Role of chemical signalling in release of motor programs during embryogenesis of freshwater snails <Emphasis Type="Italic">Lymnaea stagnalis</Emphasis> and <Emphasis Type="Italic">Helisoma trivolvis</Emphasis>

We have earlier found that freshwater pond snails Helisoma trivolvis and Lymnaea stagnalis, when reared under conditions of starvation, release chemical signals that reversibly suppress larval development of conspecific embryos. Here, we report that (i) these signals are not strictly conspecific and affect also the embryos of a closely related species, which occupies a similar environmental niche; (ii) besides the development of embryos, the signals also affect the release of main motor programs, such as locomotion, feeding, and cardiac activity; (iii) action of the signals is bidirectional: they retard the development and release of motor programs at the early larval stages (trochophore to veliger) and accelerate them at later stages (late veliger to hatching). A possible adaptive significance of the described phenomena is discussed.     

Effect of Copper Ions on Early Developmental Stages of the Mussel <Emphasis Type="Italic">Mytilus trossulus</Emphasis> (Bivalvia)

The effect of exposure to copper in seawater (0.005, 0.01, and 0.02 mg/liter) on the development of Mytilus trossulus was examined in the stages of fertilization, blastula, early veliger, veliger, and veliconch. Copper in a concentration of 0.01 and 0.02 mg/liter inhibited the development and growth of the embryos and larvae. At 0.005 mg Cu/liter, the embryos and larvae were capable of adaptation. If M. trossulus embryos and larvae were maintained for 1–2 days in seawater containing 0.01 mg Cu/liter and then transferred to clean water, the consequences were largely dependent on the developmental stage at which the exposure to copper took place. The early developmental stages were more sensitive to the effect of copper than veliger larvae.Original Russian Text Copyright ¢ 2005 by Biologiya Morya, Yaroslavtseva, Sergeeva.     

The rate of oxygen consumption during embryogenesis of Lymnaea stagnalis (Gastropoda)

We studied the rate of oxygen consumption by the Lymnaea stagnalis embryos. The rate of oxygen consumption increased consistently during embryogenesis. The volume specific rate of oxygen consumption increased initially from the early cleavage stages until the gastrula stage and then decreased gradually to the eclosion of snails. There are three periods in embryogenesis of L. stagnalis, which differ in the coefficients of allometric dependence between the rate of oxygen consumption and volume of embryos: (1) early embryogenesis, when the increase in the rate of oxygen consumption is not accompanied by the growth of volume of the embryos; (2) larval period (trochophore and veliger stages; exponential coefficient k = 0.514), and (3) postlarval period (exponential coefficient k = 0.206).     

The rate of oxygen consumption during embryogenesis of Lymnaea stagnalis (Gastropoda)

We studied the rate of oxygen consumption by the Lymnaea stagnalis embryos. The rate of oxygen consumption increased consistently during embryogenesis. The volume specific rate of oxygen consumption increased initially from the early cleavage stages until the gastrula stage and then decreased gradually to the eclosion of snails. There are three periods in embryogenesis of L. stagnalis, which differ in the coefficients of allometric dependence between the rate of oxygen consumption and volume of embryos: (1) early embryogenesis, when the increase in the rate of oxygen consumption is not accompanied by the growth of volume of the embryos; (2) larval period (trochophore and veliger stages; exponential coefficient k = 0.514), and (3) postlarval period (exponential coefficient k = 0.206).     

Spawning and larval development of Tegula euryomphala (Jones, 1844) (Trochoidea: Tegulidae) from La Herradura Bay,Chile

The spawning and larval development of Tegula euryomphala (Jones, 1844) were studied under laboratory conditions. The release of gametes was stimulated by raising temperature from 16 to 19?°C, and could be induced during several months of the year. Males always spawned first. Eggs were 155.6?μm in diameter and remained unattached to each other. Development was similar to that described for other Trochoidea and lasted 3–5?days at 19?°C. Larvae hatched approximately 24 h post-fertilization at the pre-torsional veliger stage. The larval protoconch of the metamorphic veliger measured 247?μm and was ornamented with six spiral threads and weakly ordered granulation. Larvae metamorphose without any natural substratum.     

