Climate and geographic trends in hatch delay of the treehole mosquito,Aedes triseriatus Say (Diptera: Culicidae)

Eggs of Aedes triseriatus mosquitoes are stimulated to hatch when inundated with water, but only a small fraction of eggs from the same batch will hatch for any given stimulus. Similar hatching or germination patterns are observed in desert plants, copepods, rotifers, insects, and many other species. Bet hedging theory suggests that parents stagger offspring emergence into vulnerable life history stages in order to avoid catastrophic reproductive failures. For Ae. triseriatus, a treehole breeding mosquito, immediate hatching of an entire clutch leaves all of the parent's progeny vulnerable to extinction in the event of a severe drought. Natural selection has likely favored parents that pursued a bet hedging strategy where the risk of reproductive failure is distributed over time. Considering treehole mosquitoes, bet hedging theory could be used to predict that hatch delay would be positively correlated with the likelihood of drought. To test this prediction, we collected Ae. triseriatus from habitats that varied widely in mean annual precipitation and exposed them to several hatch stimuli in the laboratory. Here we report that, as predicted, Ae. triseriatus eggs from high precipitation regions showed less hatch delay than areas of low precipitation. This strategy probably allows Ae. triseriatus to cope with the wide variety of climatic conditions that it faces in its extensive geographical range.     

To hatch or not to hatch? Egg hatch response to larval density and to larval contact in a treehole mosquito

Abstract.

• 1 We investigated the effect on egg hatch of exposure to: (1) varying larval density, and (2) larval contact in Aedes triseriatus Say (Diptera: Culicidae). For 2 days in the laboratory we submerged eggs into a treehole water medium containing 0 (control), 4, 12 or 24 larvae that could either contact the eggs directly or were separated from them by a screen. Following treatment, abundance of microorganisms on the egg surfaces, a food source for newly hatched larvae and a proposed hatching stimulus, was assessed by counts made from serial dilutions of samples.
• 2 We discovered a complex hatching response to larval contact and to larval density, and an interaction between these two factors in their effect on microbial growth. Hatching was inhibited in the 0-larva control, even though microorganisms grew abundantly on the eggs. Hatch rate, as well as microbial counts, were high for eggs in direct contact with 4 larvae. As density increased in the larval contact treatment, microorganisms disappeared from the egg surfaces and hatch rate declined.
• 3 When protected from larval grazing, eggs supported numerous microbial colonies irrespective of larval density. In contrast to the contact treatment group, egg hatch increased with increasing larval density. These observations suggest that the combination of microbial growth and a larval factor stimulates hatch. This hatching response may have evolved because both abundant microorganisms and numerous larvae reflect a habitat of good quality.

     

Temperature‐mediated trade‐offs and changes in life‐history integration in two slipper limpets (Gastropoda: Calyptraeidae) with planktotrophic development

Intraspecific variation in egg size and hatching size, and the genetic and environmental trade‐offs that contribute to variation, are the basis of the evolution of life histories. The present study examined both univariate and multivariate temperature‐mediated plasticity of life‐history traits, as well as temperature‐mediated trade‐offs in egg size and clutch size, in two planktotrophic species of marine slipper limpets, Crepidula. Previous work with two species of Crepidula with large eggs and lecithotrophic development has shown a significant effect of temperature on egg size and hatching size. To further examine the effect of temperature on egg size in Crepidula, the effects of temperature on egg size and hatching size, as well as the possible trade‐offs with other the life‐history features, were examined for two planktotrophic species: Crepidula incurva and Crepidula cf. marginalis. Field‐collected juveniles were raised at 23 or 28 °C and egg size, hatching size, capsules/brood, eggs/capsule, time to hatch, interbrood interval, and final body weight were recorded. Consistent with results for the lecithotrophic Crepidula, egg size and hatching size decreased with temperature in the planktotrophic species. The affects of maternal identity and individual brood account for more than half of the intraspecific variation in egg size and hatching size. Temperature also showed a significant effect on reproductive rate, with time to hatch and interbrood interval both decreasing with increasing temperature. However, temperature had contrasting effects on the number of offspring. Crepidula cf. marginalis has significantly more eggs/capsule and therefore more eggs per brood at 28 °C compared to 23 °C, although capsules/brood did not vary with temperature. Crepidula incurva, on the other hand, produced significantly more capsules/brood and more eggs per brood at the lower temperature, whereas the number of eggs/capsule did not vary with temperature. The phenotypic variance–covariance matrix of life‐history variables showed a greater response to temperature in C. incurva than in C. cf. marginalis, and temperature induced trade‐offs between offspring size and number differ between the species. These differences suggest that temperature changes as a result of seasonal upwelling along the coast of Panama will effect the reproduction and evolution of life histories of these two co‐occurring species differently. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ?? , ??–??.     

