White gorgonian

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White gorgonian
Eunicella singularis (Esper, 1794) 2.jpg
Systematics
Class : Flower animals (anthozoa)
Subclass : Octocorallia
Order : Soft corals (Alcyonacea)
Family : Gorgoniidae
Genre : Eunicella
Type : White gorgonian
Scientific name
Eunicella singularis
Esper , 1791

Eunicella singularis , the white gorgonian, is a colonial soft coral from the family Gorgoniidae , also known as horn corals or sea fans. Of the 20or so species of sea fan foundin the Mediterranean, it is one of the most common and the only Mediterranean sea fan that, like most stony corals , has symbiotic algae . It was first described in 1791 by Eugen Johann Christoph Esper .

Distribution area and habitat

E. singularis is mainly found in the western Mediterranean and the Adriatic , but can also be found occasionally in the eastern Mediterranean. In the sublittoral, it usually occurs at depths of 10 to 70 meters, sometimes at depths of over 100 meters, and is part of the benthos , the community of the seabed. Here it prefers to settle on hard, stony ground.

E. singularis plays an important role in its ecosystem as an "ecosystem engineer", as its morphology gives it structure and thus offers protection and habitat to many other species, i.e. it forms microhabitats, which is why it makes a relatively large contribution to the biomass and biodiversity of benthos . It also acts as a link between plankton and benthos.

morphology

E. singularis shows the typical branched growth form of the Gorgoniidae , but belongs to the species with very few branches. In the Mediterranean, it is the gorgonian with the longest and thickest branches, the fewest branches and the largest ratio of height to width, that is, it grows more in height (up to about 70 cm) than in width. When taking a closer look at the morphology, the phenotypic plasticity , which is particularly pronounced in gorgonians and many other coral species, must be taken into account, as colonies from shallow and deep depths show some differences. It is not yet fully clear whether it is an intraspecific variation or different subspecies of E. singularis . Usually one speaks of a shallow and a deep morphotype. At shallow depths of around 35 to 40 meters, the colonies are candelabra-shaped and gray-white due to the symbiotic algae they contain. The flexible, long primary branches are parallel to each other and up towards the light and have very few branches. At greater depths of around 40 meters, the colonies have a more variable morphology, they have shorter primary branches, more ramifications and a bright white color due to the lack of symbionts .

The expression of the different morphotypes represents an adaptation of sessile organisms to different environmental conditions, which for E. singularis vary particularly with depth. The most important aspects here are lighting conditions and hydrodynamics. The symbiotic algae need enough light for their photosynthesis , but this is no longer available from around 40 meters, so the deep morphotype no longer has symbionts. The shallow morphotype is adapted to stronger water currents that can be caused, for example, by storms. Due to their little branched shape, they show less water resistance, which is why the risk of being carried away by currents is lower. In addition, algae and the like carried along by currents are less likely to get caught in the branches. In addition to the water depth, the environmental conditions also vary with the different seasons. In summer, the high levels of solar radiation cause the water column to be stratified and a thermocline forms. Below this thermocline , water temperature, currents, and nutrient supply are more constant than above where the colonies are exposed to higher temperatures, stronger currents, and lower nutrient levels.

The varying environmental conditions also explain why small and young colonies dominate at shallower depths, while larger and older colonies with more regular spreading can be found at greater depths. In the depths the environmental conditions are generally more stable, the colonies are disturbed less often and can therefore get bigger and older. The frequent disturbances as well as the greater competition from rapidly growing algae impair the growth of colonies in the flat.

Polyps of Eunicella singularis

Diet

E. singularis can feed itself both autotrophically and heterotrophically . Like most stony corals, colonies at depths of up to 40 meters, where there is still enough light, have single-celled algae as symbionts. The so-called zooxanthellae , which are dinoflagellates of the genus Symbiodinium , carry out photosynthesis in the inner body layer of the gorgonian and thus supply them with the substances assimilated in the process. In addition to this autotrophic diet, E. singularis can also feed itself heterotrophically, as the polyps of the colony filter food particles such as algae and zooplankton from the water. So it is not necessarily symbiotic, the symbiotic density decreases with increasing depth. E. singularis relies on the heterotrophic diet, especially at greater depths of less than 40 meters, where it is too energy-consuming to have zooxanthellae due to poor lighting conditions . Colonies at shallower depths can maintain basic metabolism through both heterotrophy and autotrophy alone. It is believed that in summer, the high levels of solar radiation drive the majority of metabolism by autotrophy, while in winter heterotrophy compensates for the decline in photosynthesis rates. Overall, the metabolism rates decrease in summer with increased water temperatures of 22 to 24 ° C, which is an adjustment to the lower nutrient availability due to the stratification of the water column, but also a reaction to temperature-related stress. Photosynthesis of zooxanthellae is impaired at elevated temperatures, but in contrast to observations in other tropical and Mediterranean flowered animals , in which heterotrophy generally increases metabolism and maintains it during such stress phases, heterotrophy in E. singularis does not seem to act as a buffer against temperature-related stress.

