Intraspecific variation of hatchling size in Late Cretaceous ammonoids from Hokkaido,Japan: implication for planktic duration at early ontogenetic stage

Tajika, A. & Wani, R. 2011: Intraspecific variation of hatchling size in Late Cretaceous ammonoids from Hokkaido, Japan: implication for planktic duration at early ontogenetic stage. Lethaia, Vol. 44, pp. 287–298. Intraspecific variations of the early shell dimensions (ammonitella and protoconch diameters) of two Late Cretaceous (earliest Campanian) ammonoid species (Gaudryceras tenuiliratum and Hypophylloceras subramosum) from the Haboro and Ikushumbetsu areas, Hokkaido, Japan, show no significant difference between these areas that are approximately 110 km apart. The geographic distributions of G. tenuiliratum and H. subramosum are supposed to be mainly controlled by the flotation and transportation during the embryonic stage within floating egg masses and/or post‐embryonic stage because of their small hatchling sizes (1.18–1.46 mm in diameter for G. tenuiliratum, and 0.91–1.13 mm in diameter for H. subramosum), suggesting these two ammonite species at the embryonic and/or post‐embryonic stages were transported at least 110 km. Postulating that the velocity of palaeocurrent around the Haboro and Ikushumbetsu areas during the Cretaceous Period was 0.25 m/s, similar to those in the modern ocean current flowing off the eastern Pacific coast of Hokkaido, the egg masses and/or hatchlings of G. tenuiliratum and H. subramosum were buoyant and transported more than 5 days. The preliminary comparison of hatchling size through time suggests that the hatching sizes of H. subramosum in Hokkaido increased slightly from the Middle Turonian until the earliest Campanian (during about 7 Myr). □ammonoid, hatchling, paleoecology, variation, Cretaceous.     

Sutural amplitude of ammonite shells as a paleoenvironmental indicator

The Sutural Amplitude Index (SAI), obtained by measuring the maximum height of sutural elements and the length of the suture pattern on ammonoid shells, provides a useful indicator of relative habitat depth for ammonoids with similar shell morphotypes. A higher SAI indicates greater septal support for the shell wall against implosion under increased hydrostatic pressures in deeper waters. Relating the sutural amplitude indices of ammonites found in a well-studied depositional basin, such as the Cretaceous Western Interior Seaway, to morphotype distributions illustrates the utility of this index in bathymetric interpretations. □ Ammonoidea, sutures, Sutural Amplitude Index, Cretaceous, Greenhorn Cyclothem. Western Interior.     

Rhythms of ammonoid shell secretion

From measurements of the thickness of successive growth rings shell growth curves were constructed for seven ammonoid genera of different taxonomic position and geological age. All the curves have 'sinusoidal' aspect due to the alternation of maximum and minimum growth increments of the shell tunc. Growth curves made for two ammonoid shells belonging to the genera Proloxyclymenia (D₃fm) and Euphylloceras (K₁a) show regularly recurring minima and maxima at intervals of 14–16 growth rings. The fact that the minima and maxima of the growth curves recur exactly at intervals of 14–16 growth rings seems to reflect the relationship between the shell tube secretion and the formation of septa. This fact also seems to support the views about fortnightly or 28-day cycles in the construction of ammonoid hydrostatic chambers. By counting the total number of septa in the phragmocones, the duration of phragmocone secretion for 23 shells was estimated. Preliminary inferences on the presence in ammonoid shells of daily, bidaily and weekly growth rings are made. □ Ammonoidea, growth rhythms.     

Fish hatching strategies: a review

The paper presents differences in the distribution of hatching gland cells, and the location of egg envelope digestion, the significance of hatching movements, and the ways larvae escape from egg envelopes. A review of the literature on the hatching orientation of 34 fish species is compared. No correlation was seen between hatching orientation and egg diameter or newly hatched larva length, nor newly hatched larvae length ratio to egg diameter. Photographs of twelve freshwater species present the moment of hatching either head first or tail first. Some differences were shown in swelling between eggs incubated in commercial hatchery and developed in natural conditions, as well as possible effect of these differences on hatching.     