The effect of egg size variability on thermoregulation of Mallard (Anas platyrhynchos) offspring and its implications for survival

Summary There is a range of egg size phenotypes in Mallards (Anas platyrhynchos) that has a large genetic component. It was hypothesized that egg size variation could play an important role in survival of newly hatched ducklings during their first few days out of the nest when they are most susceptible to thermal stress and starvation. Precocial young must be physiologically capable of maintaining homeothermy in order to spend adequate time foraging. Duckling size at hatching was highly correlated with egg mass, and those hatching from heavier eggs were able to maintain homeothermy at colder environmental temperatures than those from lighter eggs. Heavy ducklings had significantly lower mass-specific cooling rates, but lower critical temperature did not vary significantly among ducklings of different size. Although insulation and energy reserves were not proportionally greater in larger ducklings, those hatching from heavier eggs can survive starvation longer than those from lighter eggs. The relative cold tolerance of young from light and heavy eggs will affect the ratio of time spent foraging to time spent being brooded by the female parent. Although there is no direct evidence that selection is acting on egg size, variation in this trait within a population could be maintained by fluctuating environmental conditions at hatch.     

Temperature‐mediated survival,development and hatching variation of Pacific cod Gadus macrocephalus eggs

Laboratory‐validated data on the survival, development and hatching responses of fertilized Pacific cod Gadus macrocephalus eggs from the northern Japan stock were determined through an incubation experiment. The optimum temperature for survival until hatching ranged from 4 to 8° C. No significant difference in development rates was found between the populations from Mutsu Bay, Japan, and western Canadian coastal waters even though the samples may belong to different G. macrocephalus stocks. Gadus macrocephalus larvae hatched asynchronously from egg batches despite incubation under the same environment during their development. Both incubation temperature and temperature‐mediated hatch rank affect size and yolk reserve. These data suggest that variations in water temperatures within an ecological range markedly influence the development rates, survival and hatching of the eggs, as well as the stage at hatch larvae of G. macrocephalus. Asynchronous hatching and the production of offspring with variable sizes and yolk reserves are considered evolutionary bet‐hedging strategies that enable the species to maximize their likelihood of survival in an environment with variable temperatures.     

Influence of Inoculum Density,Host, and Low-temperature Period on Delayed Hatch of Meloidogyne javanica Eggs

Most eggs of M. javanica hatch within several days when incubated in water. Those that do not are said to show delayed hatching. Several experiments were conducted to determine the effect of specific conditions on the percentage of eggs with delayed hatch. Six initial inoculum densities ranging from 100 to 20,000 eggs per pot did not influence egg hatch within a 45-day incubation period. In a 60-day test, the percentage of eggs hatching after more than 20 days was low for egg masses removed from carrot and okra and high for those from pepper and bean. Increasing exposure to cold temperature (8 C) from 7 to 30 days tended to delay hatch.     

Embryonic Development and Rates of Metabolic Activity in Early and Late Hatching Eggs of the Major Malaria Vector Anopheles gambiae

Anopheles gambiae eggs generally hatch at the completion of embryo development; two-three days post oviposition. However, staggered or delayed hatching has been observed whereby a single batch of eggs shows marked variation in time-to-hatch, with some eggs hatching 18 days post oviposition or later. The mechanism enabling delayed hatch has not been clearly elucidated but is likely mediated by environmental and genetic factors that either induce diapause or slow embryo development. This study aimed to compare metabolic activity and embryonic development between eggs collected from sub-colonies of the baseline Anopheles gambiae GAH colony previously selected for early or late time-to-hatch. Egg batches from early and late hatch sub-colonies as well as from the baseline colony were monitored for hatching. For both time-to-hatch selected sub-colonies and the baseline colony the majority of eggs hatched on day two post oviposition. Nevertheless, eggs produced by the late hatch sub-colony showed a significantly longer mean time to hatch than those produced by the early hatch sub-colony. The overall proportions that hatched were similar for all egg batches. CO₂ output between eggs from early and late hatch sub-colonies showed significant differences only at 3 and 7 days post oviposition where eggs from the early hatch and the late hatch sub-colony were more metabolically active, respectively. No qualitative differences were observed in embryo development between the sub-colonies. It is concluded that all viable embryos develop to maturity at the same rate and that a small proportion then enter a state of diapause enabling them to hatch later. As it has previously been shown that it is possible to at least partially select for late hatch, this characteristic is likely to involve genetic as well as environmental factors. Delayed hatching in An. gambiae is likely an adaptation to maximise reproductive output despite the increased risk of desiccation in an unstable aquatic environment.     

