Reversible Epithelial Adhesion Closes the Mouth of Beroe, a Carnivorous Marine Jelly

The ctenophore <i>Beroe</i> cf. <i>ovata</i> in the Caspian Sea. The beginning of a new stage in the evolution of the Caspian ecosystem?

Aim. Analysis of the first data regarding the arrival of the predatory ctenophore Beroe cf. ovata in the Caspian Sea and assessment of possible effects of its introduction for the Caspian ecosystem.Material and Methods. The material was obtained in the deep-water region of the western shelf of the middle Caspian Sea in the first ten days of October 2020. Zooplankton samples were collected using a Juday plankton net (0.1 m2 opening, 180 μm mesh size). Large ctenophores were collected using a big cone plankton net (CB) with a 0.5 m2 opening, 500 μm mesh size. Samples were taken at five stations in the 0-40 m layer. Combs larger than 5 mm were counted and measured on board the ship. The number and size of small ctenophores, larvae and eggs were determined in zooplankton samples fixed with formalin to a final concentration of 4%.Results. The presence of a new alien Beroe cf. ovata was recorded in the Dagestan shelf waters of the Caspian Sea in the beginning of October 2020. The average number of Beroe cf. ovata for five stations was 22 ind/m2 and the biomass was 6.15 g/m2. Larvae and small individuals from 5 to 20 mm in size predominated. Mnemiopsis leidyi was absent in the area investigated. The structure of mesozooplankton was dominated by the small copepod Acartia tonsa, which accounted for 92% of the zooplankton community and 78% of its biomass.Conclusion. The size structure of the B. ovata population with a predominance of larvae and small individuals indicates a recently completed breeding cycle and adaptation of the ctenophore to the conditions of the Caspian Sea. The first impacts of the new invasive ctenophore on the population of M. leidyi were demonstrated. This event may be the beginning of a new stage in the evolution of the Caspian Sea ecosystem and the restoration of its bio resources affected by the invasion of Mnemiopsis leidyi. The structure of mesozooplankton with total dominance, in abundance and biomass, of the small copepod Acartia tonsa did not differ from that in the previous period, indicating that the Caspian zoocenosis did not respond to the short-term reduction of the impact of M. leidyi. A brief analysis of the history of the introduction of ctenophores to the Black and Caspian Seas is presented and their impact on ecosystems and biological resources of these seas is studied. Based on longterm observations of the invasive ctenophores' interactions in the Black Sea, the possible outcomes of the penetration of Beroe cf. ovata in the Caspian Sea are discussed.

The ctenophore Mnemiopsis leidyi regulates egg production via conspecific communication

Background

Communication between individuals of the same species is an important aspect of mating and reproduction in most animals. In simultaneously hermaphroditic species with the ability to self-fertilize, communication with conspecifics can be essential to avoid inbreeding depression. One such behavioral adaptation observed in some simultaneous hermaphrodites is gamete trading. This behavior involves individual hermaphrodites in pairs alternating between reproducing as the male and female, and, as such, necessarily requires communication and coordination between mates. Little is known about communication in ctenophores and conspecific communication has not been described in this group; however, our previous work suggested that the ctenophore Mnemiopsis leidyi might engage in gamete trading. We tested for this possibility by constructing divided arenas (both sealed and permeable) that allowed us to measure individual egg output for paired M. leidyi.

Results

We found that, when not allowed to interact, size-matched individuals produced similar numbers of eggs on each side of the arena. However, if allowed to interact and exchange water, size-matched pairs produce significantly different numbers of eggs on each side of the arena, suggesting that these pairs use chemical communication to modulate reproduction in the presence of conspecifics as would be expected in gamete trading.

Conclusion

This finding presents exciting new possibilities for future investigations into the nature of signaling in M. leidyi. Furthermore, this first evidence of conspecific communication in Ctenophora, a group that branched off from the rest of animals more than 600 million years ago, has significant implications for the signaling ability of the last common ancestor of all animals.

Electronic supplementary material

The online version of this article (10.1186/s12898-018-0169-9) contains supplementary material, which is available to authorized users.

Dynamic control of reversible cell adhesion and actin cytoskeleton in the mouth of Beroë

Cell-cell adhesion in the various types of intercellular junctions of differentiated tissues is relatively stable and permanent. In migrating cells of embryos, or in wound closure, inflammatory responses and tumors of adult tissues, however, bonds between cells are made and broken and made again, i.e., cell-cell adhesions are transient and reversible. These nonjunctional contacts lack the organized structure of intercellular junctions, but may initiate their tissue-specific formation during development. Investigation of dynamic, nonjunctional cell-cell adhesions has been hampered by the asynchronous and heterogeneous distribution of these transient contacts among groups of moving cells. We recently discovered a novel system of reversible cell adhesion in a differentiated tissue that overcomes this difficulty. Here I review our current knowledge of this system, particularly its unique experimental advantages for investigating the mechanisms and control of dynamic cell adhesion.

A giant nerve net with multi-effector synapses underlying epithelial adhesive strips in the mouth of Beroë (Ctenophora)

We present ultrastructural evidence for the first known example of a giant nerve net in the phylum Ctenophora. The giant fibre system in Beroë underlies paired strips of adherent epithelial cells that run inside the lips. Interlocking actin-lined cell junctions between opposing adhesive strips keep Beroë's large mouth closed while the ctenophore searches for prey. The giant neurons, up to 6-8 microns in diameter, form a continuous lattice-like plexus rich in vesicles, microtubules, and 'presynaptic triads'. A novel feature is that individual giant axons make synaptic contacts with more than one type of effector, i.e. longitudinal muscle fibres and epithelial adhesive cells. Contact of prey with sensory receptors on the lips of Beroë induces rapid disappearance of the actin-lined adhesive cell junctions, and muscular opening of the mouth to ingest prey. Electron microscopy of food-opened mouths shows local thickening of longitudinal muscles and widening of the basal ends of epithelial cells in the adhesive strip, correlated with retraction of the adhesive epithelium into the mesoglea. Addition of 1% Triton X-100 to formaldehyde fixative in the absence of prey also elicits regional thickening of longitudinal muscles at the location of the adhesive strips (visualized by rhodamine-phalloidin staining). The giant neuron system may serve as a final common pathway to rapidly signal disassembly of actin-based junctions between adhesive cells as well as contractions of longitudinal muscles underlying the adhesive strips, thereby enabling Beroë to open its mouth rapidly to engulf prey.