Embryonic rotational behaviour in the pond snail Lymnaea stagnalis: influences of environmental oxygen and development stage

Responses of freshwater organisms to environmental oxygen tensions (PO₂) have focused on adult (i.e. late developmental) stages, yet responses of embryonic stages to changes in environmental PO₂ must also have implications for organismal biology. Here we assess how the rotational behaviour of the freshwater snail Lymnaea stagnalis changes during development in response to conditions of hypoxia and hyperoxia. As rotation rate is linked to gas mixing in the fluid surrounding the embryo, we predicted that it would increase under hypoxic conditions but decrease under hyperoxia. Contrary to predictions, early, veliger stage embryos showed no change in their rotation rate under hyperoxia, and later, hippo stage embryos showed only a marginally significant increase in rotation under these conditions. Predictions for hypoxia were broadly supported, however, with both veliger and hippo stages showing a marked hypoxia-related increase in their rotation rates. There were also subtle differences between developmental stages, with hippos responding at PO₂s (50% air saturation) greater than those required to elicit a similar response in veligers (20% air saturation). Differences between developmental stages also occurred on return to normoxic conditions following hypoxia: rotation in veligers returned to pre-exposure levels, whereas there was a virtual cessation in embryos at the hippo stage, likely the result of overstimulation of oxygen sensors driving ciliary movement in later, more developed embryos. Together, these findings suggest that the spinning activity of L. stagnalis embryos varies depending on environmental PO₂s and developmental stage, increasing during hypoxia to mix capsular contents and maintain a diffusive gradient for oxygen entry into the capsule from the external environment (“stir-bar” theory of embryonic rotational behaviour).     

扁玉螺早期发育的实验观察

在实验室条件下人工孵化扁玉螺(Neverita didyma)的卵块,观察了其胚胎发育和幼虫发育过程.扁玉螺的早期发育属间接发生型,其胚胎发育包括卵裂期、囊胚期、原肠胚、膜内担轮幼虫、膜内面盘幼虫;幼虫发育包括面盘幼虫、后期面盘幼虫和匍匐幼虫;匍匐幼虫经变态后发育为稚螺.在水温25～26℃条件下,受精卵发育至膜内面盘幼虫约需38h,5～6d后面盘幼虫冲破卵膜而孵化.扁玉螺面盘幼虫的显著特点是具有1对眼点和1对平衡囊,面盘呈双叶状;后期面盘幼虫的面盘为4叶,呈蝴蝶状,足发达,幼虫既能浮游,又能爬行.后期面盘幼虫进一步生长发育,逐渐转入匍匐生活.     

Larval ecology and morphology in fossil gastropods

The shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.     

Effects of Low Salinity on Adult Behavior and Larval Performance in the Intertidal Gastropod Crepipatella peruviana (Calyptraeidae)

Shallow-water coastal areas suffer frequent reductions in salinity due to heavy rains, potentially stressing the organisms found there, particularly the early stages of development (including pelagic larvae). Individual adults and newly hatched larvae of the gastropod Crepipatella peruviana were exposed to different levels of salinity stress (32(control), 25, 20 or 15), to quantify the immediate effects of exposure to low salinities on adult and larval behavior and on the physiological performance of the larvae. For adults we recorded the threshold salinity that initiates brood chamber isolation. For larvae, we measured the impact of reduced salinity on velar surface area, velum activity, swimming velocity, clearance rate (CR), oxygen consumption (OCR), and mortality (LC50); we also documented the impact of salinity discontinuities on the vertical distribution of veliger larvae in the water column. The results indicate that adults will completely isolate themselves from the external environment by clamping firmly against the substrate at salinities ≤24. Moreover, the newly hatched larvae showed increased mortality at lower salinities, while survivors showed decreased velum activity, decreased exposed velum surface area, and decreased mean swimming velocity. The clearance rates and oxygen consumption rates of stressed larvae were significantly lower than those of control individuals. Finally, salinity discontinuities affected the vertical distribution of larvae in the water column. Although adults can protect their embryos from low salinity stress until hatching, salinities <24 clearly affect survival, physiology and behavior in early larval life, which will substantially affect the fitness of the species under declining ambient salinities.     

Larval development and metamorphosis in the marine pulmonate Amphibola crenata (Mollusca: Pulmonata)

The development of the free-swimming veliger of Amphibola is followed from hatching to settlement, and the larval structures compared with those of post-metamorphic juveniles and adult snails. Observations of living specimens and light-microscope sections were combined with scanning electron microscopy to build up a composite picture of veliger structure.
Four stages in the development of veligers are recognized, each being characterized by the appearance of organ systems such as the mantle cavity, larval heart, adult heart and kidney, and larval pallial gland. At or after metamorphosis, the larval systems (heart, kidney and pallial gland) disappear, and the developing adult organs move to the positions characteristic of adult snails.
Organogenesis in Amphibola veligers is compared with that of prosobranch and opisthobranch larvae, and with that of pulmonate larvae with direct development. The closest similarity is seen to be with opisthobranch veligers.     