Nautilid nurseries: hatchlings and juveniles of Eutrephoceras dekayi from the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east‐central Montana

The nautilid Eutrephoceras dekayi (Morton 1834) is relatively abundant in the lower Maastrichtian (Upper Cretaceous) Pierre Shale of east‐central Montana. We analysed the morphology, size frequency distribution, and isotope composition of a large collection of 220 well‐preserved specimens including hatchlings, juveniles and adults. The newly hatched shell is approximately 14 mm in diameter with a body chamber one‐third whorl in angular length terminating in the nepionic constriction. Internally, the embryonic shell contains five septa. Juveniles are abundant and comprise two‐thirds of the sample whereas sub‐adults, defined by the incipient flattening of the venter, are rare. Adults comprise approximately one‐third of the sample and average 100 mm in diameter. The co‐occurrence of newly hatched shells, small juveniles and adults suggests that the eggs were laid in the same area in which the hatchlings developed. Based on the excellent preservation of the juveniles, we conclude that they did not float into the area after death, but lived in the region, implying that this area served as a nursery for young animals. The calculated temperatures of the embryonic shells are similar to those of the post‐embryonic shells and generally range from 16 to 18 °C. Upon hatching, the nautilids probably followed a demersal mode of life and lived in well‐oxygenated water ≤50 m deep. An examination of lethal injuries (puncture holes) suggests that all ontogenetic stages were equally vulnerable to predation. The proximity of the site to the Sheridan Delta suggests that the specimens were smothered by sudden pulses of sediment transported into the area by major storms.     

The role of suture complexity in diminishing strain and stress in ammonoid phragmocones

Several hypotheses have been put forward to explain the sinuosity and complexity of suture lines in Ammonoidea. At present, the two principal opponent views maintain either that high complexity was a requisite to reinforce the shell in response to hydrostatic pressure, or that complexity augmented the attachment area for muscles. By using finite element calculations and analytical estimates of simplified ammonoid shell geometries, it is shown that complex suture lines reduced dramatically the strain and the stress in the phragmocone. The calculations lend support to the hypothesis that high sinuosity is an evolutionary response to external pressure. Additionally, it is found that without complex septa, the inward deformation of an ammonoid with thin shell would cause it to shrink in response to pressure and to lose buoyancy by a non-negligible amount.     

