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

The other gastropod larvae: larval morphogenesis in a marine neritimorph

Two of the three major gastropod clades with feeding larvae are sister groups and larval morphogenesis for members of these clades, the Caenogastropoda and Heterobranchia, has been well studied. The third clade, the Neritimorpha, has an unstable phylogenetic position and little is known about development of their planktotrophic larvae. Information about larval morphology of neritimorphs and resolution of their controversial phylogenetic placement is critically important for understanding evolution of larval feeding within the Gastropoda. We describe larval morphogenesis to metamorphic competence for laboratory-reared larvae of Nerita melanotragus (Smith, 1884) (Neritimorpha: Neritidae). Preliminary observations suggest that prehatch larvae are capable of delayed hatching, possibly by entering a diapause state. Our description of larval morphogenesis, as based on tissue sections for light and transmission electron microscopy, scanning electron microscopy, three-dimensional-reconstructions of sectioned tissue, and labeling of muscles with fluorphore-tagged phalloidin, revealed four features that are unprecedented among both feeding and nonfeeding gastropod larvae. Larvae of N. melanotragus have muscles on the left and right side that both meet current criteria of a larval retractor muscle; shell-anchored muscles with oblique striations that project inside the visceral nerve loop to insert mainly on the velar lobes. They also have left and right digestive glands of similar size and a left and right hypobranchial gland. A larval "heart" is absent, but water circulation through the mantle cavity may be facilitated by large circular orifices, lined by patches of motile cilia, leading in and out of the mantle cavity. Comparison of larval traits among all three groups of gastropods with feeding larvae indicates that larvae of N. melanotragus have many unique characteristics, but they show more similarities to caenogastropod than to heterobranch larvae. These results are a significant step toward the goal of identifying primitive versus derived larval traits among feeding gastropod larvae.

Development of cardiovascular function in the marine gastropod Littorina obtusata (Linnaeus)

The molluscan cardiovascular system typically incorporates a transient extracardiac structure, the larval heart, early in development, but the functional importance of this structure is unclear. We documented the ontogeny and regulatory ability of the larval heart in relation to two other circulatory structures, the true heart and the velum, in the intertidal gastropod Littorina obtusata. There was a mismatch between the appearance of the larval heart and the velum. Velar lobes appeared early in development (day 4), but the larval heart did not begin beating until day 13. The beating of the larval heart reached a maximum on day 17 and then decreased until the structure itself disappeared (day 24). The true heart began to beat on day 17. Its rate of beating increased as that of the larval heart decreased, possibly suggesting a gradual shift from a larval heart-driven to a true heart-driven circulation. The true heart was not sensitive to acutely declining PO2 shortly after it began to beat, but increased in activity in response to acutely declining PO2 by day 21. Larval heart responses were similar to those of the true heart, with early insensitivity to declining PO2 (day 13) followed by a response by day 15. Increased velum-driven rotational activity under acutely declining PO2 was greatest in early developmental stages. Together, these findings point to cardiovascular function in L. obtusata larvae being the result of a complex interaction between velum, larval and true heart activities, with the functions of the three structures coinciding but their relative importance changing throughout larval development.

Molecular phylogenetic analysis of mudflat snails (Gastropoda: Euthyneura: Amphiboloidea) supports an Australasian centre of origin

Amphiboloidea is a small but widespread group of snails found exclusively, and often abundantly, in mudflat and associated salt marsh or mangrove habitat. This study uses molecular data from three loci (COI, 16S and 28S) to infer phylogenetic relationships in Amphiboloidea and examine its position in Euthyneura. All but two of the named extant species of Amphiboloidea and additional undescribed taxa from across Southeast Asia and the Arabian Gulf were sampled. In contrast to the current morphology-based classification dividing Amphiboloidea into three families, analysis of molecular data supports revision of the classification to comprise two families. Maningrididae is a monotypic family basal to Amphibolidae, which is revised to comprise three subfamilies: Amphibolinae, Phallomedusinae and Salinatorinae. Sequence divergence between Asian populations of Naranjia is relatively large and possibly indicative of species complexes divergent across the Strait of Malacca. Salinatorrosacea and Salinator burmana do not cluster with other Salinator species, and require generic reassignment. In addition, sequences were obtained from an undescribed species of Lactiforis from the Malay Peninsula. Reconstruction of ancestral distributions indicates a plesiomorphic distribution and centre of origin in Australasia, with two genera subsequently diversifying throughout Asia. Increasing the sampling density of amphiboloid taxa in a phylogenetic analysis of Euthyneura did not resolve the identity of the sister taxon to Amphibolidae, but confirmed its inclusion in Pulmonata/Panpulmonata.

