Bioluminescence of ctenophore Pleurobrachia pileus (O. F. Müller 1776)
Authors:   A.O. Kovalevsky Institute of Biology of the Southern Seas of RAS, Melnik Alexander V. , Silakov Mihail I. ,   A.O. Kovalevsky Institute of Biology of the Southern Seas of RAS, Mashukova Olga V. ,   A.O. Kovalevsky Institute of Biology of the Southern Seas of RAS, Melnik Lidia A. ,   A.O. Kovalevsky Institute of Biology of the Southern Seas of RAS
Published: October 2024

Since the 1980s, in the Black Sea, there has been carried out intensive research of the ctenophores. The features of their distribution over the sea areas, in connection with depth, temperature and salinity, those of nutrition, respiration and reproduction, and luminescence under experimental conditions are being investigated. Until recently, it was believed that the autochthonous inhabitant of the Black Sea, Pleurobrachia pileus, (O.F. Muller 1776) is not a luminous species, therefore, in addition to studying the distribution and vital activity of this species, its bioluminescence has not been studied. In addition, throughout the history of studying the bioluminescence of the waters of the Black Sea, bathyphotometers were rarely lowered deeper than the photic zone in in situ studies. As a result, the bioluminescence of the lower part of the oxygen zone of the Black Sea has not been largely studied. However, at the end of the last century redoxclina became known to have dense layers of copepods and crests. Therefore, it was suggested that the deep peaks of bioluminescence at the lower boundary of the oxygen zone are associated with the glow of zooplankton organisms, among which the most likely contender for this role could be pleurobrachia crests. P. pileus (Ctenophora: Cydippida) is found in the North Atlantic Ocean and along the northwestern coasts of Europe. The range of the combtail includes the Baltic Sea, Skagerrak, Kattegat and the North Sea. It is a pelagic species that lives in open waters, but sometimes it is found in rock pools or on the coast. P. pileus is also found off the eastern Atlantic coasts of North America and in the Black Sea (See Photo 1). This yellow-bodied organism is distributed all along the coast of Europe in early summer. P. pileus has a spherical body up to 15 mm long. On the surface of the body there are eight rows of rowing plates or cten, starting near the aboral pole and extending more than three-quarters of the distance to the mouth. The combtail is seasonally one of the dominant predators and can regulate the number of zooplankton. The problem was that these animals were considered non-luminous. Therefore, additional studies were required, which showed that these species also glow. These works can be divided into two areas: in situ bioluminescence studies and laboratory measurements of the luminescence intensity of individual organisms. This paper presents the results of laboratory experiments and in situ studies of the Pleurobrachia pileus comb, (O.F. Muller 1776), which showed that this species has bioluminescent properties. The bioluminescence of P. pileus was studied during the 116th voyage of the RV Professor Vodyanitsky (See Fig. 1), onboard the vessel. Before the start of sampling, live samples were probed with the Salpa MA+ probe (See Fig. 4) to register the daily maximum glow in redoxcline, which was recorded in this zone, as a rule, in the depth range of 60-70 m, where dense clusters of P. pileus crests formed at that time. Samples of ctenophores were taken with a Bogorov–Russ net. It was shown that only at a temperature not exceeding 14°C, the pleurobrachia remained alive for 2-3 days. These data indicate that the temperature above 14°C is close to the maximum permissible for P. pileus, therefore, it was at this temperature (14°C) that chemical and mechanical stimulation experiments were carried out to highlight the crests, which revealed a glow, but the nature of the signals was significantly different (See Fig. 5). The photosensitivity of P. pileus has not yet been studied enough at this stage of research, and since bioluminescence is inhibited in sunlight in many crests and this photoinhibition is reversible when living samples are kept in the dark, experiments to study the bioluminescence of P. pileus were carried out in complete darkness during the daytime. In total, 37 experiments on highlighting pleurobrachial crests were carried out onboard during the 116th voyage of the RV Professor Vodyanitsky. Typical bioluminescent signals of crests do not differ and represent one or two intense peaks with a steep rise front and the same attenuation front, while sharper flashes of greater amplitude, rapidly reaching a maximum and also rapidly decreasing, are observed during chemical stimulation. The total percentage of luminous organisms from the entire catch was 32.43% (See Table), which unequivocally proves that pleurobrachia glows and makes a significant contribution to the intensity of the glow at great depths in redoxklin. It has been established that bioluminescence in redoxycline has a diurnal rhythm associated with the migrations of crests: during the day it increases when they smoothly migrate to the lower boundary of the oxygen zone, and at night this glow disappears. The dynamics of ctenophore bioluminescence in redoxycline is in the opposite phase to that in the surface layers, where it is associated with phytoplankton bioluminescence. The discovery of this new phenomenon makes it possible to use bioluminescent methods to quickly assess the depth of daytime zooplankton layers for subsequent trapping by plankton nets. This significantly expands the possibilities of studying the structure and functioning of the pelagic ecosystem of the Black Sea and other marine basins with redoxcline.