Determining Obelia spp. diversity and population dynamics in Thau lagoon (Northwestern Mediterranean Sea, France).

The jellyfish Obelia belongs to the family Campanulariidae (Hydrozoa, Leptomedusae), an important and widely distributed family of hydrozoans. Obelia is a very popular and widespread medusa, however first report on its complete life cycle was only published in the late nineties in Northern Japan. This is probably due to its size, as the medusa umbrella diameter is lower than 1mm. This Hydrozoa presents a bentho-pelagic life cycle with both polyp and medusa stages. In Thau lagoon, Northwestern Mediterranean Sea, populations of Obelia occur each year. Some polyps colonies were identified and sampled in the lagoon on Zostera noltii leaves, indicating that the populations realise their full development cycle within the lagoon. Morphological and genetical (COI) approaches, on both polyps and medusae, have revealed that main populations are composed by O. dichotoma individuals (side branches typically irregular in length; hydroteca bell-shaped, usually not very deep, thin walled, often thrown into fine longitudinal folds; hydrotecal rim with smooth or with shallow cusps \u2013crenate, slightly flared; diaphragm transverse to oblique) but that O. bidentata (lateral branches roughly in right angles pairs are given on both sides; slightly oblique diaphragm; hydrotecal rim with bimucronate cusps) is also present in lower abundances. An in situ pelagic monitoring every two weeks since 2008 have allowed understanding the populations dynamics of the genus and the environmental factors that seemed to be particularly appropriated for promoting optimum growth conditions. Baseline information regarding seasonal cycles and historical abundances were provided from a 8 years monitoring, which has allowed detecting blooms. Indeed, the medusae were found at very low densities during the study period with main abundances under 5 ind.m-3, in accordance to previous reports for Obelia spp. medusae around the world. Nevertheless, 2 exceptional events took place in June 2008 and May 2013 were blooms reached 1232 and 660 ind.m-3 respectively. Such data raise questions regarding the potential importance of this small and understudied hydrozoa in the pelagic community of Thau lagoon. Consequently, it is necessary to improve the available databases documenting medusae blooms, as this will provide baseline information about these little-studied events

Mapping distribution and habitats of Aurelia sp. polyps in Thau lagoon, north-western Mediterranean Sea (France)

Aurelia spp. is a cosmopolite scyphozoan species and likely the most studied jellyfish in the world. Its pelagic–benthic life cycle is complex, and the benthic asexual reproducing stage (polyp) is acknowledged fundamentally in bloom onset. Despite this, field investigations remain scarce and are mainly restrained to the western Pacific Ocean. Thau lagoon (43°23′59.10″ N 3°36′37.15″ E), a semi-enclosed system that harbours a resident population of Aurelia sp., is in essence a natural laboratory that offers an ideal framework to investigate the life cycle of the species. We here used a non-destructive approach consisting on a field survey over the entire lagoon (ca. 7 ha) and several substrate types by free diving to examine the distribution and habitat use of Aurelia sp. benthic population. We show that polyps were largely distributed over the entire lagoon, settled mainly on artificial hard substrates, thereby stressing the promoting role of anthropogenic perturbations in coastal areas, i.e. habitat modification, for jellyfish proliferations. Therefore, our study suggest a potential increase in Aurelia sp. benthic populations as an outcome of mounting coastal constructions in the near future; the consequences of which ultimately might promote an increase in jellyfish outbreaks

Dynamics and asexual reproduction of the jellyfish Aurelia coerulea benthic life stage in the Thau lagoon (northwestern Mediterranean)