Larval development and metamorphic transformation of the feeding system in the kleptoparasitic snailTrichotropis cancellata(Mollusca, Caenogastropoda)

Trichotropis cancellata Hinds, 1849 has a planktonic larval stage that feeds on microalgae and a benthic stage that feeds both by ctenidial suspension feeding and by stealing food ("kleptoparasitism") from several species of suspension-feeding, tube-dwelling polychaete worms. We used scanning electron microscopy, histological sections, and observations on live animals to document the sequence and timing of morphogenetic events during larval and metamorphic development of T. cancellata. These data were compared with other accounts of gastropod development to test for differences in the timing of developmental events among feeding larvae of two major gastropod clades: the caeno gastropods and heterobranchs. In T. cancellata, as in feeding larvae of previously studied caenogastropods, components of the post-metamorphic body plan differentiate at an earlier stage of larval development (relative to times of hatching and ability to undergo metamorphosis) than in feeding heterobranch larvae. Metamorphosis of T. cancellata was induced by polychaete hosts of this snail's kleptoparasitic benthic stage, and young juveniles of T. cancellata could steal food from these polychaetes within a day after snail metamorphosis began. Rapid onset of kleptoparasitic feeding following metamorphosis of T. cancellata was permitted by development of a specialized feeding structure, the pseudo proboscis, during the larval stage. This novel embellishment of larval development was likely preceded during evolution by selective larval induction by polychaete hosts.

Development and evolution of adult feeding structures in Caenogastropods: overcoming larval functional constraints

Comparative study of the developing foregut in three species of caenogastropods, including an herbivorous grazer (Lacuna vincta) and two carnivores (Euspira [Polinices] lewisii and Nassarius mendicus), suggests how the specialized adult foregut of a carnivorous neogastropod evolved within a life cycle having a planktotrophic larva. Postmetamorphic feeding structures (buccal cavity and radular sac) in all three species achieve advanced differentiation in the larval stage, permitting juvenile feeding at 3 days postmetamorphosis. Recent phylogenetic hypotheses for the Gastropoda predict that foregut developmental patterns in E. lewisii and N. mendicus are derived, relative to that of L. vincta. In hatching larvae of these three, the anlage of postmetamorphic feeding structures is a small patch of nonciliated cells embedded in the ventral wall of the larval foregut and the patch soon forms an outpocketing. During subsequent morphogenesis, Euspira lewisii and N. mendicus share a developmental novelty that involves semi-isolation of the developing, postmetamorphic buccal cavity and radular sac from the larval foregut and formation of a new, definitive mouth at metamorphosis. Nassarius mendicus, a neogastropod, embellishes this novelty by adding the entire anterior esophagus and valve of Leiblein (de novo structures) to the semi-isolated buccal cavity. Therefore, a valve and long stretch of muscular anterior esophagus, which are necessary for feeding with a pleurembolic proboscis, are preformed in the larval stage of this neogastropod without interfering with larval feeding. The inferred evolutionary events leading to postmetamorphic feeding specialization in N. mendicus are invisible in adults; they require reconstruction from comparative developmental analysis.

The gastropod nervous system in metamorphosis

Many gastropods, including the sea hare Aplysia californica, undergo metamorphosis in passing from the larval to the juvenile phases of their life cycle. During metamorphosis, the gastropod nervous system is affected by both progressive and regressive neuronal events. In addition to this metamorphic reorganization, the nervous system appears to be centrally involved in initiating metamorphosis. We propose that gastropods not only possess temporally distinct neuronal adaptations for the specific needs of the larval and juvenile phases, but also another transient neuronal adaptation specialized to subserve the metamorphic episode.