belong to the same family, differences in gill morphology were found and are related to different lifestyles. S. rostellatus is the more active species and therefore has more filaments per gill arch, more lamellae per filament, wider projecting lamellae and a more extreme utilisation of available space in the gill cavity through a very densely packed gill sieve. N. ophidion has a stationary mode of life and therefore has a less extreme gill anatomy.     

The occurrence, distribution and pathology associated with gnathiid isopod larvae infecting the epaulette shark, Hemiscyllium ocellatum

Gnathiid isopod praniza larvae were found to infect the epaulette shark Hemiscyllium ocellatum. All sharks carried larvae on their external body surface, with the preferred attachment site in both sexes around the cloaca (P<0.05). The claspers were the second site of preference in male sharks. Within the buccal and branchial cavities, about 16% of larvae were attached to the roof and floor of the mouth and 84% attached to the gills. A significant positive correlation existed between larval number and fish size. Histological examination showed that larval attachment in the buccal cavity elicited variable responses, the most severe being a loss of epithelium and compression of underlying tissue. No host cellular response or tissue proliferation was observed. Praniza attached preferentially to the efferent side of gill filaments (relative to blood flow), and caused loss of epithelium, compression of tissue, and a small amount of connective tissue proliferation. Attachment to the gill septum or to the afferent side of the gill filament caused lamellar disruption, a cellular inflammatory response, and connective tissue proliferation. Scanning electron microscopy showed little obvious praniza-induced gill damage, other than localised tissue distortion to form "pockets" around larvae attached between filaments. The results suggest that praniza larvae do not cause sufficient tissue damage to adversely affect the health of this shark species.     

Hatching timing,oxygen availability,and external gill regression in the tree frog,Agalychnis callidryas

The physiological role of the embryonic external gills in anurans is equivocal. In some species, diffusion alone is clearly sufficient to supply oxygen throughout the embryonic period. In others, morphological elaboration and environmental regulation of the external gills suggest functional importance. Since oxygen stress is a common trigger of hatching, I examined the relationships among hatching timing, oxygen stress, and external gill loss. I worked with the red-eyed tree frog, Agalychnis callidryas, a species with arboreal eggs and aquatic tadpoles in which gill regression is associated with hatching, and hatching timing affects posthatching survival with aquatic predators. Both exposure to a hypoxic gas mixture and submergence in water, a natural context in which hypoxic stress can occur, induced early hatching. Exposure to hyperoxic gas mixtures induced regression of external gills, and subsequent exposure to air induced early hatching. Prostaglandin-induced external gill regression also induced hatching, and this effect was partially ameliorated by exposure to hyperoxic gas. Together, these results suggest that external gills enhance the oxygen uptake of embryos and are necessary to extend embryonic development past the onset of hatching competence.     

Brooding in the Chilean Oyster Ostrea chilensis: Unexpected Complexity in the Movements of Brooded Offspring within the Mantle Cavity

Brooding in invertebrates serves to protect embryos from stressful external conditions by retaining progeny inside the female body, effectively reducing the risk of pelagic stages being exposed to predation or other environmental stressors, but with accompanying changes in pallial fluid characteristics, including reduced oxygen availability. Brooded embryos are usually immobile and often encapsulated, but in some Ostrea species the embryos move freely inside the female pallial cavity in close association with the mother’s gills for as long as eight weeks. We used endoscopic techniques to characterize the circulation pattern of embryos brooded by females of the oyster, Ostrea chilensis. Progeny at embryonic and veliger stages typically circulated in established patterns that included the use of dorsal and ventral food grooves (DFG, VFG) to move anteriorly on the gills. Both embryos and veligers accumulated around the mother’s palps, and remained there until an active maternal countercurrent moved them to the gill inhalant area. Both food grooves were able to move embryos, veligers, and food-particle aggregates anteriorly, but the DFG was more important in progeny transport; early embryos were moved more rapidly than veligers in the DFG. A microcirculation pattern of embryos was apparent when they were moved by gill lamellae: when they were close to the VFG, most embryos lost gill contact and ´´fell´´ down to the DFG. Those that actually reached the DFG moved anteriorly, but others came into contact with the base of the lamellae and again moved towards the VFG. The circulation pattern of the progeny appears well-suited for both cleaning them and directing them posteriorly to an area where there is more oxygen and food than in the palp region. This process for actively circulating progeny involves the feeding structures (gill and palps) and appears to be energetically costly for the female. It also interferes with feeding, which could explain the poor energy balance previously documented for brooding females of this species.     