Reproduction and life cycle

Every year there is a reproductive phase ( iteroparia ) and the colonies of E. singularis are separate sexes ( gonochor ), which means that each colony is either female or male and therefore produces egg cells or sperm . Hermaphroditic ( hermaphroditism ) is unlike many hard corals untypical of sea fans. The formation of the egg cells ( oogenesis ) takes about 13-17 months, starting in February to June, in the following year between the end of May and July the relatively large egg cells in the female polyps have fully matured. The formation of the sperm ( spermatogenesis ) only takes about five to six months, the mature sperm are released into the sea water at the end of May to June.

The fact that sex cells only mature once a year has already been observed in other gorgonians from temperate regions; in tropical species this often happens several times a year. The strong seasonal change in the supply of nutrients in temperate regions could be responsible for this, since the formation of sex cells ( gametogenesis ) is primarily dependent on temperature and nutrient situation. That is why E. singularis invests heavily in the development of sex cells, especially in January to May. The breeding period in summer also maximizes the survival of the offspring, since there is less danger of falling victim to predators and there is more subsoil available for settlement, as algae growth declines.

Eunicella singularis is overgrown by Alcyonium coralloides

The sperm pass through the mouth opening into the interior of the female polyp, where fertilization of the yolk-rich, spherical, light-pink egg cells takes place. After holoblastic cleavage of the zygote , the worm- to pear-shaped planula larvae , which are also light pink in color, develop and are released again through the mouth of the female polyps. If the mother colony has zooxanthellae, these are transferred directly to the endoderm of the planula larva. The larvae now sink and can be driven away by water currents, but they can also actively move underground with cilia in order to find a suitable place to settle. In doing so, they are mainly based on the roughness of the subsurface and the lighting conditions; well-lit locations are preferred. If a suitable location has been found after several hours to days, the metamorphosis begins from the planula larva to the primary polyp, which takes about four days. However, only a fraction of the larvae achieve complete metamorphosis, as many are eaten or are on unsuitable subsoil. And of the primary polyps, only about 1–2% survive, as many are covered by algae or sediment. The primary polyp now reproduces asexually, a new colony grows through budding .

The high potential for regeneration of E. singularis is also due to its ability to bud , as individual polyps die, but can also be replaced. Therefore aging really is not considered a cause of death, this is mostly external conditions: If living tissue of the sea fan is damaged, for example by predators, diseases or mechanical effects, can affect the exposed skeleton Ride-organisms ( epibionts ) such as algae and various invertebrates like Snails, bryozoa or other octocorallia colonize, which then push back and overgrow the remaining living tissue of the gorgonian. For example, the soft coral Alcyonium coralloides destroys the tissue of E. singularis and grows along its branches. This also increases the water resistance of the colony, which is why the risk of being washed away by water currents, which alone often leads to the death of gorgonians, increases. It is estimated that E. singularis can reach an age of 25 to 30 years under given conditions.

Danger

Like many tropical hard corals, E. singularis is also at great risk from climate change , especially the associated warming of the oceans. In the last few decades, several mass death events of E. singularis have been documented (1999, 2003, 2008/09, 2015/16), which could be due to increased water temperatures for too long a period, but also to other stress factors such as diseases, which could arise can spread faster in already weakened populations. Even at temperatures from 24 ° C for several weeks, the gorgonians are under temperature stress, which has a negative effect on several levels. On the one hand, the polyps withdraw, which reduces the heterotrophic food supply, and a stratification of the water column prevents the upwelling of nutrients and thus the formation of zoo and phytoplankton that can be caught by polyps. On the other hand, the zooxanthellae are severely restricted in their ability to photosynthesize and coral bleaching occurs , during which the symbionts are expelled and the autotrophic food source is lost. This deficit can be withstood for a few weeks by reducing the respiration rate, but if the high temperatures persist for too long and the symbionts are therefore not taken up again, the gorgonians will eventually starve to death. Populations at shallower depths seem to be more sensitive to high temperatures, presumably because temperature fluctuations have more frequent and stronger effects here, or also because of the associated oxidative stress caused by the additional high radiation intensity . Since the conditions at greater depths are more stable and colonies of E. singularis usually have no symbionts here, they seem to be more resistant.