Patterns in the abundance,phenology, and hatching of the resting egg stage of the invasive zooplankter <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis>: implications for establishment

To examine how dormancy contributes to the establishment and persistence of Bythotrephes longimanus, we investigated resting egg production and hatching in relation to the demography of the planktonic stage and environmental conditions in Island Lake Reservoir (USA). During a 3-year study, the largest contribution to the egg bank occurred in autumn and most eggs hatched in spring, but we also detected some resting egg production and hatching in summer. The difference between summer and late autumn densities of eggs in sediments averaged 47% (range 0–98%) for 18 sites throughout the reservoir, which was similar to experimental estimates of in situ hatching fraction of 67% for eggs in the spring and summer following their production. Based on emergence traps, neonates hatch in the field during May and June. We estimated mortality rates of 64% for resting eggs and embryos, and 59% for newly emerged neonates. Although hatching fraction saturated at the same level, eggs incubated offshore hatched later than those nearshore where water temperature was warmer and light was detectable at the sediment surface. Low dissolved oxygen concentration did not significantly reduce hatching fraction but resulted in some eggs that initiated development but failed to hatch. Collectively, our results demonstrate substantial annual turnover in the resting egg bank of B. longimanus and high mortality of resting eggs during recruitment from the egg to the first molt of the planktonic stage. These patterns suggest that propagule pressure in the form of resting eggs requires large numbers for establishment, and that considerable post-establishment resting egg production is necessary for inter-annual persistence.     

Extended hatching periods in the subantarctic lithodid crabs<Emphasis Type="Italic"> Lithodes santolla</Emphasis> and<Emphasis Type="Italic"> Paralomis granulosa</Emphasis> (Crustacea: Decapoda: Lithodidae)

Temporal pattern of hatching was studied in the subantarctic lithodid crabs Lithodes santolla (Molina) and Paralomis granulosa (Jaquinot) from the Argentine Beagle Channel. In both species, larval hatching occurred in low daily numbers over an extended period of up to several weeks, depending on hatch size. Low daily hatching activity and low oxygen-consumption rates in freshly hatched P. granulosa larvae are discussed as life history adaptations to, and/or physiological constraints by, the environmental conditions of high latitudes. Communicated by H.-D. Franke     

Effects of embryonic responses to clutch mates on egg hatching patterns of Pentatomidae (Heteroptera)

Stink bugs and shield bugs of the family Pentatomidae (Heteroptera) generally produce a clutch of densely deposited eggs. In a few species of this family, embryos hatch in response to some form of cues associated with the preceding hatching to synchronize egg hatching with clutch mates. The aim of the present study is to obtain a family‐wide understanding of the extent to which the hatching response to clutch mates accelerates hatching within egg clutches. Accordingly, the hatching patterns in intact egg clutches and eggs individually detached from egg clutches are compared in eight species among different genera. In Halyomorpha halys, hatching is significantly and highly synchronized by the effect of the hatching response: when eggs are not attached to each other, the hatching rate is only 3.8% at 15 min and exceeds 95% at 200 min. By contrast, when eggs are attached to each other, the hatching rate reaches more than 95% at 15 min. Hatching is also significantly synchronized by the hatching response in Nezara viridula (which shows relatively high hatching synchronization) and in Piezodorus hybneri and Plautia stali (both of which show milder hatching synchronization). Synchronization of hatching is not found to be promoted by a hatching response in Aelia fieberi, Dolycoris baccarum, Eurydema rugosum or Palomena angulosa. These findings reveal that the hatching response varies depending on the species in Pentatomidae, with a wide spectrum of effects on the hatching patterns of the egg clutches.     