Recent advances in heteromorph ammonoid palaeobiology

Heteromorphs are ammonoids forming a conch with detached whorls (open coiling) or non-planispiral coiling. Such aberrant forms appeared convergently four times within this extinct group of cephalopods. Since Wiedmann's seminal paper in this journal, the palaeobiology of heteromorphs has advanced substantially. Combining direct evidence from their fossil record, indirect insights from phylogenetic bracketing, and physical as well as virtual models, we reach an improved understanding of heteromorph ammonoid palaeobiology. Their anatomy, buoyancy, locomotion, predators, diet, palaeoecology, and extinction are discussed. Based on phylogenetic bracketing with nautiloids and coleoids, heteromorphs like other ammonoids had 10 arms, a well-developed brain, lens eyes, a buccal mass with a radula and a smaller upper as well as a larger lower jaw, and ammonia in their soft tissue. Heteromorphs likely lacked arm suckers, hooks, tentacles, a hood, and an ink sac. All Cretaceous heteromorphs share an aptychus-type lower jaw with a lamellar calcitic covering. Differences in radular tooth morphology and size in heteromorphs suggest a microphagous diet. Stomach contents of heteromorphs comprise planktic crustaceans, gastropods, and crinoids, suggesting a zooplanktic diet. Forms with a U-shaped body chamber (ancylocone) are regarded as suspension feeders, whereas orthoconic forms additionally might have consumed benthic prey. Heteromorphs could achieve near-neutral buoyancy regardless of conch shape or ontogeny. Orthoconic heteromorphs likely had a vertical orientation, whereas ancylocone heteromorphs had a near-horizontal aperture pointing upwards. Heteromorphs with a U-shaped body chamber are more stable hydrodynamically than modern Nautilus and were unable substantially to modify their orientation by active locomotion, i.e. they had no or limited access to benthic prey at adulthood. Pathologies reported for heteromorphs were likely inflicted by crustaceans, fish, marine reptiles, and other cephalopods. Pathologies on Ptychoceras corroborates an external shell and rejects the endocochleate hypothesis. Devonian, Triassic, and Jurassic heteromorphs had a preference for deep-subtidal to offshore facies but are rare in shallow-subtidal, slope, and bathyal facies. Early Cretaceous heteromorphs preferred deep-subtidal to bathyal facies. Late Cretaceous heteromorphs are common in shallow-subtidal to offshore facies. Oxygen isotope data suggest rapid growth and a demersal habitat for adult Discoscaphites and Baculites. A benthic embryonic stage, planktic hatchlings, and a habitat change after one whorl is proposed for Hoploscaphites. Carbon isotope data indicate that some Baculites lived throughout their lives at cold seeps. Adaptation to a planktic life habit potentially drove selection towards smaller hatchlings, implying high fecundity and an ecological role of the hatchlings as micro- and mesoplankton. The Chicxulub impact at the Cretaceous/Paleogene (K/Pg) boundary 66 million years ago is the likely trigger for the extinction of ammonoids. Ammonoids likely persisted after this event for 40–500 thousand years and are exclusively represented by heteromorphs. The ammonoid extinction is linked to their small hatchling sizes, planktotrophic diets, and higher metabolic rates than in nautilids, which survived the K/Pg mass extinction event.     

Buckman's Paradox: variability and constraints on ammonoid ornament and shell shape

Buckman's Law of Covariation states that ammonoid shell shape and ornamentation are typically correlated, such that compressed, involute forms have light ornament while more inflated, evolute forms have heavier ornament. Such covariation has been observed in many ammonoid groups, and implies a link between the morphogenesis of shell shape and ornamentation. However, other evidence suggests that while ornament growth is controlled by the genetic‐developmental program of the ammonoid, shell shape is strongly influenced by environmental factors. These differing viewpoints lead to Buckman's Paradox – are ornamentation and shell shape tightly linked, as implied by Buckman's covariation, or is the morphogenesis of ornament controlled genetically, while shell shape is controlled environmentally? To address this issue, the variability of shell shape and rib morphology has been compared for a group of endemic acanthoceratid ammonites from the Cretaceous Western Interior Seaway of North America. If Buckman's Law holds due to a morphogenetic connection between shell shape and ornamentation, we would expect taxa with more variable shell shapes to also show more variable rib features and growth. Morphometric analysis of seven shell shape and two rib characters for the Western Interior acanthoceratids finds no such correlation, suggesting that shell shape and rib growth are controlled by different processes. Indeed, rib growth appears to be more constrained than shell shape, consistent with the view that ornamentation is more tightly controlled by the developmental‐genetic growth program of the ammonoid. These results emphasize the complexity of ammonoid morphogenesis and highlight our limited understanding of the causes underlying Buckman's Law.     

Rhyncholites and conchorhynchs as calcified jaw elements in some late Cretaceous ammonites

