Embryos In Capsules Research Articles

Perils of drifting encapsulated embryos of the periwinkle Littorina scutulata from failures at launch and unscheduled landings

Unusually for benthic animals, mothers in many species of littorinid snails release drifting egg capsules that experience hazards of either the plankton or seafloor. At a site sheltered from large waves, numerous egg capsules of Littorina scutulata remained in high intertidal pools near mothers, and the embryos died. Mortality was also high for embryos in capsules caught in high intertidal clumps of algae. Capsules in shallow water were at early cleavage stages, indicating exposure to intertidal predators only for the hours of early development. In the laboratory, small intertidal pagurid hermit crabs and grapsid crabs ate capsules, and barnacles captured capsules, though with ingestion uncertain. An anemone and a mussel were not hazards. Embryos in capsules exposed to microbial and physical hazards on submerged sediments suffered little mortality, in contrast to previously observed high mortality at the same site for embryos of an echinoid. Predation rates could not be estimated for any location, but other hazards were less on low intertidal and subtidal sediments than in the high intertidal zone. The number of embryos per capsule and capsule sizes varied within a mother’s spawn, between spawns of mothers of similar size, and with maternal condition. Packaging several embryos in a capsule did not increase sinking speeds much beyond sinking reported for planktonic single unhatched blastulae in other taxa. Some embryos died through failure to launch, but capsules provided low sinking speeds during drift, brief residence in shallow intertidal waters, and protection from microbial and chemical hazards on sediments when sunk.

Brood Reduction, Not Poecilogony, in a Vermetid Gastropod with Two Developmental Outcomes within Egg Capsules

A small vermetid gastropod broods capsules containing nurse eggs and embryos that develop into small veligers. A few of these veligers continue development and growth while nurse eggs and developmentally arrested sibling veligers disappear. Survivors hatch as crawling pediveligers and juveniles. None of the veligers, if removed from capsules, swim in a directed way or withdraw into their shells, indicating that even the developing veligers are unsuited for extracapsular life until they can crawl. The shells of arrested veligers decalcify while their siblings grow. Few of the developmentally arrested veligers that were isolated from siblings and fed algal cells resumed detectable growth. Nurse eggs rather than cannibalism provide most of the food, but full growth of developing veligers depends on limited sharing; arrest of some siblings is a necessary adjunct of the nurse-egg feeding. Here, two developmental outcomes for larvae produced by developmental arrest of some (often termed poecilogony) serves instead as a means of brood reduction. Brood reduction is often attributed to family conflicts resulting from genetic differences. Another hypothesis is that a mother who cannot accurately sort numbers of nurse eggs and developing eggs into capsules could rely on brood reduction to adjust food for her offspring. At the extreme, an entirely random packaging would produce a binomial distribution of embryos in capsules, a very uneven distribution of food per embryo, and some capsules with no embryos. Males have yet to be found in this species, but even if reproduction is asexual, selection could still favor brood reduction.

Should I stay or should I go: predator- and conspecific-induced hatching in a marine snail

Predator-induced hatching plasticity has been demonstrated in many species of amphibians. However, animals from other clades (e.g., marine species of molluscs and annelids) also place their embryos in capsules or gelatinous masses and might also exhibit hatching plasticity to predators. To date there is no evidence of predator-induced hatching plasticity from any marine species or a major clade of bilateria animals, the Lophotrochozoa. We studied predator-induced hatching plasticity of Nucella lamellosa, a carnivorous marine snail that deposits embryos in capsules. We used two experiments to investigate the effects of two types of predator, crabs and isopods, on developing embryos. In the first experiment, we quantified proportion of hatched embryos from capsules through time exposed to water-borne chemicals of crabs and isopods. Crabs delayed time-to-hatching, and the effects of predators were additive. In the second experiment, we quantified proportion of hatched embryos from capsules through time, developmental stage, and size of embryos in capsules exposed to water-borne chemicals of crabs and conspecifics. With this experiment, we wanted to answer: (1) whether a delay in hatching corresponded to embryos developing slower, and (2) whether the general products of metabolic waste from organisms can delay hatching. We unexpectedly observed that adult conspecific snails accelerated hatching but not developmental rate-the few past studies on the effects of conspecifics have all demonstrated that conspecifics delay time-to-hatching and rate of development. The results were also inconsistent with metabolic waste in general causing a delay in hatching, although the effect of conspecifics does weaken this inference. This study demonstrates that predators delay time-to-hatching in a marine mollusc, and suggests that predator-induced hatching plasticity is widespread among animals and likely evolved multiple times within the bilateria. In addition, conspecifics accelerated time-to-hatching in a marine mollusc, which suggests that conspecifics, like predators, might commonly influence when embryos hatch.

