• Abstract and Figures
• Public Full-text

INTERSPECIFIC AGGRESSION BY SCLERACTINIAN CORALS. 1. THE REDISCOVERY OF SCOLYMIA CUBENSlS (MILNE EDWARDS & HAIME) JUDITH LANG Smithsonian Tropical Research Institute, P. O. Box 2072, Balboa, Canal Zone, and Discovery Bay Marine Laboratory, Discovery Bay, Jamaica ABSTRACT The solitary sc1eractinian coral, Scolymia cubensis, was recently con- sidered to be an "ecovariant" of S. lacera. As the latter invariably kills the tissues of S. cubensis by an extracoelenteric feeding response, while con- trol individuals of S. lacera never damage each other, S. cubensis is again separated as a distinct species. S. lacera is one of the two most "aggres- sive" corals in Jamaica, while S. cubensis ranks ninth in the "aggression hierarchy." Small aggressors such as the Scolymias damage subordinate species which compete for their space and light. The system of classification of the stony corals in present use is based almost entirely upon skeletal characters (Vaughan & Wells, 1943; Wells, 1956; Goreau, 1961). In practice this method suffers from severe limita- tions, as it often cannot discriminate habitat-induced variations within a species from diversity between species. Variations in the shape of the coralla of reef corals may be due to environmental factors such as light, currents, sedimentation, substrate, and interactions with neighboring or- ganisms. The scleractinian coral Scolymia is one example of a genus within which the morphological and ecological characteristics of two distinct species so overlap that their differentiation was doubted until aggressive interactions were discovered to occur between them. Catala (1964) first observed the interactions of two Pacific corals, Trachyphyllia geoffreyi and Cynarina lachrymalis, in his aquaria at New Caledonia. He described a "bridge" of tentacles which joined the polyps together for several days until both died. However, in the illustrative photo- graph (Catala, 1964: pI. 1, f. 4), they appear rather to be connected to each other by mesenterial filaments. According to Catala, this phenomenon, which he called "reciprocal cannibalism," only occurred among certain species in overcrowded aquaria. I was not aware of Catala's observation when I placed two supposed "ecovariants" of the solitary mussid coral Scolymia lacera (Pallas) be- side each other in Jamaica in 1967. The tissues of the polyp of one of these "ecovariants" were more fleshy, verrucose, and lighter green than those of the other. After several hours, mesenterial filaments of this "fleshy" 1971] Lang: I nterspecijic Aggression by Scleractinian Corals 953 FIGURE I. The extracoelenteric feeding response of Scolymia lacera (lighter coral, on the left) to Scolymia cubensis. Sc1eroseptaof the damaged S. cubensis can be seen underlying the extruded mesenterial filaments of the S. lacera. Scolymia were extruded through temporary openings in the polyp wall, and penetrated the adjacent part of the coelenteron, or body cavity, of the less fleshy, smooth variant. In less than 12 hours, they had completely digested the tissues of the latter within a radius of about 2 cm from the zone of contact, thereby exposing the underlying skeleton. Elsewhere the coeno- sarc of the "victimized" coral remained healthy. Initially, mesenterial fila- ments of the "smooth" Scolymia appeared at the ectodermal surface, but they were evidently unable to damage the filaments of the fleshy Scolymia, and were later withdrawn from the area under attack. This act of agres- sion was basically an extracoelenteric feeding response, such as Carpenter (1910), Yonge (1930), and Goreau (1956) have described in mussid and faviid corals presented with pieces of animal food which were too large to be transported into the stomodaeum by the ectodermal feeding currents. Further experiments were made with over 400 individuals of Scolymia from the northern and southern coasts of Jamaica. The fleshy, verrucose individuals of S. lacera invariably digested those tissues of the smooth variants that were within reach of their mesenterial filaments (Fig. 1). 954 Bulletin at Marine Science [21( 4) TABLE 1 AGGRESSIVE INTERACTIONS AMONG Mussa angulosa, 1sophyllia sinuosa, Scolymia lacera AND S. cubensis (Number of trials: More than 100 of each.) S. lacera S. cubensis ("fleshy ("smooth M. angulosa I. sil1110sa Scolymia") Scolymia") M. angulosa 0 0 0 ~ 1. sinuosa 0 0 0 ~ S. lacera 0 0 0 ~ ("fleshy Scolymia") S. cubensis ~ ~ ~ 0 ("smooth Scolymia") ...