International audienc

Jellyfish as an alternative source of food for opportunistic fishes

Although scientific interest on jellyfish ecology has substantially increased in the last decades, little is known on the role of potential predators shaping their population dynamics. Jellyfish were long considered as ‘dead ends’ within food webs, and therefore overlooked as potential food source for higher trophic levels, e.g. fishes. Here this question is tackled by using comprehensive laboratory experiments assessing fish predation on jellyfish. The approach included all the life stages (polyps, ephyrae and medusa) of Aurelia sp. versus more traditional aquaculture feeds in an easily farmed opportunistic fish, the gilthead seabream Sparus aurata (L.). Results revealed that all life stages of Aurelia sp. were accepted as a source of food by S. aurata, whose grazing pressure varies depending on the jellyfish life stage. Higher ingestion rates were observed on young stages (i.e. small medusa) indicating their higher vulnerability to fish predation and the potential negative impact this may have on Aurelia sp. population dynamics. These results provide new insights on the so far underestimated role fish predation can have on jellyfish population dynamics. In particular, opportunistic fish species, such as S. aurata may contribute to control jellyfish blooms, through top-down regulations of jellyfish biomass

Le système d’observation et de recherche en environnement côtier de Thau – RECThau

International audienceCe Système d’Observation (SO) a été créé en 2009 au démarrage de l’OSUOREME à Montpellier. Il avait le double objectif de 1.structurer et pérenniserdes observations dans la lagune et sur la côte jusqu’alors ponctuelles voiresporadiques et 2. initier une coordination des différentes observations sur lelittoral languedocien dans l’espace géographique de la lagune de Thau afind’engendrer de nouvelles collaborations aussi bien en termes méthodologiquesque de nouveaux projets de recherche.Depuis 2015, le SO affiche une nouvelle appellation et une nouvellestructuration en trois Tâches d’Observation (TO) :- la TO du Bassin-Versant de la lagune de Thau (TO BV-Thau) : regroupe lesobservations météo-hydrologiques sur le bassin versant de la lagune de Thau.Ces observations sont mises en place depuis plus de 15 ans sur la Vène,principal contributeur du bassin.- la TO de la lagune de Thau (TO Suivi-Thau) : regroupe les observations deparamètres météorologiques et de paramètres physico-chimiques etbiologiques effectuées dans la colonne d’eau de la lagune de Thau. Cette TOest en partie en relation avec les expérimentations effectuées dans lesmésocosmes in situ de MEDIMEER installés sur la lagune de Thau,- la TO côtière au large de la lagune de Thau et sur le plateau continental enface de la ville de Sète (SOMLIT-Sète) : regroupe les observations desvariables physico-chimiques et biologiques sous 30 m de fond au large deSète, jusqu’alors dénommée Suivi-Côte. Elle a intégré le réseau SOMLIT(Service national d’Observation du Milieu LITtoral) en 2015. Les mesures sefont également à Haute Fréquence sur la station BESSète et elle est ainsimembre fondateur du réseau COAST-H

Déterminisme du recrutement larvaire de l’huître creuse (Crassostrea gigas) dans la lagune de Thau.