Embryonic encapsulation and maternal incubation: Requirements for survival of the early stages of the estuarine gastropod Crepipatella dilatata

ABSTRACT During their reproductive period, females of Crepipatella dilatata deposit their embryos in capsules that they then brood in the pallial cavity until juveniles emerge several weeks later, after passing through a transient veliger “larval” stage. Artificially excapsulated veligers of this species experimentally exposed to a wide range of salinities (5, 10, 15, 20, 25, and 30 psu) for six hours showed reduced activity at salinities of 15 and 20 psu, whereas encapsulated veligers exposed to those same salinities showed no reduction of activity. Artificially excapsulated veligers showed high mortality at salinities of 5 and 10 psu; encapsulated embryonic stages also showed high mortalities at 5 psu and serious sublethal effects at 10 psu in tests excluding maternal protection, showing that encapsulation alone does not provide complete protection from low salinity stress. Natural tidal cycles in the Quempillen River estuary also reduced embryonic survival at salinities of ≤ 10 psu when the capsules were exposed without maternal protection. In contrast, encapsulated embryos protected by their mothers survived well regardless of the salinity to which they were exposed, under both natural and laboratory-simulated estuarine tidal cycles. C. dilatata are able to develop in the estuary only because of maternal protection, since salinity levels in this environment sometimes decline to as low as 7 psu. Successful embryonic development in this estuary reflects the capacity of C. dilatata adults to detect dangerously low salinity levels and then seal themselves off from the environment for up to 50 hrs (O. Chaparro pers. obs.) when the salinity drops below 22.5 psu, allowing salinity to remain above this level within the pallial cavity despite continued salinity declines in the surrounding seawater.

Effect of oxygen availability in determining clutch size in Acanthina monodon

Most taxa of aquatic invertebrates exhibit (1) strong variation in number and size of off- spring, and (2) an association between adult size and mode of development, and also between mode of development and size of the clutch. These patterns suggest that a similar set of constraints affects both critical life-history traits and physiology of most taxa. Evidence suggests that oxygen is limiting during early development in aggregated egg masses of marine invertebrates. We studied whether oxygen availability during early development could be a determinant of clutch size in the gastropod Acanthia monodon. We estimated intra- and extracapsular oxygen availability throughout embryo development using optic fibers and incubated capsules at different oxygen partial pressures (hypoxia, normoxia and hyperoxia) throughout development. We also compared the ratio between nurse eggs and embryos in capsules collected in situ as well as in experimental capsules. We found that intracapsular oxygen availability decreased as embryos developed (80% air saturation in early stages to 40% air saturation in late stages). The mean number of embryos increased as oxygen avail- ability increased from hypoxia to hyperoxia, while the total number of eggs remained constant. The mean number of embryos that developed under normoxia was similar to that recorded in field- collected capsules. We suggest that oxygen availability determines clutch size in this species, prob- ably as a result of oxygen competition among siblings. This may also occur in other taxa of marine invertebrates, since oxygen is limiting in egg masses in general.