Embryonic Calls as Care-soliciting Signals in Budgerigars,Melopsittacus undulatus

Embryonic calls occur 1–3 d before hatching among precocial and some altricial birds. In precocial species, calls may synchronize hatching among siblings or, in semi-precocial species, elicit parental attention to, and often thermoregulation of, the hatching egg. Much less is known about the functional significance of calls in fully altricial species. In this study, naturalistic observations and laboratory experiments were used to document factors affecting calling and the parental responses to calls in one altricial species, the budgerigar Melopsittacus undulatus. Budgerigar chicks hatch asynchronously and vocalize 24–48 h before hatching. Embryonic calling rates increase at higher egg temperatures, and also as embryos near hatching. Parents easily locate a calling egg in their clutch, even among a large brood of much older, vocalizing nestlings. Furthermore, they actively assist in the last stages of hatching by helping to break the shell along the crack in the egg. Both observational and experimental evidence suggests that embryonic vocalizations are distinctive signals that increase parental attention and care, and may stimulate hatching assistance to a calling egg.     

Cyst bank life-history model for a fairy shrimp from ephemeral ponds

1. Ephemeral wetland habitats provide a useful model system for studying how life‐history patterns enable populations to persist despite high environmental variation. One important life‐history trait of both plants and crustaceans in such habitats involves hatching/germination of only some of the eggs/seeds at any time. This bet‐hedging leads to the development of a bank composed of dormant propagules of many ages. 2. The San Diego fairy shrimp, Branchinecta sandiegonensis (Crustacea: Anostraca), a dominant faunal element of ephemeral ponds in San Diego, California, is a suitable organism for studying the consequences of highly fluctuating environmental conditions. As a result of large‐scale habitat loss, the species is also endangered, and this motivated our specific study towards understanding the hatching dynamics of its egg bank for planning conservation efforts such as pool restoration and re‐creation. 3. We formulated a matrix population model using egg age within the bank to study the relationship between adult survival and reproduction, and survival in and hatching from the egg bank. As vital rates for fairy shrimp are only poorly known, we generated 48 matrices with parameters encompassing ranges of likely values for the vital rates of B. sandiegonensis. We calculated population growth rates and eigenvalue elasticities both for a static model and a model with periodic reproductive failure. 4. The model shows that in good filling events, population growth rate is very high and the egg bank is increased dramatically. While population growth rate is insensitive to long‐term survival in the egg bank in our static deterministic model, it becomes sensitive to survival in the egg bank when a regime of periodically failed reproductive events is imposed. 5. Under favourable conditions, it was best for shrimp to hatch from eggs as soon as possible. However, under a regime where failed reproductive events were common, it was best to hatch after several pool fillings. Because conditions change from favourable to unfavourable unpredictably, variation in age within the egg bank appears to be critical for the persistence of the population. This attribute needs to be carefully considered when restoring or creating new pools for conservation purposes.     

Testing Predictions of Displacement of Native Aedes by the Invasive Asian Tiger Mosquito Aedes Albopictus in Florida, USA

The Asian Tiger Mosquito Aedes albopictus arrived in the USA in 1985 in used automobile tires from Japan and became established in Texas. This species has since spread to become the most abundant container-inhabiting mosquito in the southeastern USA, including Florida, where it has reduced the range of another non-indigenous mosquito, Aedes aegypti. To assess the accuracy of predictions that A. albopictus would competitively exclude the native Eastern Treehole Mosquito Aedes triseriatus from tires but not from treeholes (Livdahl and Willey (1991) Science 253: 189–191), we extensively monitored the abundances of mosquito immatures before and after the Asian Tiger invaded these habitats in south Florida. These field data failed to demonstrate exclusion of A. triseriatus from treeholes following the establishment of A. albopictus in this microhabitat in 1991. However, A. albopictus had significantly higher metamorphic success and showed a significant increase in mean crowding on A. triseriatus in treeholes monitored from 1991 to 1999. In urban and suburban sites, A. triseriatus was uncommon in abandoned tires even before the arrival of A. albopictus. In some wooded sites, there is evidence for a decline in numbers of A. triseriatus in used tires and cemetery vases, but the native species has not been excluded from these habitats. Overall, the negative effect of A. albopictus on A. triseriatus has been less severe than that on A. aegypti. Experiments outdoors in surrogate treeholes showed that A. albopictus was more successful than A. triseriatus in survival to emergence in the presence of predatory larvae of the native mosquito Toxorhynchites rutilus when first instar predators encountered both prey species shortly after their hatch. Eggs of A. albopictus also hatched more rapidly than those of A. triseriatus, giving larvae of the invasive species an initial developmental advantage to escape predation. Biological traits that may favor A. albopictus are offset partly by greater treehole occupancy by A. triseriatus and the infrequency of the invasive mosquito species in undisturbed woodlands, which mitigates against displacement of the native mosquito in these habitats.     