Since spat mortality events that were firstly detected in 2008, Mediterranean shellfish industry seeks to exploit the Pacific oyster spat from lagoon natural recruitment. The CRCM, Cepralmar and Ifremer developed accordingly a research project named PRONAMED Phase 1 and 2 - 2010-2014, in order to define the economic potential of spatfall and improve knowledge of the reproductive cycle of Crassostrea gigas in this specific environmental context. The results of the past three year‘s study (2012, 2013, 2014) showed that broodstock have the ability to provide larvae after sexual maturation and spawning. A coincidence was noted between moon phase (new and full moon) and spawnings with an amplifying effect of storms on the intensity of spawning (Ubertini et al. In prep). The works exposed here focus on larval cycle and spat recruitment. The results showed that larval development until pediveliger settlement, influenced by water temperature and abundance of Chaetoceros sp, faced no major biological lock. The supply of larvae from different areas of the lagoon has been governed by hydrodynamics and a new method of lagoon larval connectivity assessment was thus developed: the dynamic connectivity. A good fit was noted between high dynamic connectivity areas and the most effective collection stations (Lagarde et al. 2015). It was shown that survival, during metamorphosis, relied on the abundance of total nanophytoplankton (pico + nanophytoplankton). Moreover, shellfish farming areas were less efficient for spat collection that some others located off shellfish farms (Lagarde et al. 2015). An additional experiment on collectors management showed that their deployment in growing area under the condition of dewatered, arranged in line, horizontally, led to the best results. This report is a first work of exploration, explanation and synthesis on reproduction and larval recruitment determinism of the Pacific oyster in Mediterranean lagoon. The abilities of Thau lagoon to provide oyster spat has been explored and now the next objectives are to refine its economically viability and delineate the best spatfall areas. From a scientific point of view, the resulting conceptual diagram of this three-year-study should be confirmed by a detailed analysis of the life history of recorded larval cohorts combined to a more reliable simulation model development. At the interface between social demand and research, PRONAMED project enables the first steps towards a new type of sustainable exploitation of shellfish resource in Mediterranean lagoon.Depuis les évènements de surmortalité de naissains apparus en 2008, la filière conchylicole méditerranéenne cherche à exploiter le naissain d’huître creuse issu de captage naturel en lagune. Pour ce faire, le CRCM, le Cépralmar et l’Ifremer ont élaboré un projet de recherche, PRONAMED phase 1 et 2 - 2010-2014, dans le but de définir le potentiel économique de cette activité et améliorer les connaissances du cycle de reproduction dans ce contexte environnemental spécifique. Les résultats de travaux de recherche des années 2012, 2013 et 2014 montrent que les huîtres adultes ont la capacité de fournir des larves après maturation sexuelle et ponte. Une coïncidence a été remarquée entre phase lunaire (nouvelle et pleine lune) et ponte ainsi qu’un effet amplificateur des orages sur l’intensité des pontes dans certains cas (Ubertini et al. in prep). Les travaux présentés ici portent sur le cycle larvaire et le recrutement de naissain. Les résultats montrent que la phase de développement larvaire jusqu’à la fixation des pédivéligères, sous influence de la température de l’eau et de l’abondance de Chaetoceros sp, ne rencontre pas de verrou biologique majeur. L’approvisionnement en larves des différentes zones de la lagune est régi par l’hydrodynamique et une nouvelle méthode d’évaluation de la connectivité larvaire plus adaptée au contexte lagunaire a dû être développée : la connectivité dynamique. Une bonne adéquation est à noter entre les zones de connectivité dynamique élevées et les stations de captage les plus efficaces (Lagarde et al. 2015). Il est montré que la survie pendant la métamorphose est sous influence de l’abondance de nanophytoplancton total (pico+nanophytoplancton) et que d’une manière générale, les zones conchylicoles sont moins favorables au captage naturel de naissains d’huîtres que certaines zones situées en dehors des parcs conchylicoles (Lagarde et al. 2015). Une expérience complémentaire portant sur la gestion de collecteur à coupelle montre qu’un déploiement des collecteurs, en zone conchylicole sous condition d’exondation, disposés en ligne (pas en fagot) et à l’horizontale est optimal. Ce rapport constitue un premier travail d’exploration, d’explications et de synthèse du déterminisme de la reproduction et du recrutement larvaire de l‘huître creuse en lagune méditerranéenne. L’objectif de la filière conchylicole est maintenant de pouvoir exploiter au mieux les capacités de la lagune de Thau à fournir du naissain d’huître creuse. Il conviendra maintenant de non seulement définir précisément le périmètre des zones de captage économiquement viables en lagune de Thau mais aussi caractériser les zones de nurserie dans les zones ostréicoles. D’un point de vue scientifique, le schéma conceptuel résultant de ces trois années d’acquisition de données devra être testé par une analyse fine des traits de vie des cohortes larvaires enregistrées et fiabilisé en vue du développement de modèle de simulation, in fine. A l’interface entre demande sociétale et recherche, le projet PRONAMED permet les premiers pas vers un nouveau type d’exploitation sobre et durable d’une ressource conchylicole aujourd’hui encore en crise en lagune méditerranéenne