Embryonic chirality and the evolution of spiralian left–right asymmetries

The group Spiralia includes species with one of the most significant cases of left–right asymmetries in animals: the coiling of the shell of gastropod molluscs (snails). In this animal group, an early event of embryonic chirality controlled by cytoskeleton dynamics and the subsequent differential activation of the genes nodal and Pitx determine the left–right axis of snails, and thus the direction of coiling of the shell. Despite progressive advances in our understanding of left–right axis specification in molluscs, little is known about left–right development in other spiralian taxa. Here, we identify and characterize the expression of nodal and Pitx orthologues in three different spiralian animals—the brachiopod Novocrania anomala , the annelid Owenia fusiformis and the nemertean Lineus ruber —and demonstrate embryonic chirality in the biradial-cleaving spiralian embryo of the bryozoan Membranipora membranacea . We show asymmetric expression of nodal and Pitx in the brachiopod and annelid, respectively, and symmetric expression of Pitx in the nemertean. Our findings indicate that early embryonic chirality is widespread and independent of the cleavage programme in the Spiralia. Additionally, our study illuminates the evolution of nodal and Pitx signalling by demonstrating embryonic asymmetric expression in lineages without obvious adult left–right asymmetries. This article is part of the themed issue ‘Provocative questions in left–right asymmetry’.

Predation on Juvenile Lagoonal Mud Snails (Hydrobia Neglecta)

Lagoonal mud snails have been shown to suffer approximately 50% mortality between hatching and adulthood. Experiments were conducted in the laboratory to assess the role of predators on the survival of juvenile Hydrobia neglecta (Mollusca: Gastropoda). Adult snails were collected from a lagoon in the Reedland Marshes system at Dunwich, Suffolk, and their eggs hatched in the laboratory, providing a population of juvenile snails which could be presented as a food source. The vessels, kept at environmental regimes close to that of the natural environment, were seeded with juvenile snails at field density. Predator preference was established by exposing the juvenile snails to combinations of potential predators found in the lagoon. One experiment also gave predators a choice of various juvenile snail sizes and other prey species. Juvenile snails remaining alive after the four week exposure to predators were removed, counted and measured. In the absence of other prey, the opisthobranch snail Retusa obtusa preferentially consumed 50% of the available juvenile H. neglecta over a four week period and the nemertean Lineus ruber up to 90%, making them the major contributors to juvenile H. neglecta mortality. The infaunal anemone Nematostella vectensis, did not have a significant impact on snail survival in the experiments but was observed (incidentally) in a separate non-experimental situation engulfing juvenile H. neglecta. The stickleback Gasterosteus aculeatus and the lagoonal prawn Palaemonetes varians, did not contribute directly to the observed mortality of juvenile mud snails, but were found to consume other predators, producing a system of prey preferences in the food chain. Understanding the role of predation contributes to the understanding of ecological processes occurring in rare lagoonal systems.

Localisation du réflexe lucifuge dans les ganglions cérébroïdes dorsaux de la némerte marine, Lineus ruber (O. F. Müller): étude expérimentale

The anterior part of Lineus ruber contains four successive "segments", an antecerebral segment, a second segment containing the cerebral ganglia, a third segment containing the cerebral organs, and lastly a preoesophageal segment. An experimental study involving microsurgical removal followed or not by grafts of different segments has shown that the negative phototaxis of this marine nemertean has its origins in the dorsal cerebral ganglia, in the photoreceptive structure that we have described.