Embryonic learning and developmental carry-over effects in an invasive anuran

Carry-over effects influence trait responses in later life stages as a result of early experience with environmental cues. Predation risk is an influential stressor and selection exists for early recognition of threats. In particular, invasive species may benefit from carry-over effects by preemptively recognizing and responding to novel predators via latent developmental changes and embryonic learning. In a factorial experiment, we conditioned invasive American bullfrog embryos (Lithobates catesbeianus) to the odor of a novel fish predator, largemouth bass (Micropterus salmoides) alone or in combination with injured conspecific cues. We quantified developmental carryover in the larval life stage and found that individuals conditioned to the highest risk (fish and injured conspecific cues) grew into longer bodied larvae relative to larvae from lower risk treatments. We also assessed embryonic learning, a behavioral carry-over effect, and found an interaction between embryonic conditioning and larval exposure. Behavioral responses were only found in scenarios when predation risk varied in intensity across life history stages, thus requiring a more flexible antipredator strategy. This indicates a potential trade-off between the two strategies in larval growth and development rates, and time until metamorphosis. Our results suggest that early predator exposure and carry-over effects have significant impacts on life history trajectories for American bullfrogs. This research contributes to our understanding of a potentially important invasion mechanism in an anuran species of conservation concern.     

Histochemical localization of FMRFamide, serotonin and catecholamines in embryonic Crepidula fornicata (Gastropoda, Prosobranchia)

 Recent reports indicate that neuronal elements develop in early larval stages of some Gastropoda from the Pulmonata and Opisthobranchia prior to the appearance of any ganglia of the future adult central nervous system (CNS). The present study describes similar early neuronal elements in Crepidula fornicata. A posterior FMRFamide-like immunoreactive (LIR) cell with anteriorly projected fibers was observed in the trochophore stage. Additional FMRFamide-LIR and serotonin-LIR cells and fibers were found in the apical organ in the trochophore and early veliger stages. FMRFamide-LIR and serotonin-LIR projections to the velum and foot were also detected at this time. As the veliger developed, peripheral FMRFamide-LIR and later catecholaminergic cells were located in the foot region. Also during this stage, catecholaminergic cells and processes were observed near the mouth. In addition, this study tentatively identified the first serotonin- and FMRFamide-LIR cells and fibers within the developing ganglia of the adult CNS, which appeared in close proximity to the earlier developing elements. These observations are consistent with the hypothesis that, in addition to its presumed role in the control of larval behaviors, the larval nervous system guides the development of the adult CNS. Larvae from the class Bivalvia and other invertebrate phyla also have neuronal elements marked by the presence of FMRFamide, serotonin, and catecholamines, and, therefore, this study may provide additional insights into phylogenetic relationships of the Gastropoda with other representatives of the Mollusca and different invertebrate phyla. Accepted: 10 February 1999     

Individual growth of the great ramshorn snail <Emphasis Type="Italic">Planorbarius corneus</Emphasis> (Gastropoda,Planorbidae) embryos

Individual growth of the great ramshorn snail Planorbarius corneus has been studied by intravital video imaging. As has been observed, the types of growth change over the embryogenesis. The linear dimensions slightly but in a statistically significant manner decrease during the stages of cleavage to blastula. Starting from the stage of blastula to trochophore, the embryo diameter remains constant to commence increasing at the stage of middle trochophore. During the larval stages (trochophore and veliger), the growth is synchronous (in Dettlaffs, biological time units) for the embryos in both the same clutch and different clutches. The growth at that time is exponential but later desynchronizes in individual clutches. The embryos in eight clutches grew and developed slower and hatched later as compared with the remaining five egg clutches. An accelerated growth follows an asymptomatic pattern according to the von Bertalanffy equation. A retarded growth is describable with a linear equation. The observed differences are likely to be associated with the number of embryos in a clutch. All types of changes in the linear dimensions observed in the great ramshorn snail embryogenesis can be described with the same united equation.     

Simultaneous measurements of oxygen consumption and ammonia-N excretion in embryos and larvae of marine invertebrates

The quantification of oxygen consumption and ammonia-N excretion rates is essential in determining energy requirements for development of larval invertebrates. In larval energetics, there is a need for accurate and uncomplicated techniques to quantify metabolic rates. A method for simultaneous measurements of oxygen and ammonia-N concentrations is presented. It employs sealed respirometric chambers (ca. 30 ml) in which embryos and larvae are incubated. Analysis is carried out in end-point samples by Winkler's titration and indophenol-blue for oxygen and ammonia-N, respectively. Water is sampled into volume-calibrated glass syringes and oxygen consumption and ammonia-N excretion rates were determined by the difference between experimental and control (no animals) units. The method was successfully used to measure metabolic rates in embryo and larval stages of the shrimp Farfantepenaeus paulensis and in veliger of the mussel Perna perna. The accuracy denoted by the coefficient of variation is comparable to previous results on larval metabolic rates. A biomass: volume (microg ml(-1)) is proposed to extend its application to further species of marine invertebrates. The method is simple to operate, involves non-expensive material and is portable enough for field work. A substantial number of replicates can be analyzed at the same time and O:N ratio, an indicator of the catabolized substrate, can be calculated.     

