Balanophyllia europaea one inhabitant of our primitive and unknown sea
by Stefano Goffredo
Corals are primordial animals because they have a relatively simple corporeal organizzation. Observed “from close” , with the help of a powerful electronic microscope, they reveal interesting and spectacular differentiations of shapes and functions.
Barrier Reef and tropical atolls, precious red corals, soft and hard corals; but what are corals? They were just between the first pluricellular animals evolved in seas. They populated the oceans already in Ordovician Age, in other words almost 500 million years ago: On the purpose it is curious to remember that the first vertebrates (fishes) appeared in the Silurian Age (approximately 400 million years ago) while man completed the first steps on earth only in Quaternarius Age (0,5 – 1 million years ago).
One coral, as a rule, is a sessile organism, that means that it lives adherent to a hard substrate, with a body shape of a sack, more or less cylindrical. The body is bordered, on the upper part with a crown of tentacles, on the center of which opens the mouth. This communicates with an inner cavity blind-bottomed (celenteron), that has a digestive and transport function. Lower down, the individual adheres on a rocky bottom, by means of a pedal disc.The body organized like this takes the name of “polyp”.
Corals can be colonials or solitary. In the first case, many polyps cooperates together, for example for the provision of food. On the second case, every single polyp lives isolated like an independent organism.
Corals are carnivorous and prey upon small planctonic animals. The capture takes place with a direct contact: the small prey, transported by the running waters, bangs against the polyp’s tentacle and is stunned by the toxic liquid unloaded by the filaments avolved by batteries of urticant cells (cnidoblasti). The following straggling of the stunned victim stimulates the coral to digest. Moreover, many corals, give hospitality to unicellular simbionts algas (zooxanthellae), that provides also the nutriment of the polyp. During the photosinthetic process, in fact, the zooxanthellae, pick up the solar energy that uses to synthetize the organic substance. A great part of this energy, exit from the alga in order to be used from the polyp as a nutriment. The fotosinthesis of zooxanthellae more over accelerates the precipitation of calcium carbonate, in the corals which posses an external calcareous skeleton (Madreporari). For this reason these corals have an growth speed of the skeleton that is 2 or 3 times higher in comparison with other corals without zooxanthellae. Because of this symbiosi, the madreporari of tropical seas have rates of growth higher than the erosion one (this erosion due to the actions of the waves and of the animals that are nurished by the corals) and so they have the possibility to create barrier reefs and atolls, for which today the scuba dive turism aims.
Corals reproduction can be sexual (through spermatozoons and eggs) or non-sexual too. In case of sexual reproduction, in the most simple condition, exist males and females colonies that, in a determinate period of the year, release in the sea respectively spermatozoons and eggs. And so fertilization takes place in free waters. The successive embryonic development leads on the formation of a planctonic larva (planula). This larva, after a dispersion period like a pelagic organism transported by the running waters, fixes to the bottom, on a rigid substrate, originating a polyp, and from this first polyp, through the asexual reproduction by gemmation, will grow up the new colony. On the body of the first polyp, one draft of cells will form (gemma) from which will be born the second polyp and after, on the same way, will form the third, the forth, untill achieving a high number of polyps(thousands) and creating an entire new colony.
Corals are present in Mediterranean Sea with some dozen of species. The biology of this animals of our sea is unknown in many cases. On April of 1997 the research group of Francesco Zaccanti, Full Professor of the Department of Evolutionary and experimental Biology of the University of Bologna, with the undersigned like responsable of the project, in collaboration with the Scuba Schools International Italy, has begun a research, having the aim to clear the salient aspects of the biology of one of this species: Balanophyllia europaea, of the Madreporari group.
Balanophyllia europaea is a solitary coral that colonizes, in water less depth, the rocky coasts of the Mediterranean. It is also signaled along the Atlantic coasts of Spain. Our attempt is to study the growth, mortallity, natality, reproduction, in other words, using an scientifical term, the population dynamics.
The first research site was selected in Calafuria, a famous scuba diving site ten kilometers south of Lenghorn. We first made a monitoring, to check the abundance and bathimetric distribution of Balanophyllia europaea in this place. It resulted that in Calafuria this species is distributed from 2 to 12 meters deep, with maximum density around 7 m, where it reaches peaks of 140 corals per m2. These data explain how B. europaea cololonizes, in Calafuria, only superficial waters, where it reaches a quite high population density. Its distribution is limited to less deep and most lighted waters, for the symbiosis with zooxanthellae, requesting light for the photosynthesis. For this reasons, this coral results to be an important element of benthonic population of the first 10 meters of depth.