Intergenomic epistasis causes asynchronous hatch times in whitefish hybrids,but only when parental ecotypes differ

Support for the theory of ecological speciation requires evidence for ecological divergence between species which directly or indirectly causes reproductive isolation. This study investigates effects of ecological vs. genetic disparity of parental species on the presence of endogenous selection (deformation and mortality rates) and potential sources of exogenous selection (growth rates and hatch timing) on hybrids. Hybrid embryonic development is analysed in a common‐garden full‐sib cross of three species belonging to two different ecotypes within the Coregonus lavaretus species flock in the central Alpine region of Europe. Although hatch timing was similar across the three species, embryonic growth rates and egg sizes differed between ecotypes. This led to a mismatch between embryonic growth rate and egg size in hybrid crosses that reveals epistasis between the maternal and embryonic genomes and transgressive hatch times that were asynchronous with control crosses. A strong constraint of egg size to embryo size at late development was also evident. We argue that this demonstrates potential for coadaptation of a maternal trait (egg size) with offspring growth rate to be an important source of selection against hybridization between ecotypes with different egg sizes. Implications for the measurement and quantification of early life‐history traits affected by this additive relationship, such as hatch day and larval size, are also discussed.     

Seasonal declines in reproductive success of the common tern Sterna hirundo: timing or parental quality?

Reproductive success declines over the course of the breeding season in many bird species. Two categories of hypothesis have been evoked to explain this decline. The “timing” hypothesis suggests that seasonal declines in breeding success are attributable to the date of laying. The “parental quality” hypothesis suggests that seasonal declines result from the fact that young, inexperienced, or low quality birds breed later in the season. To evaluate the relative importance of timing and parental quality, egg exchanges and removals were used to manipulate hatching dates of common terns Sterna hirundo. Indices of quality, attendance, provisioning rates, and reproductive success of birds in three experimental groups (delayed hatch pairs, advanced hatch pairs, and pairs induced to relay) were compared to those of date‐matched controls. Pairs that hatched chicks early raised more chicks than pairs hatching chicks late in the season, regardless of initial laying date. This suggests that hatching chicks early is advantageous in itself. Our results, however, also support the parental quality hypothesis. There was a significant negative relationship between natural laying date and fledging success, independent of hatching date. Differences in chick growth and survival suggest that higher quality adults may be able to compensate for the disadvantages of late hatching dates and achieve similar reproductive success to that of pairs hatching chicks early. We found that pairs hatching chicks late in the season were subject to more incidents of kleptoparasitism than those hatching chicks early. This may be a proximate factor contributing to seasonal declines in reproductive success for common terns, although such a mechanism would not be likely in non‐colonial species. Failure to control for egg quality may have biased the results of some prior egg exchange experiments. Additionally, altered cost of incubation may be an unavoidable confounding factor in studies designed to manipulate timing of breeding.     

The hatching time of Locusta migratoria under outdoor conditions: role of temperature and adaptive significance

The present study investigates the time of hatching of the migratory locust Locusta migratoria using egg pods that are artificially buried in the soil under outdoor conditions. Most eggs hatch in the mid‐morning, with a peak between 11.00 and 12.00 h, and none hatch before 09.00 or after 16.00 h. Furthermore, most egg pods complete hatching within a day, although some take 2 or 3 days, and egg hatching is interrupted by rain. There are no large differences in hatching time from May to September. Laboratory experiments in which the eggs are exposed to temperatures simulating outdoor conditions show that soil temperature is the main factor controlling hatching activity. The increase in temperature in the morning appears to trigger egg hatching, as confirmed by laboratory experiments, which may explain the similar hatching times between seasons. The seasonal patterns of temperature variation and hatching time suggest that the hatching time of L. migratoria eggs may be adjusted to allow the hatchlings to be exposed to high temperatures in the afternoon so that they can harden their bodies quickly.     

