Symbioses in Amphipholis squamata (Echinodermata, Ophiuroidea, Amphiuridae): geographical variation of infestation and effect of symbionts on the host's light production

Genetic variation in the brooding brittle-star: a global hybrid polyploid complex?

The widespread and abundant brooding brittle-star (Amphipholis squamata) is a simultaneous hermaphrodite with a complex mitochondrial phylogeography of multiple divergent overlapping mtDNA lineages, high levels of inbreeding or clonality and unusual sperm morphology. We use exon-capture and transcriptome data to show that the nuclear genome comprises multiple (greater than 3) divergent (π > 6%) expressed components occurring across samples characterized by highly divergent (greater than 20%) mitochondrial lineages, and encompassing several other genera, including diploid dioecious species. We report a massive sperm genome size in A. squamata, an order of magnitude larger than that present in other brittle-stars, and consistent with our SNP-based measure of greatly elevated ploidy. Similarity of these genetic signatures to well-known animal systems suggests that A. squamata (and related taxa) is a hybrid polyploid asexual complex of variable subgenome origins, ploidy and reproductive mode. We discuss enigmatic aspects of A. squamata biology in this light. This putative allopolyploid complex would be the first to be reported from the phylum Echinodermata.

Different phylogenomic methods support monophyly of enigmatic 'Mesozoa' (Dicyemida + Orthonectida, Lophotrochozoa)

Dicyemids and orthonectids were traditionally classified in a group called Mesozoa, but their placement in a single clade has been contested and their position(s) within Metazoa is uncertain. Here, we assembled a comprehensive matrix of Lophotrochozoa (Metazoa) and investigated the position of Dicyemida (= Rhombozoa) and Orthonectida, employing multiple phylogenomic approaches. We sequenced seven new transcriptomes and one draft genome from dicyemids (Dicyema, Dicyemennea) and two transcriptomes from orthonectids (Rhopalura). Using these and published data, we assembled and analysed contamination-filtered datasets with up to 987 genes. Our results recover Mesozoa monophyletic and as a close relative of Platyhelminthes or Gnathifera. Because of the tendency of the long-branch mesozoans to group with other long-branch taxa in our analyses, we explored the impact of approaches purported to help alleviate long-branch attraction (e.g. taxon removal, coalescent inference, gene targeting). None of these were able to break the association of Orthonectida with Dicyemida in the maximum-likelihood trees. Contrastingly, the Bayesian analysis and site-specific frequency model in maximum-likelihood did not recover a monophyletic Mesozoa (but only when using a specific 50 gene matrix). The classic hypothesis on monophyletic Mesozoa is possibly reborn and should be further tested.

A New Species of Orthonectida That Parasitizes Xenoturbella bocki: Implications for Studies on Xenoturbella

Orthonectida is a phylum of marine invertebrates known to parasitize many invertebrate animals. Because of its simple body plan, it was suggested that it belong to Mesozoa, together with Dicyemida, and that it represent the evolutionary step between unicellular organisms and multicellular animals. Recent studies, including analyses of its genomes, have clarified its phylogenetic position as a member of the Protostomia, but details such as the species diversity within the phylum and how it infects the host remain unknown. Here we report orthonectids discovered from the marine worm Xenoturbella bocki. Orthonectids were found from sections of four xenoturbellid specimens, collected eight years apart. Live females were also discovered on three separate occasions. These recurring instances of orthonectids found from Xenoturbella show that they are parasitic to the animal and not just chance contaminations. Based on morphological characters such as the presence of sexual dimorphism, the arrangement of oocytes within the female body, and the presence of crystalline inclusions in the male epidermal cells, we regard this orthonectid as a new species, Rhopalura xenoturbellae sp. nov. Since orthonectids are present within the xenoturbellid adult body, caution is needed when interpreting morphological, molecular, and experimental data from X. bocki. Further studies on R. xenoturbellae will yield important information on the fundamental biological details of orthonectids that remain unknown.

Evidence of seasonal variation in bioluminescence of Amphipholis squamata (Ophiuroidea, Echinodermata): effects of environmental factors

The bioluminescence of Amphipholis squamata was assessed from freshly collected individuals for 16 successive months, and from individuals maintained in the laboratory under various experimental conditions of salinity, temperature and photoperiodic regime. Field investigations showed that bioluminescence intensity and kinetics varied seasonally, with the light produced being brighter and faster in winter and summer. The seasonal variation was not correlated with changes of ambient salinity. However, it was correlated with changes in temperature, the luminescence being brighter and faster in coldest and warmest seasons, and with the changes of photoperiod, the luminescence being brighter and faster in seasons with shortest and longest day length. Laboratory investigations also demonstrated that luminescence was not affected by salinity conditions. Conversely, luminescence was affected by temperature, the light production being brighter and faster in warmer conditions (in agreement with field observations) and dimmer and slower in colder conditions (in disagreement with field observations). Light production was also affected by photoperiod since experimental changes of natural light:dark regime caused the bioluminescence to decrease. Considering that photoperiod guides the biology of A. squamata and that reproduction takes place during coldest months in the species, an endogenous factor of neurophysiological nature linked to the ophiuroid reproductive cycle is proposed to induce the luminescence to peak in winter. This was confirmed by the fact that seasonal variation of luminescence was different between adult and juveniles, the latter showing no winter peak of luminescence. It is suggested that the luminescence normally associated with defense could also be part of an intraspecific visual signal related to individuals aggregating for reproduction during winter.

Alteration of bioluminescence in Amphipholis squamata (Ophiuroidea: Echinodermata) by heavy metals contamination: a field study

The ophiuroid Amphipholis squamata (Echinodermata) is a bioluminescent species whose light production varies with physico-chemical parameters of the medium. Individuals collected in the bay of Portman along a gradient of heavy metal contamination show different patterns of light production: the ones from the highest contaminated area showing a bioluminescence weaker and slower than those from the lowest contaminated area. Individuals that were transferred for 3 days from the lowest to the highest contaminated area displayed a light production that became weaker and slower. It is suggested that the decrease of the bioluminescent capability due to heavy metal pollution could indirectly affect the ophiuroid ecological success (bioluminescence is associated with defense functions in ophiuroids.