Functional anatomy of the internal gills of the tadpole of Litoria ewingii (Anura,Hylidae)

Summary (1) Scanning electron microscopy and vascular casting were used to study the morphology and vascular anatomy of the fully developed internal gills of Litoria ewingii tadpoles. — (2) The four pairs of gills were located in two branchial baskets on either side of the heart. Each gill consisted of a branchial arch with gill tufts projecting ventrally and gill filters running dorsally. The gills bore a variable number of gill tufts in which a complex three-dimensional array of capillary loops, of varying lengths and diameters, was trailed in the path of the ventilatory current. — (3) The evidence presented in this paper suggests that the gill tufts have greater potential as gas exchangers than either the gill filters or skin. — (4) The study revealed structural and functional evidence for the existence of branchial shunts between afferent and efferent branchial arteries.     

Bystrow’s Paradox – gills,fossils, and the fish‐to‐tetrapod transition

Schoch, R.R. and Witzmann, F. 2011. Bystrow’s Paradox – gills, fossils, and the fish‐to‐tetrapod transition. —Acta Zoologica (Stockholm) 92 : 251–265. The issue of which breathing mechanism was used by the earliest tetrapods is still unsolved. Recent discoveries of stem tetrapods suggest the presence of internal gills and fish‐like underwater breathing. The same osteological features were used by Bystrow to infer a salamander‐like breathing through external gills in temnospondyl amphibians. This apparent contradiction – here called Bystrow’s Paradox – is resolved by reviewing the primary fossil evidence and the anatomy of the two gill types in extant taxa. Rather unexpectedly, we find that internal gills were present in a range of early crown tetrapods (temnospondyls), based on the anatomy of gill lamellae and location of branchial arteries on the ventral side of gill arch elements (ceratobranchials). Although it remains to be clarified which components are homologous in external and internal gills, both gill types are likely to have been present in Palaeozoic tetrapods – internal gills in aquatic adults of some taxa, and external gills in the larvae of these taxa and in larvae of numerous forms with terrestrial adults, which resorbed the external gills after the larval phase. Future developmental studies will hopefully clarify which mechanistic pathways are involved in gill formation and how these might have evolved.     

CO2‐metabolism in early tetrapods revisited: inferences from osteological correlates of gills,skin and lung ventilation in the fossil record

One of the most important physiological changes during the conquest of land by vertebrates was the increasing reliance on lung breathing, with the concomitant decrease in importance of gill breathing. The main problem involved here was to cope with the excessive accumulation of CO₂ in the body and to avoid respiratory acidosis. In the past, several often mutually contradicting hypotheses of CO₂‐elimination via skin, lungs and gills in early tetrapods have been proposed, based on theoretical physiological considerations and comparison with extant air‐breathing fishes and amphibians. This study proposes a revised scenario of CO₂‐elimination in early tetrapods based on fossil evidence, that is recently identified osteological correlates of gills, skin structure and mode of lung ventilation. In stem tetrapods of the Devonian and Carboniferous, O₂‐uptake via the lungs by buccal pumping was decoupled from CO₂‐release via internal gills, and the rather gas‐impermeable skin played a minor role in gaseous exchange. The two main lineages of crown‐group tetrapods, the amphibian and amniote lineage, used different strategies of CO₂‐elimination. As in stem tetrapods, O₂‐uptake and CO₂‐release remained always largely decoupled in temnospondyls, which ventilated their lungs via buccal pumping and relied mainly on their internal gills for CO₂‐release. Temnospondyls were not able to reduce their internal gills before their skin became more gas permeable and their body size was reduced, to shift from internal gills to the skin as the major site of CO₂‐elimination, a pattern that is retained in most lissamphibians. In contrast, internal gills were lost very early in stem amniote evolution. This was associated with the evolution of the more effective aspiration pump that allowed the elimination of the bulk of CO₂ via the lungs, leading to a coupled O₂‐uptake and CO₂‐loss in stem amniotes and later in amniotes.     