Conspicuous calcareous coverings are present in the anterior region of 17 fossil jaws from late Cretaceous rocks of Hokkaido (Japan) and Sakhalin (U.S.S.R.). The jaws were preserved in calcareous nodules either in situ in body chambers of ammonites or in close association with identifiable ammonite conch remains. From the morphologic similarity between in situ and isolated jaws, they may be attributed to Tetragonites glabrus, Gaudryceras tenuiliratum, G. denseplicatum, G. sp., and Neophylloceras subramosum. The jaw apparatus of these species is composed of two three-dimensional black walls of carbonate apatite, which might be a diagenetic replacement of chitinous material. The calcareous coverings in both upper and lower jaws closely resemble those of upper (rhyncholite) and lower (conchorhynch) jaws of modern Nautilus as well as rhyncholite and conchorhynch fossils in their gross morphology, microstructure, and chemical composition. Calcified remains of cephalopod jaws known as rhyncholites and conchorhynchs have been reported from late Paleozoic to Recent. The present discovery of ammonoid rhyncholites and conchorhynchs suggests that at least some previously known late Paleozoic and Mesozoic counterparts belong to the Ammonoidea. The essential similarity of jaw elements of some Late Cretaceous ammonites and modern Nautilus gives reliable information on the feeding habits of the former. The sharp and thick ammonoid rhyncholites and conchorhynchs may have had a special function for cutting up food, similar to those of Nautilus.     

Increased lifetime reproductive success of first‐hatched siblings in Common Kestrels Falco tinnunculus

Order of birth has profound consequences on offspring across taxa during development and can have effects on individuals later in life. In birds, differential maternal allocation and investment in their progeny lead to variance in the environmental conditions that offspring experience during growth within the brood. In particular, laying and hatching order have been proposed to influence individual quality during the growing period, but little is known about the fitness consequences that these two factors have for offspring from a lifetime perspective. We explored the effect of laying and hatching order on post‐fledgling survival (measured as recruitment probability) and lifetime reproductive success (LRS) in Common Kestrels Falco tinnunculus, using a long‐term and individual‐based dataset. First‐hatched chicks showed higher survival probability and LRS than their siblings. This effect was not due to body condition of the individuals at adulthood, the quality of their mates or the reproductive outcome compared with later‐hatched individuals. Instead, first‐hatched chicks had a higher recruitment probability. This could be explained by the higher body condition attained by first‐hatched chicks at the end of the nesting period, perhaps due to an enhanced competitive advantage for food over their siblings at the time of hatching. Laying order, in contrast to hatching order, appeared to have little or no effect on LRS. Our results suggest that hatching order within siblings predicts fitness, and that better early‐life conditions during growth experienced by first‐hatched chicks improve first survival and then recruitment, resulting in an enhanced LRS.     

Maternally derived yolk testosterone enhances the development of the hatching muscle in the red-winged blackbird Agelaius phoeniceus

Hatching asynchrony in avian species often leads to the formation of a size hierarchy that places last-hatched nestlings at a significant disadvantage. The hatching muscle (musculus complexus) is responsible for breaking the shell during hatching and for dorsal flexion of the neck during begging. An increase in its strength in last-hatched nestlings could mitigate the effects of hatching asynchrony by reducing the time required for hatching or enhancing the effectiveness of begging for parentally delivered food or both. We have previously found that yolk testosterone concentration increases with laying order in the red-winged blackbird Agelaius phoeniceus. In this study, we investigated the hypothesis that yolk testosterone has anabolic effects on the development of the complexus, thereby influencing competition among asynchronously hatched nestlings. We found that both yolk testosterone concentration and relative complexus mass (complexus mass/nestling body mass) increased with laying order and that these two variables were positively correlated in both newly hatched nestlings and in two-day-old broods. Moreover, direct injections of testosterone into egg yolks resulted in an increase in relative complexus mass, while injections of flutamide, a testosterone antagonist, resulted in a decrease in relative complexus mass. Neither yolk testosterone concentration nor relative complexus mass differed between male and female nestlings.     

