The origin of the common name for the reef-forming coral, Diploria labyrinthiformis, "brain coral", is readily apparent: this species forms large clumps, 6 to 8 feet (2 to 2.5 meters) in diameter, with a deeply convoluted surface reminiscent of a human brain. It is brownish yellow in life (Voss 1980). This "brain" is actually a colony of tiny cnidarian polyps (sea anemone-like animals) that secrete a hard calcareous skeleton. The polyps feed by catching food with their tentacles, as well as obtaining nutrients from symbiotic photosynthesizing dinoflagellate "algae".
Logan and Tomascik (1991) studied the growth rate of Diploria labyrinthiformis on several high latitude coral reefs around Bermuda. Growth rates showed an inverse curvilinear relationship with depth, with highest growth rates in shallow inshore waters and lowest at the edge of the Bermuda platform and on the adjacent fore-reef slope. Annual density bands formed seasonal couplets, with narrow, high density bands appearing to form in the spring-summer months and wider, low density bands over the rest of the year . Comparison of the extension rates of D. labyrinthiformis from Bermuda with published rates from lower latitudes indicates that reefs at lower latitudes (i.e., closer to the Equator) have higher extension rates than reefs at higher latitudes (Logan et al. 1994).
The genus Diploria is a conspicuous, common, and abundant reef-building group throughout the wider Caribbean. It is endemic to (i.e., found only in) the Atlantic-Caribbean (Weil and Vargas 2010).
D. labyrinthiformis (Linnaeus 1758) form crusts, plates, and sub-massive and massive boulders along a wide depth distribution (0–35 m) and is often abundant (Weil and Vargas 2010).
Alvarado et al. (2004) studied the sexual reproduction of Diploria labyrinthiformis in Colombia and found that it is hermaphroditic (i.e., a single individual produces both eggs and sperm) and releases its gametes (eggs and sperm) in the spring, in contrast to the summer spawning of the other two Diploria species, D. strigosa and D. clivosa. Weil and Vargas (2010) studied the reproductive biology of all three species in Puerto Rico. primatily at the San Cristobal reef complex, where all three species had abundant large colonies. All three species were found to be simultaneous hermaphrodites (i.e., an individual produces both male and female gametes at the same time). Diploria strigosa and D. clivosa released gametes during August and/or September, but D. labyrinthiformis released its gametes in April and/or May (consistent with the findings of Alvarado et al. from Colombia). Of the three species, D. labyrinthiformis had the highest fecundity (an average of 36.5 eggs/polyp versus 27.2 for D. strigosa and 20.2 for D. clivosa). All three of these species are broadcast spawners, releasing their gametes into the water and providing no parental care (in some other coral species, known as brooders, fertilization and early development of larvae are internal, with larvae eventually released rather than gametes). Synchronized release of gametes (sperm and eggs) appears to be linked to lunar cycles, as well as other environmental cues.
There are three described species currently placed in the genus Diploria. In addition to D. labyrinthiformis, these are D. strigosa, which is the most common and abundant of the three and has a growth habit similar to that of D. labyrinthiformis, and D. clivosa, which is mostly sub-massive and crustose with a distribution restricted to shallow, high energy, exposed reef platforms, back reef, and rocky habitats (Weil and Vargas 2010).
Cohen et al. (2004) investigated the potential for using chemical and microstructural analysis of the skeletons of brain corals (Diploria labyrinthiformis) to provide proxy records of wintertime sea surface temperature (SST) variability in the subtropical North Atlantic. Although this species has a slow growth rate (less than one half of Pacific Porites species) and complex skeletal architecture, D. labyrinthiformis is an appealing potential archive of paleo-SST because of its abundance throughout the Caribbean; its tendency to build massive, long-lived colonies; and the presence of strong annual growth bands in the skeleton (Cohen et al. 2004).