Even several years after such disturbances, populations are impaired in their recovery from such mass mortality events, both at shallow and at greater depths. A direct aftereffect of the death of large parts of a population is the overgrowth of the freed areas with dense lawns of various types of benthic algae, which can grow quickly. They make repopulation of the areas difficult by E. singularis larvae , as they take away their space. In addition, they overgrow weakened and young colonies and thus disrupt their growth, similar to tropical hard corals after a large coral bleaching. Invasive algae species in particular are a problem because, unlike native species, they remain present all year round.

Due to the important role of E. singularis as an "ecosystem engineer", environmental disturbances can easily have a negative impact on the entire ecosystem and impair its organization and function. On the Red List is considered E. singularis not as vulnerable right now, but is on the verge and the population trend is clearly decreasing. The ever worsening climate change could ensure that E. singularis is soon even more endangered.

Medical importance

Many soft corals and especially gorgonians have come more and more into the focus of medicine and pharmacy in recent years. They are being investigated in order to find new, natural active ingredients that are better tolerated by humans and have fewer side effects than synthetically produced active ingredients. The genus Eunicella is known for having pharmacologically interesting active ingredients with antiproliferative and antibacterial effects. In this context, E. singularis was also examined for substances that could potentially be anti-inflammatory and pain-relieving as well as good for the stomach. In some studies it has already been proven experimentally that the ethanol fractions extracted from E. singularis and the diterpenoids and sterols contained therein could provide the desired positive effects.

Individual evidence

  1. ↑ Cordeiro, R., McFadden, C., van Ofwegen, L., Williams, G. 2020. World List of Octocorallia. Eunicella singularis (Esper, 1791). Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=taxdetails&id=125365 (accessed March 4, 2020)
  2. ↑ a b c Gori, A., Bramanti, L., López-González, P. et al. 2012. Characterization of the zooxanthellate and azooxanthellate morphotypes of the Mediterranean gorgonian Eunicella singularis . Marine Biology 159: 1485-1496. https://doi.org/10.1007/s00227-012-1928-3
  3. ↑ a b c d Ezzat, L., Merle, PL, Furla, P. et al. 2013. The response of the Mediterranean gorgonian Eunicella singularis to thermal stress is independent of its nutritional regime. PloS One, 8 (5), e64370. https://doi.org/10.1371/journal.pone.0064370
  4. ↑ Gorgone blanche. Retrieved March 4, 2020 (French).
  5. ↑ a b Gori, A., Rossi, S., Linares, C. et al. 2011. Size and spatial structure in deep versus shallow populations of the Mediterranean gorgonian Eunicella singularis (Cap de Creus, northwestern Mediterranean Sea). Marine Biology 158: 1721-1732. https://doi.org/10.1007/s00227-011-1686-7
  6. ↑ a b Ribes, M., Coma, R., Rossi, S. et al. 2007. Cycle of gonadal development in Eunicella singularis (Cnidaria: Octocorallia): trends in sexual reproduction in gorgonians. Invertebrate Biology 126 (4): 307-317. https://doi.org/10.1111/j.1744-7410.2007.00101.x
  7. ↑ a b c Weinberg, S., Weinberg, F. The cycle of a gorgonian: Eunicella singularis (Esper, 1794). Bijdragen tot de Dierkunde 48 (2): 127-137.
  8. ↑ Alcyon encroûtant. Retrieved March 4, 2020 (French).
  9. ↑ Linares, C., Cebrian, E., Coma, R. 2012. Effects of turf algae on recruitment and juvenile survival of gorgonian corals. Marine Ecology Progress Series 452: 81-88. https://doi.org/10.3354/meps09586
  10. ↑ Kipson, S., Linares, CL, Betti, F., Bo, M., Terrón-Sigler, A., Garrabou, J., Caroselli, E. & Cerrano, C. 2015. Eunicella singularis. The IUCN Red List of Threatened Species 2015: e.T50012188A50609244. (accessed March 4, 2020)
  11. ↑ Deghrigue, M., Festa, C., Ghribi, L. et al. 2014. Pharmacological evaluation of the semi-purified fractions from the soft coral Eunicella singularis and isolation of pure compounds. Daru: journal of Faculty of Pharmacy, Tehran University of Medical Sciences, 22 (1): 64. https://doi.org/10.1186/s40199-014-0064-7
  12. ↑ Deghrigue, M., Festa, C., Ghribi, L. et al. 2015. Anti-inflammatory and analgesic activities with gastroprotective effect of semi-purified fractions and isolation of pure compounds from Mediterranean gorgonian Eunicella singularis . Asian Pacific Journal of Tropical Medicine 8: 606-611. https://doi.org/10.1016/j.apjtm.2015.07.019