Effects of sessile Protozoa on intracapsular oxygen tension and embryonic shell calcification in the muricid Chorus giganteus

Clusters of egg capsules deposited by some common marine Mollusca may suffer problems of a low diffusive oxygen supply to the embryos they contain, especially if the capsules are exposed to hypoxic seawater or attachment and growth of marine biofouling organisms. The present study was undertaken to determine the effects of severe biofouling by sessile Protozoa on intracapsular oxygen tension (IPO 2 ) and the development of embryos contained in the egg capsules of the muricid snail Chorus giganteus. We also investigated the effects of ambient oxygen tension (EPO2) on IPO 2 . The presence of sessile Protozoa attached to the outer wall of the egg capsules significantly reduced the IPO 2 compared to capsules not fouled by Protozoa. Clean capsules containing embryos showed an IPO 2 of about 105 mm Hg, compared with about 92 mm Hg for protozoan-fouled capsules when both were immersed in air-saturated seawater at 12°C. The embryos in capsules without Protozoa grew normally, hatching in about 70 d as veliconch larvae, whereas the development of larvae in protozoan-fouled capsules showed impairment of shell formation and delay in hatching for up to 5 mo. Pre-hatch embryos at 60 d measured about 922 μm and had an ash content near 18 μg embryo -1 embryos in capsules covered by micro-organisms measured only about 783 pm, with an ash content of about 3 μg embryo -1 over the same time period. Our study suggested that the lack of larval calcification observed in the presence of sessile Protozoa on the outer wall of the egg capsules was probably related to reduced IPO 2 . Similarly, any factor reducing oxygen supply to encapsulated embryos (i.e. exposure to water masses with low oxygen content, biofouling, reduced water movement) could impair embryonic development, a significant phenomenon thus far not reported in C. giganteus.

The biology of Dendropoma corallinaceum and Serpulorbis natalensis, two South African vermetid gastropods

D. corallinacewn forms sheet-like colonies at Mean Low Water (MLW) on exposed rocky shores: S. natalensis forms loose aggregations under stones below MLW in more sheltered situations. Both species released young from July— December. D. corallinaceun has single embryos in capsules brooded free in the mantle cavity; S. natalensis has 20–40 embryos per capsule, the latter being attached to the roof of the shell and hanging through a dorsal slit in the mantle. The morphology of the crawling young is similar in both species, resembling adult errant prosobranchs. D. corallinaceum has a stouter, more streamlined protoconch than S. natalensis correlated with the more turbulent water of its habitat. The crawling young secrete a sticky mucous trail for adhesion to the substratum. They are strongly photopositive until settlement. D. corallinaceum can settle within 24 hours but may delay settlement to 4–5 days if suitable substrata are absent. Metamorphosis is complete 2 days after settlement. D. corallinaceum prefer to settle on Lithothamnion which is easily eroded by the radula to provide a groove for rapid, firm attachments to the substratum. Both species feed from mucous nets, S. natalensis having a slower feeding rhythm commensurate with its larger size. Contiguous pairs of S. natalensis have synchronised feeding rhythms, probably to reduce mutual net robbing. Particles are also filtered out of suspension; many particles are rejected in the pseudofaeces but some are ingested. D. corallinaceum and S. natalensis have many similar ecologically equivalent species throughout the warm temperate and tropical seas of the world.

DO INTERTIDAL SNAILS SPAWN IN THE RIGHT PLACES?

Most marine snails enclose their embryos in capsules, attach the capsules to hard surfaces, and then abandon them. The embryos will survive only if their parents have chosen suitable attachment sites. Some embryos are left below the tidal zone, and the major danger they face is discovery by a predator. These embryos may survive on almost any type of site. Other snails attach capsules to intertidal surfaces, and their embryos may be exposed to repeated dehydration and overheating, as well as to predators. Chances for survival at these sites will depend on how long the embryos must remain exposed. Some intertidal snails require 3-5 months to complete development (Spight, 1975). These embryos will survive only if the parents choose relatively protected attachment sites. The types of sites used by the rocky shore snail, Thais lamellosa, were tallied for a six year period and subsequent embryo mortality was monitored for four of the years. Surprisingly, the snails consistently chose sites at which few embryos survived, although better sites were always available. The discussion will hypothesize why the sites used are the right places for the snails even though they are not right for the embryos.