Observation of some key stages of the embryonic development of Biomphalaria straminea (Dunker, 1848) (Molluska,Planorbidae)

Summary

Biomphalaria straminea is a freshwater snail and one of the intermediate hosts of the trematode parasite which causes schistosomiasis in Brazil. The main stages of embryonic development were analyzed with confocal laser scanning microscopy (CLSM), using the fluorescent probe DiOC and Nile Red as a vital stain in in vivo preparations. In fixed preparations nuclei were stained by the Feulgen reaction or the fluorescent DNA probe, Höechst 33342. Results obtained from the analysis of embryos at the stages of early cleavage, morula, blastula, gastrula, early trocophore and veliger showed that these stages are similar to those described for Biomphalaria glabrata and Biomphalaria tenagophila. CLSM optical sections of early trochophore showed important morphological structures such as the blastopore, stomodeum and shell gland; and in early veliger, internal organs such as the esophagus, stomach and male and female ducts were also clearly identified.     

DNA-RNA hybridization studies of gene activity during the development of the gastropod, Acmaea scutum

DNA-RNA molecular hybridization experiments were performed to compare sequences of RNA that are present in the unfertilized egg, gastrula, trochophore, early veliger, mid-veliger, and adult with that population that is being synthesized by the midveliger. The conditions of annealing were designed to examine the more common RNA sequences that are complementary to the highly reiterated sequences of the genome. It was found that the RNA of the unfertilized egg competes extensively with RNA sequences that are being synthesized by this relatively late larval stage. These RNA sequences are present at a lower concentration in RNA preparations extracted from unfertilized eggs as compared to later stages. Autoradiographs of ³H-uridine labeled veligers indicate that the pulse-labeled RNA used in these experiments represents that synthesized by a wide variety of larval tissues. These results are discussed with respect to differences in RNA metabolism and determination between regulative and mosaic embryos.     

Plasticity in newt metamorphosis: the effect of predation at embryonic and larval stages

1. Some organisms under variable predator pressure show induced antipredator defences, whose development incurs costs and may be associated with changes to later performance. This may be of especial relevance to animals with complex life histories involving metamorphosis. 2. This study examines the effect of predation environment, experienced both during embryonic and larval stages, on palmate newt (Triturus helveticus) metamorphosis. Newt eggs were raised until hatching with or without exposure to chemical cues from brown trout (Salmo trutta), and larval development was monitored in the presence or absence of the cues. 3. Exposure to predator cues during the embryonic stage resulted in higher growth rates at the larval stage, reduced time to metamorphosis and size at metamorphosis. Metamorphs also had narrower heads and shorter forelimbs than those from predator‐free treatments. In contrast, exposure to predator cues during the larval stage did not affect metamorph characteristics. 4. These results indicate that developing embryos are sensitive to predator chemical cues and that the responses can extend to later stages. Reversion of induced defences when predation risk ceased was not detected. We discuss the possible adaptive significance of these responses.     

Role of Sensory Experience in Functional Development of Drosophila Motor Circuits

Neuronal circuits are formed according to a genetically predetermined program and then reconstructed in an experience-dependent manner. While the existence of experience-dependent plasticity has been demonstrated for the visual and other sensory systems, it remains unknown whether this is also the case for motor systems. Here we examined the effects of eliminating sensory inputs on the development of peristaltic movements in Drosophila embryos and larvae. The peristalsis is initially slow and uncoordinated, but gradually develops into a mature pattern during late embryonic stages. We tested whether inhibiting the transmission of specific sensory neurons during this period would have lasting effects on the properties of the sensorimotor circuits. We applied Shibire-mediated inhibition for six hours during embryonic development (15–21 h after egg laying [AEL]) and studied its effects on peristalsis in the mature second- and third-instar larvae. We found that inhibition of chordotonal organs, but not multidendritic neurons, led to a lasting decrease in the speed of larval locomotion. To narrow down the sensitive period, we applied shorter inhibition at various embryonic and larval stages and found that two-hour inhibition during 16–20 h AEL, but not at earlier or later stages, was sufficient to cause the effect. These results suggest that neural activity mediated by specific sensory neurons is involved in the maturation of sensorimotor circuits in Drosophila and that there is a critical period for this plastic change. Consistent with a role of chordotonal neurons in sensory feedback, these neurons were activated during larval peristalsis and acute inhibition of their activity decreased the speed of larval locomotion.