Effects of Temperature and Root Leachates on Embryogenic Development and Hatching of Meloidogyne chitwoodi and M. hapla

At 20 C the duration of the embryogenic development of Meloiclogyne chitwoodi and M. hapla was about 20 days. At 10 C the embryogenic development was 82-84 days for M. chitwoodi and 95-97 days for M. hapla. The effect of distilled water and root leachates of potato cv. Russet Burbank, tomato cv. Columbian, and wheat cv. Hyslop on the hatching of eggs of the two root-knot nematode species was investigated at 4, 7, 10, 15, 20, and 25 C (± 1 C). Cumulative egg taatch was no greater in root leachates titan in distilled water, but temperature did significantly affect egg hatch (P = 0.05). Less than 1% of the eggs of both nematode species hatched at 4 C. The percent cumulative hatch at 10 C was significantly less (P = 0.05) than at higher temperatures for both nematodes and significantly more (P = 0.05) M. chitwoodi eggs hatched than did M. hapla eggs. At 15 G the percent cumulative hatch of both species was significantly lower (P = 0.05) than that at 20 and 25 C. The percent cumulative egg hatch of two species did not differ at 25 C, but was higher (P = 0.05) at 25 C than at 20 C. At 7 C the emergence of M. chitwoodi juveniles was about seven times (P = 0.01) greater than that of M. hapla in distilled water.     

Report of a cohesive gelatinous egg mass produced by a tropical marine bivalve

Abstract. Gelatinous egg masses are common in a number of animal phyla. However, they are virtually unknown in marine bivalves, with structures that could be thought of as gelatinous egg masses being reported for only five species. We describe the gelatinous egg mass and intracapsular development in the tropical lucinid Phacoides pectinatus. The embryos developed within individual capsules embedded in a large flimsy, spherical mass. Swimming veligers hatch at 198 μm shell length. They did not feed, settled within several days of hatching, and metamorphosis was completed within 2 weeks of hatching. Gelatinous egg masses might be detected in members of more lucinid species if studies of development included field or in vivo observations of reproduction in addition to producing embryos by stripping the gonads.     

Influence of temperature and photoperiod on embryonic development in the dragonfly Sympetrum striolatum (Odonata: Libellulidae)

Temperature and photoperiod play major roles in insect ecology. Many insect species have fixed degree‐days for embryogenesis, with minimum and maximum temperature thresholds for egg and larval development and hatching. Often, photoperiodic changes trigger the transfer into the next life‐cycle stadium. However, it is not known whether this distinct pattern also exist in a species with a high level of phenotypic plasticity in life‐history traits. In the present study, eggs of the dragonfly Sympetrum striolatum Charpentier (Odonata: Libellulidae) are reared under different constant and fluctuating temperatures and photoperiodic conditions in several laboratory and field experiments. In general, and as expected, higher temperatures cause faster egg development. However, no general temperature or light‐days for eyespot development and hatching are found. The minimum temperature thresholds are distinguished for survival (2 °C), embryogenesis (6 °C) and larval hatching (above 6 °C). Low winter temperatures synchronize hatching. Above 36 °C, no eyespots are visible and no larvae hatch. In laboratory experiments, light is neither necessary for eyespot development, nor for hatching. By contrast to the laboratory experiments, the field experiment show that naturally changing temperature and photoperiod play a significant role in the seasonal regulation of embryonic development. The post‐eyespot development is more variable and influenced by temperature and photoperiod than the pre‐eyespot development. This developmental plasticity at the end of the embryogenesis might be a general pattern in the Libellulidae, helping them to cope with variation in environmental conditions.     

Effects of environmental calcium deprivation on the egg masses of Physa marmorata Guilding (Gastropoda: Physidae) and Biomphalaria glabrata Say (Gastropoda: Planorbidae)

Eggs of the basommatophoran snails Physa marmorata and Biomphalaria glabrata were cultured in low concentrations of calcium to determine effects on growth and development. In both species there was some development in media with 0.12 mg/l Ca²⁺ but embryos were unable to hatch. 61.04% of embryos of P. marmorata could develop to hatching in 0.22 mg/l Ca²⁺ but those of B. glabrata required a level of 0.42 mg/l Ca²⁺, to attain even a 31.07% hatch. Marked effects on growth rate, embryo size and on time taken to achieve hatching were noted in both species at very low calcium levels. The possibility of cation-controlling mechanisms in the egg membrane is discussed.