Feedback between size balance and consumption strongly affects the consequences of hatching phenology in size‐dependent predator–prey interactions

In many size‐dependent predator–prey systems, hatching phenology strongly affects predator–prey interaction outcomes. Early‐hatched predators can easily consume prey when they first interact because they encounter smaller prey. However, this process by itself may be insufficient to explain all predator–prey interaction outcomes over the whole interaction period because the predator–prey size balance changes dynamically throughout their ontogeny. We hypothesized that hatching phenology influences predator–prey interactions via a feedback mechanism between the predator–prey size balance and prey consumption by predators. We experimentally tested this hypothesis in an amphibian predator–prey model system. Frog tadpoles Rana pirica were exposed to a predatory salamander larva Hynobius retardatus that had hatched 5, 12, 19 or 26 days after the frog tadpoles hatched. We investigated how the salamander hatch timing affected the dynamics of prey mortality, size changes of both predator and prey, and their subsequent life history (larval period and size at metamorphosis). The predator–prey size balance favoured earlier hatched salamanders, which just after hatching could successfully consume more frog tadpoles than later hatched salamanders. The early‐hatched salamanders grew rapidly and their accelerated growth enabled them to maintain the predator‐superior size balance; thus, they continued to exert strong predation pressure on the frog tadpoles in the subsequent period. Furthermore, frog tadpoles exposed to the early‐hatched salamanders were larger at metamorphosis and had a longer larval period than other frog tadpoles. These results suggest that feedback between the predator‐superior size balance and prey consumption is a critical mechanism that strongly affects the impacts of early hatching of predators in the short‐term population dynamics and life history of the prey. Because consumption of large nutrient‐rich prey items supports the growth of predators, a similar feedback mechanism may be common and have strong impacts on phenological shifts in size‐dependent trophic relationships.     

Verification of the eclossion mark on the sagittal otolith of the larvae of Sardinella aurita (Pisces: Clupeidae)

The study of otolith in larvae is important to determine fish age and growth, essential parameters in the study and management of fisheries resources. In this study, the formation of the hatching mark in Sardinella aurita was verified on ichthyoplankton samples collected off southern Cubagua island, Venezuela, from May 1998 to January 1999. The embryos were kept alive using a culture system until they hatched and daily a group of 10 to 30 larvae were fixed in 95% ethanol. An image analysis system was used to measure morphometric characteristics of larvae and sagittal otoliths. Following are mean values in newly hatched larvae: otolith hatching mark distance from nucleus 4.78 m (I.C. 0.36 m, p 0.05 n = 30), increase width 1.46 m (I.C. 0.17 microm, p 0.05, n = 30) and diameter 14.28 m (IC 1.11 m, p 0.05, n = 30). The mean standard length of larvae at age 0 was 3.31 mm (I.C. 0.08 mm, p 0.05, n = 200). The identification of the hatching mark allows the exact calculation of the number of rings in larvae from the natural environment.     

Sheltered preservation: a peculiar mode of ammonite occurrence in the Cretaceous Yezo Group, Hokkaido, north Japan

Maeda, H. 1991 01 15: Sheltered preservation: a peculiar mode of ammonitc occurrence in the Cretaceous Yeto Group, Hokkaido. north Japan. Lethaia , Vol. 24, pp. 69–82. Oslo. ISSN 0024–1164.
'Sheltered preservation', in which many small ammonites ('refugees') arc preserved in the inside and/or the lower umbilical void of a large ammonite shell ('shelter'), is a common preseivational style in the Cretaceous Yezo Group in Hokkaido, north Japan. The best example is large Calycoceras of Middle Cenomanian age, attaining 300 to 500 mm diameter, whose shell usually hours more than 100 individuals of Desmoceras comprising mainly juveniles less than 12 mm in diameter. In contrdst with other preservational styles of Desmoceras from the same horizon. the shells of refugee ammonites have not suffered phragmoeone collapse, and the inner whorls and a protoconch are intact. Many refugee ammonites are allochthonous, as indicated by the incomplete preservation of their living chamber and the absence of a jaw apparatus. The sheltered preservation is, therefore, a kind of 'Konzcntrat Lagerstatten' completed by both pre- and post-burial processes. Acting as a 'concentration trap' on the sea floor, a large empty ammonite shell probably triggered off such accumulation of the small ammonite shells in post-mortem transport across the sea floor. This biostratinomical process biased the original thanatococnosis of ammonites, so that its immature specimens were selectively accumulated and preserved in the shelter. Ammonoidea, taphonomy, Konzentrat Lagerstätten'. differenrial preservation, Cretuceous. Hokkaido .     