From the next study of sexual reproduction of B. europaea, we had the first important results. This coral, in Calafuria, is a simultaneos hermaphrodite; in other words, the same specimen produces, during the reproductive period, both spermatozoons and eggs. This is the first time ever that an hermaphroditism condition is described in Balanophyllia: any other congeneric species, whose reproduction was studied, has separated sexes, which means with male and femal individuals. Moreover, B. europaea, in Calafuria, is viviparous, in other words, the adults animals incubate the embryos inside the internal cavity (celenteron) until much advanced development phases. During this study on reproduction, we also were able to follow all embryonic phases. These studies will be published soon on an international scientific paper.
In the light of these results, we spontaneously ask the question: why this Balanophyllia in Calafuria is hermaphrodite, wjile in other places its closest relatives are with separated sexes? Which is the factor that selcted hermaphroditism in Calafuria? Finding an answer to this question is not simple. First of all it would be interesting to study other populations of B. europaea in places with significantly different enviromental condiotions, verifying if the hermaphroditism condition is mantained there, or if, otherwise, it is replaced by a separated sexes condition. In this last case, it would then be interesting to compare the conditions of the place where the species is hermaphrodite, with the ones where it is with separated sexes, trying to detect the factor selectioning the different sexual condition in the two zones. For this we are now searching a population of B. europaea in high Adriatic sea.
By the way, we have some population density data (n° of individuals per m2) that offer a possible answer to why Balanophyllia is a simultaneous hermaphroditein Calafuria. Along Pacific coast, from California to Canada, lives another species of the genus Balanophyllia: B. elegans. In central California, where this is been studied, it has separated sexes, and an average population density 31 times higher than the one of B. europaea in Calafuria (563 corals per m2 vs 18 corals per m2). The higher pupulation density promotes encounters of males and females, otherwise the low population density hinders it, decreasing the reproductive success probability. In a situation of low density like the one in Calafuria, a simultaneous hermaphroditism condition, for which any specimen produces both spermatozoons and eggs, increases the probability of a fertile germinal cells encounter, increasing the population survival probability.
During the last months, we involved in general B. europaea polyp morphology study, using electronic scanning microscopic technics. We obtained some high zoomed impressive images of the tentacles, with urticant nematocysts batteries, of spermatozoons, eggs and skeleton, that we purpose you here.
With transmission electronic microscopic technics, we are also studying the spermatogenesis and oogenesis in B. europaea. In other words we want to follow, in this primitive animal, all maturation phases that the cells must pass through, transforming from undifferentiated and “banal”, to elements responsible of the birth of a new individual.
Finally, in Calafuria, some underwater signals are now present, delimiting many little areas at different depths, on which individuals of B. europaea live. These individuals are measured about every 3 months. It will be interesting to verify if there are different growth rates amongst different seasons and depths, and in that case to relate them with enviromental prameters trends.
The research is planned to continue until 2000. Some important results have already been anchieved, and certainly others will come in the future. Difficultly, in this relatively short period, we will be able to light all the aspects of this animal’s biology that we find interesting. The study of relatively simple organisms has already demonstrated in the past its importance in giving answers and models valid also for the comprehension of more evolved organisms.
A section of the spermatozoon Balanophyllia europaea seen with electronic microscope. The spermatozoon is the cell who brings in the egg the male genetic information. Spermatozoons are usually made of a head, an intermediate zone, and a tail. The head hosts the nucleus of the cell, that contains the substance hosting the DNA, called chromatin, and some cellular organs used during the penetration of the spermatozoon inside the egg; in the intermediate zone there are others cellular organs (i.e. vescicles full of reserve substances, respiratory organs), while the tail, provided with active movement, is used by the cell to swim.
The head of the spermatozoon of Balanophyllia europaea has a conical shape and is about 0.002mm long: the top of the cone is the front of the cell. Inside this spermatozoon it is possible to observe some cellular organs. Ap, is the front process, that organ that is used to brake external egg membrane, so that the spermatozoon can enter it, chr is the chromatin, the substance localized into the nucleus that brings the genetic information, for it hosts the DNA; lv is the lipidic vescicle, the fuel tank of the cell, a mass of fat that gives the energy used by the spermatozoon to live and swim in the water; m is the mytochondrion, the “engine” of the spermatozoon, the place where reserve substances are burned (the fat above), in other words the organ where cell respiration, and so production of chemical energy for living and swimming, takes place; t are the tails of some spermatozoons near the mentioned one.
This image is been published on the international scientific magazine, published in Germany, called ZOOMORPHOLOGY, in volume 119, pages 231-240; title of the article is “Ultrastructural observations of the spermatogenesis of the hermaphroditic solitary coral Balanophyllia europaea (Anthozoa, Scleractinia)“; the authors are Stefano Goffredo, Tiziana Telò and Franca Scanabissi. In this research Bologna Scuba Team and its divers gave logistic support in the dives used during the sampling.