Myxobolus cuneus n. sp. (Myxosporea) infecting the connective tissue of Piaractus mesopotamicus (Pisces: Characidae) in Brazil: histopathology and ultrastructure

The characteristics of Myxobolus cuneus n. sp. and its relationship to the host Piaractus mesopotamicus are described based on light and electron microscopy and histological observations. Polysporic plasmodia measuring 20 microm to 2.1 mm in size were found in 63.3 % of the P. mesopotamicus examined. The parasite was found in the gall bladder, urinary bladder, gills, spleen, fins, head surface, liver and heart. Generative cells and disporoblastic pansporoblasts occurred along the periphery of the plasmodia, and mature spores were found in the internal region. The mature spores had a pear shaped body in frontal view, with a total length of 10.0 +/- 0.6 microm and a width of 5.1 +/- 0.3 microm (mean +/- SD). The spore wall was smooth with sutural folds. The polar capsules were elongated, were pear shaped, and equal in size (length 5.7 +/- 03 microm; width 1.7 +/- 0.2 microm), with the anterior ends close to each other. The polar filaments were tightly coiled in 8-9 turns perpendicular to the axis of the capsule. The plasmodia were always found in connective tissue (wall of the arterioles of the gill filaments, serous capsule of the gall bladder, middle layer and subepithelial connective tissue of the urinary bladder, connective tissue between the rays of the fins, subcutaneous tissue of the head surface and fibrous capsule spleen). The parasite caused important damage in the gills, where development occurred in the wall of gill filament arterioles; a mild macrophage infiltrate was also observed. In advanced developmental stages, the plasmodia caused deformation of the arteriole structure, with a reduction and, in some cases, obstruction of the lumen. The parasite was found throughout the period studied and its prevalence was unaffected by host size, season or water properties.     

Capsule walls as barriers to oxygen availability: Implications for the development of brooded embryos by the estuarine gastropod Crepipatella dilatata (Calyptraeidae)

Encapsulation of developing embryos imposes potential restrictions, because the capsule wall must allow for adequate inward diffusion of oxygen and for increased diffusion of oxygen as metabolic demand increases with continued development. Samples of egg capsules from the gastropod Crepipatella dilatata were used to document surface characteristics, composition of the different capsule wall layers, and alterations in wall thickness during development. The diffusion coefficient and capsule wall permeability were determined experimentally for capsules containing embryos at different developmental stages. We also determined oxygen consumption rates for various embryonic stages and for nurse eggs, which provide food for embryos during development. The capsule wall of C. dilatata possesses 2 differentiated layers: the external capsular wall (ECW) and the internal capsular wall (ICW). The ECW is compact and fibrous, features that remain invariable during development, and lacks surface features that might make some portions of the capsule wall more permeable to oxygen than others. On the other hand, the ICW is initially spongy and thick, but significantly decreases in thickness over time, particularly before the embryos begin feeding on nurse eggs. Although the capsule wall is a serious barrier to diffusion, permeability to oxygen increases over time by 112% due to the dramatic thinning of the inner capsule wall layer. Nurse eggs consume oxygen but at very low rates, supporting the idea that they correspond to living embryonic cells that have stopped their development. Respiration measurements indicated that embryos are initially supplied with enough oxygen within the egg capsules to carry out the activities characteristic of embryogenesis, even though the capsular walls show their maximum thickness and lowest permeability at this time. However, as the embryo develops its velum and becomes more active, capsule wall thickness decreases and capsule permeability to oxygen increases. Correspondingly, the oxygen demands of metamorphosed but still encapsulated specimens are approximately 135% higher than those of pre-metamorphosed sibling embryos.     

Oxygen, gills, and embryo behavior: mechanisms of adaptive plasticity in hatching.

Many species alter the timing of hatching in response to egg or larval predators, pathogens, or physical risks. This plasticity depends on separation between the onset of hatching competence and physiological limits to embryonic development. I present a framework based on heterokairy to categorize developmental mechanisms and identify traits contributing to and limiting hatching plasticity, then apply it to a case of predator-induced hatching. Red-eyed treefrogs have arboreal eggs, and tadpoles fall into ponds upon hatching. Egg and tadpole predators select for earlier and later hatching, respectively. Embryos hatch up to 30% early in predator attacks, and later if undisturbed. They maintain large external gills throughout the plastic hatching period, delaying gill regression while development otherwise continues. Rapid gill regression occurs upon hatching. Prolonged embryonic development depends on external gills; inducing gill regression causes hatching. External hypoxia retards development, kills eggs, and induces hatching. Nonetheless, embryos develop synchronously and without hatching prematurely across a broad range of perivitelline PO2, from 0.5-12.5 kPa. Embryos exploit spatial variation of PO2 within eggs by positioning gills against patches of air-exposed surface. Respiratory plasticity and oxygen-sensitive behavior appear critical for the hatching plasticity that balances a predation risk trade-off across life stages.