Montastraea cavernosa Linnaeus, 1767
Great star coral
Montastraea cavernosa is a reef-building coral of the tropical Atlantic (Nunes et al. 2009). It is abundant in the Caribbean, where it has received considerable attention from coral researchers.
Kelmanson and Matz (2003) studied the molecular basis of color diversity in Montastraea cavernosa. Typically, each natural pigment in corals and other anthozoans is essentially determined by a sequence of a single protein, homologous to the green fluorescent protein (GFP) fromthe jellyfish Aequorea victoria. [The discovery of GFP in A. victoria in 1962, and its subsequent development as a powerful tool in molecular biology, were the the basis for the 2008 Nobel Prize in Chemistry.] Kelmanson and Matz studied three colonies of Montastraea cavernosa representing distinct color morphs. Unexpectedly, these specimens were found to express the same collection of GFP-like proteins, produced by at least four, and possibly up to seven, different genetic loci. These genes code for three basic colors—cyan, green, and red—and are expressed differently relative to one another in different morphs.
As Kelmanson and Matz note, two basic alternative mechanisms of generating diversity of coloration are possible in principle: genetic polymorphism and polyphenism. Genetic polymorphism is the circumstance in which differences (such as color) can be traced to differences in the genome (i.e., different morphs have different DNA sequences coding for the trait in question). If the Montastraea cavernosa color variation is the result of a simple genetic polymorphism, then the color should be largely determined at the moment of zygote formation (fertilization of an egg), with little possibility for it to change afterwards, except for the intensity. In contrast, "polyphenism" refers to the ability of a single genotype to produce two or more alternative morphologies within a single population in response to an environmental cue. In the case of Montastraea cavernosa, the observed color variation would be an example of polyphenism if the same set of genes coding for GFP-like proteins are present in two or more color morphs, with the differences in color appearance being due to the changes in relative levels of expression of these genes. (The color diversity could also result from a combination of these two models.) In Montastraea cavernosa, the color differences between colonies can be explained by varying levels of expression (specifically, transcript abundances) of a set of genes coding for GFP-like proteins. The set includes at least four, and probably up to seven, separate genetic loci and encodes proteins emitting at three general wavebands: cyan (wide emission peak at around 495 nm), green (narrow emission peak at 505 to 520 nm), and red (emission at 580 nm). Kelmanson and Matz found that two different color morphs of Montastraea cavernosa, green and red, contained and expressed the same functional suite of color genes. No environmental cues are yet known that would confirm this color variation as environmentally-based polyphenism, but the authors suggest that depth could be one factor affecting color expression (Kelmanson and Matz 2003).
Montastraea cavernosa is an abundant reef builder in the Caribbean, being found throughout the region, from Panama to Florida, east to the islands of the Lesser Antilles, and as far north as Bermuda in the north Atlantic. This species is also widespread in the South Atlantic, being common along the coast of Brazil from Cabedelo, Paraiba to Vitoria, Espırito Santo. Montastraea cavernosa has also been reported from the offshore island of Fernando de Noronha, as well as Parcel Manuel Luiz, an offshore reef located 500 km east of the Amazon outflow. Although this species was previously unknown off the north coast of Brazil, Nunes et al. discovered abundant colonies on the offshore reef of Pedra da Risca do Meio at depths of about 25 m. Montastraea cavernosa is also one of the most common coral species in the islands of Sao Tome, Prıncipe and Annobon in the Gulf of Guinea, West Africa, but it has not been reported along the West African mainland nor in the Cape Verde islands farther north. (Nunes et al. 2009 and references therein)
Colonies of the Caribbean coral Montastraea cavernosa that harbor endosymbiotic cyanobacteria can fix nitrogen, whereas conspecifics without these symbionts cannot. Zooxanthellae (the coral's symbiotic photosynthetic dinoflagellate algae partners) appear to be somehow able to use these supplementary sources of nitrogen and increase their growth rates without compromising the integrity of the symbiosis (Lesser et al. 2007). Nitrogen limitation has long been proposed to contribute to the stability of the coral-zooxanthellae symbiosis. How this symbiosis is maintained despite nitrogen supplementation from nitrogen-fixing cyanobacteria is an open question.
In contrast to most coral species in the family Faviidae, colonies of Montastraea cavernosa are generally either all-male or all-female. They are broadcast spawners. In a study on the Caribbean coast of Colombia (October 1990 to October 1991), Acosta and Zea (1997) found that there was a single gametogenic (egg- and sperm-producing) cycle per year in both sexes. The duration of the oogenic cycle was ~11 months, with oogenesis (egg production) in all months except just after spawning (Szmant 1991). Of the colonies with gonads sampled in November 1990, all female eggs were already in Stage I. Development of eggs to Stage II (yolk comprising up to 50% of the cytoplasm) was evident in late December 1990 to early January 1991, and development of Stage III eggs began in March 1991. A marked increase in the mean gonad index did not occur until after July 1991. In contrast to the female cycle, the spermatogenic cycle lasted only 2 to 4 months. Male gonads, were first evident in October and November 1990, disappeared until the full moon of June, when testes with most spermatic cysts in Stage I were observed. Stages II and III cysts were evident from the beginning of the cycle, but the latter constituted a large percent of the sex products only after August 1991. The mean gonad index increased from June through October. By and after the full moon of October 1990 and 1991, most cysts of male colonies were in Stage III. Spawning did not occur synchronously within the population, since maturing, fully mature and spawned colonies were found simultaneously on several sampling dates. (Acosta and Zea 1997 and references therein). Zooxanthellae are not present in the eggs and must be acquired from the water column de novo during the planktonic larval phase or after settlement (Szmant 1991). Broadcast spawning is now recognized as typical for reef-building corals, but for years brooding with multiple planulation cycles (production within, and release from, coral polyps) of planula larvae per year was thought to be the norm for reef corals (Szmamt 1991).
