Does the Invasive Predator Mnemiopsis leidyi A. Agassiz, 1868 Control Copepod Abundances in the Seine Estuary?

The indigenous ctenophore Pleurobrachia pileus (O.F. Müller, 1776) was common in the coastal waters of the English Channel in the early 1990s and showed very abundant populations in the downstream part of the Seine estuary. In 2005, the non-indigenous ctenophore Mnemiopsis leidyi A. Agassiz, 1868, a species native to the Western Atlantic, was reported for the first time in Europe in Norwegian fjords and in Le Havre harbour (Seine estuary, France). More recently, in 2017, both Pleurobrachia pileus and Mnemiopsis leidyi were recorded during suprabenthos and zooplankton sampling in the Seine estuary along a downstream-upstream transect. Both species show more abundant populations in May than in September. Conversely, copepods show a spatial distribution depending on the ctenophore distribution, with low copepod abundances in the downstream part of the estuary being associated with high ctenophore abundances, while high copepod abundances are recorded where ctenophores are absent or display low abundances. We propose that the intense predation of ctenophores on copepods is related to changes in hydrological conditions over the two last decades. This may explain the dramatic decline of copepod abundance in the Seine estuary, which could have a negative effect on its nursery role.

Characterisation of Geometric Variance in the Epithelial Nerve Net of the Ctenophore Pleurobrachia pileus

Neuroscience currently lacks a diverse repertoire of model organisms, resulting in an incomplete understanding of what principles of neural function generalise and what are species-specific. Ctenophores display many neurobiological and experimental features which make them a promising candidate to fill this gap. They possess a nerve net distributed across their outer body surface, just beneath the epithelial layer. There is a long-held assumption that nerve nets are ‘simple’ and random while lacking distinct organisational principles. We want to challenge this assumption and determine how stereotyped the structure of this network really is. We validated an approach to estimate body surface area in Pleurobrachia pileus using custom Optical Projection Tomography and Light Sheet Morphometry imaging systems. We used an antibody against tyrosylated α-tubulin to visualise the nerve net in situ. We used an automated segmentation approach to extract the morphological features of the nerve net. We characterised organisational rules of the epithelial nerve net in P. pileus in animals of different sizes and at different regions of the body. We found that specific morphological features within the nerve net are largely un-changed during growth. These properties must be essential to the functionality of the nervous system and therefore are maintained during a change in body size. We have also established the principles of organisation of the network and showed that some of the geometric properties are variable across different parts of the body. This suggests that there may be different functions occurring in regions with different structural characteristics. This is the most comprehensive structural description of a nerve net to date. This study also demonstrates the amenability of the ctenophore P. pileus for whole organism network analysis and shows their promise as a model organism for neuroscience, which may provide insights into the foundational principles of nervous systems.

Characterising the Behaviour of the Ctenophore Pleurobrachia pileus in a Laboratory Aquaculture System

Neurobiological research focuses on a small number of model organisms, broadening the pool of animals used in research may lead to important insights into the evolution of nervous systems. The ctenophore is emerging as a promising model, but we are currently lacking an understanding into the relationship between behaviour and environment which is in part due to a lack of a standardised long-term laboratory husbandry system. We established a collection and husbandry system for wild caught Pleurobrachia pileus. We examined the behavioural profile of the animals over time in this controlled environment. We could reliably catch them on a seasonal basis, and we could keep the animals alive in our specialised aquarium system for months at a time. P. pileus spends most of the time in an inactive ‘drifting’ state which is interspersed with periods of one of 5 active behaviours. The most common active behaviours are tentacle resetting and feeding. The longest duration behaviours include swimming up or down. Time of day does not appear to alter their behavioural profile. Gaining a better understanding of the behaviour of these animals has important implications for systems and evolutionary neuroscience.

First record of Beroe gracilis Künne, 1939 (Ctenophora: Beroida: Beroidae) from Norway, found in a Mnemiopsis leidyi A. Agassiz, 1865 bloom

In September 2014 an unusual mixture of ctenophores was recorded at Arboretet, south-western Norway and at Flødevigen, near Arendal on the south coast of Norway. In addition to the invasive American lobate ctenophore,Mnemiopsis leidyi, the common northern lobate ctenophoreBolinopsis infundibulumand the cydippidPleurobrachia pileus, two beroid ctenophores, were noted –Beroe cucumisandBeroegracilis. The latter species had not been documented before in Norwegian waters.