Phylogeny of the aptychi-possessing Neoammonoidea (Aptychophora nov., Cephalopoda)

The different forms of the aptychi (opercula, homologous with lower jaws) of the Ammonoidea are used for the first time in a phylogenetic analysis of part of the classic Ammonoidea phylogeny. The results indicate that the aptychi-possessing ammonoids form a monophylum for which we propose the informal name Aptychophora nov. Among the Jurassic ammonoids, it is possible to recognize several monophyletic groups. In part, our results support existing superfamilies (e.g. Hildocerataceae, Haplocerataceae) by new synapomorphies. However, the Perisphinctaceae can now be much more clearly differentiated than in the previously established phylogenetic tree. The Upper Cretaceous ammonoid superfamilies cannot be derived from the Haplocerataceae, but are descendants of a 'primitive' perisphinctacean possessing a praestriaptychus. Nor can they be derived from the 'higher' perisphinctaceans (family Perisphinctidae) because that clade is characterized by granulaptychi. The consequence of these results is that the quadrilobate primary suture of the 'Ancyloceratina' must have evolved more than once by reduction from an ancestral quinquelobate primary suture. The Ancyloceratidae have praestraptychi or aptychi types which can be derived from praestriaptychi, whereas the Crioceratitinae have longitudinally striated anaptychi.     

Dimorphism in a Tethyan early Jurassic Juraphyllites

A lappeted microconch specimen of the phylloceratid ammonoid Juraphyllites from the Tethyan Jurassic of Anatolia (Turkey) indicates that sexual dimorphism in this group was established as early as in the early Lower Pliensbachian (Lower Jurassic) and that dimorphism in this genus of the Phylloceratina follows the same pattern as that proven [or many groups of the Ammonitina. This is the earliest record of well-developed lateral lappets in the Mesozoic Ammonoidea     

Perfectly nested or significantly nested – an important difference for conservation management

The trade off between age and size at emergence, which plays a central role in life history theory, is hypothesized to be more pronounced under stressful conditions, especially when these conditions are combined. Empirical evidence for this is equivocal. We tested the hypothesis by imposing combinations of two types of time stress (pond drying and late hatching date) in larvae of the damselfly Lestes viridis . Larvae from a temporary pond and a permanent pond population were reared in outdoor tubs from egg hatching until emergence. Unexpectedly, larvae did not accelerate their life history in response to simulation of pond drying. Instead, larvae reared in temporary tubs generally had a slower development and growth than larvae reared in permanent tubs. Probably deteriorating growth conditions in temporary tubs associated with higher densities and lower food levels caused this pattern. In agreement with a higher time stress in late hatched larvae, they generally had faster development and growth than larvae that hatched early in the season. Drying regime and hatching date shaped the covariation pattern between age and size at emergence, but the tradeoff was only apparent when time stress was relaxed. The tradeoff between age and size at emergence was only present in early hatched larvae, especially in permanent tubs (lowest time stress). Conversely, in late hatched larvae there was a strongly negative relationship between age and size at emergence, especially in temporary tubs (highest time stress). Our results support an alternative hypothesis that deteriorating growth conditions (i.e. pond drying) may decouple the tradeoff under time stress. The absence of a tradeoff in more time-stressed late hatched larvae can be explained by their higher intrinsic growth rates, independent of deteriorating growth conditions. We hypothesize that the pattern of less clear tradeoffs under the imposed types of time stress may be general.     