In a study of seven "massive" Caribbean corals, Soong (1993) identified major differences in reproductive behavior between species with large maximum colony size (>100 cm2 in surface area), including Montastraea cavernosa, and species with small maximum colony size. The four large species studied broadcast gametes during a short spawning season; the two smaller-sized and one medium-sized species brooded larvae during an extended season (year-round in Panama).
Prior to the early 1980s, for over 200 years, all corals were believed to be viviparous (brooding). It is now known that most reef-building corals release, or "broadcast", eggs and sperm into the water column during periodic and often synchronous spawning events. For decades researchers have speculated about and worked to identify environmental entrainment factors that might influence sexual reproduction and the eventual release of gametes. This synchronization is generally believed to operate on at least three interrelated temporal levels: (1) the time of the year; (2) the lunar cycle; and (3) the time of night. It is clear that nighttime is required for gamete release, but a consistent global relationship between lunar phase and the timing of spawning is less clear, given that most corals on the Great Barrier Reef in Australia spawn at neap tides, while the same species in southern Japan spawn at spring tides. It has seemed reasonable to assume that the time of the year for gamete release is linked to optimal sea surface temperature (SST). van Woesik et al. (2006), however, have argued that solar insolation (energy from the sun), is a better predictor of gamete production for many corals.They tested this hypothesis using data for 12 species of corals distributed throughout the Caribbean (tropical west Atlantic), including Montastraea cavernosa. Regarding temperature, they found that the cumulative dose of SST measured through time and the rate of change in temperature correlated poorly with the timing of coral spawning, although the average temperature during the month of spawning was significantly correlated with spawning. For solar insolation, they found that the rate of change and the cumulative response of solar insolation cycles was a better predictor of gamete release, although solar insolation intensity at the time of spawning was not. All of the coral species they examined showed highly significant positive relationships between spawning date and the cumulative dose of solar insolation, and 11 of 12 species, including Montastraea cavernosa, showed a significant response to the rate of change in solar insolation. Solar insolation and temperature are obviously related phenomena since solar irradiance ultimately drives SST, but because of the high specific heat capacity of water, maximum SST generally lags 1 to 2 months (or more) behind maximum solar insolation. Time delays in SST fluctuations are latitudinally predictable but vary with cloud-cover and windstrength. van Woesik et al. concluded that solar insolation influences the reproductive schedules of Caribbean corals, but water temperatures must be optimal (28–30 C) to allow maturation and gamete release. (van Woesik et al. 2006 and referencess therein)
Vize (2006) asserts that for shallow water corals, annual water temperature cycles set the month, lunar periodicity the day, and sunset time the hour of spawning. This tight temporal regulation is critical for achieving high fertilization rates in a pelagic environment. Given the differences in light and temperature that occur with depth and the importance of these parameters in regulating spawn timing, it it had been unclear whether corals in deeper water can respond to the same environmental cues that regulate spawning behaviour in shallower coral. Vize used a remotely operated vehicle to monitor coral spawning (including that of Montastraea cavernosa) activity at the Flower Garden Banks (northwest Gulf of Mexico) at depths from 33 to 45 meters. All recorded spawning events were within the same temporal windows as shallower conspecifics. These data indicate that deep corals at this location either sense the same environmental parameters, despite local attenuation, or communicate with shallower colonies that can sense such spawning cues.
Evolution and Systematics
Nunes et al. (2009) compared the levels of genetic diversity and connectivity in the coral Montastraea cavernosa among both central and peripheral populations throughout its range in the Atlantic. Genetic data from one mitochondrial and two nuclear loci in 191 individuals show that M. cavernosa is subdivided into three genetically distinct regions in the Atlantic: Caribbean-North Atlantic, Western South Atlantic (Brazil) and Eastern Tropical Atlantic (West Africa). Within each region, populations have similar allele frequencies and levels of genetic diversity. No significant differentiation was found between populations separated by as much as 3000 km, suggesting that this coral species has the ability to disperse over large distances. Gene flow within regions does not, however, translate into connectivity across the entire Atlantic. Instead, substantial differences in allele frequencies across regions suggest that genetic exchange is infrequent between the Caribbean, Brazil, and West Africa. Furthermore, markedly lower levels of genetic diversity are observed in the Brazilian and West African populations. Genetic diversity and connectivity may contribute to the resilience of a coral population to disturbance. Isolated peripheral populations may be more vulnerable to human impacts, disease, or climate change relative to those in the genetically diverse Caribbean-North Atlantic region. Although limited dispersal and connectivity in marine organisms can have negative fitness effects in populations that are small and isolated, reduced genetic exchange may also promote the potential for local adaptation.