Survival and sublethal responses of early life stages of pike exposed to low pH in artificial post-mining lake water

The threshold pH for survival of early life stages of pike Esox lucius in post-mining lakes was determined in a laboratory experiment using artificial water characterized by high concentrations of dissolved Ca, Al, Fe, Mn and SO2/4⁻. At pH 3·50–4·00, high mortality was observed before hatching. At pH 4·00 and 4·25, hatching rates were reduced compared with the controls (pH 7·40), and many embryos died in a partly hatched state. Hatching success of embryos exposed to pH levels of 4·50 or higher was not affected. However, at pH 5·00 many newly hatched embryos were deformed. Furthermore, pike exposed to pH 4·00–5·00 did not start feeding. At pH 4·75 and below, mortality increased to 100%, and at pH 5·00, only few eleutheroembryos which were in extremely poor condition survived to the end of the experiment. At pH 5·50, survival was in the same range as in the control group, but growth was reduced. Therefore, early life stages of pike are expected to survive in Lustian post-mining lakes when a pH of 5·50 and above is reached and maintained.     

Environmental context shapes immediate and cumulative costs of risk-induced early hatching

In animals with complex life cycles, fitness trade-offs across life stages determine the optimal time for transitions between stages. If these trade-offs vary predictably, adaptive plasticity in the timing of life history transitions may evolve. For instance, embryos of many species are capable of accelerating hatching to escape from egg predation and other hazards, but for plasticity in hatching timing to be selectively maintained, early hatching must also entail costs, probably in subsequent life stages. However the post-hatching environment, which influences this cost, is variable in nature. We assessed how two elements of the post-hatching environment, predator species and age structure created by hatching age plasticity, affect costs of hatching early in red-eyed treefrogs, Agalychnis callidryas. Red-eyed treefrog embryos were induced to hatch at the onset of hatching competence or near the peak of spontaneous hatching and exposed to one of three insect predators in single or mixed hatching-age treatments. Age structure created by hatching-age plasticity did not affect tadpole survivorship or growth; however, the consequences of hatching timing depended on predator species and foraging mode. Tadpoles that were induced to hatch early experienced initially higher mortality rates only with the more actively foraging predator. Nonetheless, mortality costs of accelerated hatching were apparent with all predators once we factored in the longer duration of exposure that early hatchlings experience in nature. This study suggests that extended exposure of young larvae to predators may be a general cost of early hatching, explaining why spontaneous hatching occurs later in life across variable environmental contexts.     

Seasonal variation in dry weight and elemental composition of the early developmental stages of Petrolisthes laevigatus (Guérin, 1835) (Decapoda: Porcellanidae) in the Seno de Reloncaví, southern Chile

In the Seno de Reloncaví, southern Chile, seasonal changes in dry weight (DW) and elemental composition (CHN) were studied in embryo (initial embryonic stage), newly hatched zoeae, and newly settled megalopae of a porcelain crab, Petrolisthes laevigatus. Samples were taken throughout the seasons of egg laying (March-December), hatching (August-February), and settlement (October–February). Values of DW and CHN per embryo or larva, respectively, were consistently minimum in the middle of each season and maximum near its beginning and end. Patterns of seasonal variation in early embryonic biomass may thus be carried over to larvae at hatching and, possibly, to the settlement stage. Such carry-over effects may be selectively advantageous, as zoeae released at the beginning or near the end of the hatching season face conditions of poor planktonic food availability in combination with low winter temperatures or decreasing temperatures at the end of summer (enforcing long development duration). Hence, an enhanced female energy allocation into egg production may subsequently translate to enhanced yolk reserves remaining at hatching, allowing for a larval development under unfavourable winter conditions. In summer, by contrast, plankton productivity and temperatures are generally high, allowing for fast larval growth and development. This coincides with minimal biomass and energy contents both at hatching and settlement. In conclusion, our data suggest that seasonal patterns in the biomass of early developmental stages of P. laevigatus may reflect phenotypic variability as an adaptive response to predictable variations in environmental conditions, allowing this species to reproduce in temperate regions with marked seasonality in water temperature and plankton productivity.