• The coral in the horizontal column injures the coral in the vertical column. ~ The coral in the vertical column injures the coral in the horizontal column. o No interaction. As shown in Table 1, the controls, which were pairs of either fleshy or smooth individuals, never attacked each other. When a number of differ- ent corals were brought into contact, the fleshy Scolymia damaged every other Jamaican species, except two colonial mussids, Mussa angulosa (Pal- las), and Isophyllia sinuosa (Ellis & Solander), with which it did not inter- act. Under the same conditions, the smooth Scolymia was attacked by four other species of mussid and meandrinid corals in addition to M. angulosa, T.sinuosa and the fleshy Scolymia. The skeletons of the "aggressive" Scolymia have thick primary septa, coarse septal teeth, concave or flattened calices, and a maximum diameter of at least 15 em. The "subordinate" Scolymia has flat or convex calices, narrow septa, smaller septal teeth, and a maximum diameter of about 10 em. The coralla of both forms become columniform in muddy environ- ments, raising the polyps above the region of disturbed sediments. Goreau, who found these two Scolymias in Jamaica, had believed that they were ecovariants of the same species, because the smooth form is often found growing under conditions of lower ambient illumination than the fleshy variant. However, the absolute correlation of morphological and behavioral differences suggests now that separate species are involved. Wells (1971) has made a detailed examination of the coralla of the two forms, confirm- ing that the aggressive Scolymia is S. lacera. The smooth subordinate form turns out to be Scolymia cubensis, originally described by Milne Edwards & Haime in 1849 from specimens in the Paris Museum. These remained the only known examples of this species until its rediscovery in Jamaica. Extracoelenteric feeding interactions, which are found to occur between 1971] Lang: Interspecific Aggression by Scleractinian Corals 955 many species of Jamaican reef corals, have a definite and consistent hi- erarchical structure (Lang, manuscript). S. lacera and M. angulosa are the most aggressive, while S. cubensis ranks ninth, in a total of 52 species. Most aggressors are contained within the families Mussidae, Meandrinidae and Faviidae (all suborder Faviina), members of which were thought by Gareau (1956) to extrude mesenterial filaments and feed by means of extracoelenteric forms of digestion more readily than other corals. S. lacera injures S. cubensis and other subordinates whenever their polyps are in contact, and may simultaneously attack two or more neighboring corals. Aggressors may prove to have chemoreceptors on their ectodermal surfaces which are specifically stimulated by particular molecules asso- ciated with the surfaces of subordinate corals, for S. lacera does not inter- act with M. angulosa and with l. sinl/osa, or with other individuals of S. lacera, even when it is killing other species of corals. When mesenterial filaments on one side of a S. cubensis are damaging a less aggressive coral, in another area the same individual may be under attack from S. lacera or other species higher in the aggression hierarchy. Interspecific aggression in Scolymia persists even when the animals are subjected to extreme physiological stresses, such as starvation and con- tinued darkness. For example, two individuals of S. lacera and two of S. cubensis were kept without being fed in a closed, aerated aquarium for 155 days. Their polyps slowly regressed, and the individuals of S. lacera in particular lost most of their zooxanthellae. Nevertheless, when these starved corals were put together, the S. lacera completely digested the adjacent tis- sues of the S. cubensis a few hours after contact. Other individuals of S. lacera, from which zooxanthellae were removed by bleaching in darkened aquaria, or by placing in dark crevices on the fore-reef slope, attacked all normal, or bleached individuals of S. cubensis. A S. lacera, which was kept in a running sea-water aquarium under low light intensi- ties, gradually bleached over a period of a year, and its tissues slowly re- gressed to such an extent that the polyp was eventually detached from its corallum without damage (as described by Goreau & Goreau, 1959). This skeletonless polyp fed extracoelenterically on subordinate corals, such as S. cubensis. Extracoelenteric feeding interactions were initially studied among corals in aquaria. By transplanting specimens collected at 50 meters on the fore- reef slope, S. lacera has been made to interact underwater with S. cubensis in various habitat zones of the reef, at depths ranging from just below the surface to more than 65 meters. However, when Scolymias are transferred to an unshaded part of the reef flat, at a depth of less than one meter, S. lacera loses some of its aggressiveness, and may fail to attack S. cubensis. Under these conditions, both species bleach due to mass expulsion of zoo- xanthellae, their tissues contract, and mesenterial filaments may be ex- 956 Bulletin of Marine Science [21 (4) FIGURE 2. Scolymia {acera (smaller coral, on the right) growing beside S. cubensis, which it has killed at the region of contact, at 30 meters on a sill reef at Discovery Bay, Jamaica. truded through their stomodaea and polyp walls. S. cubensis usually dies within 4 to 7 days, while S. lacera may survive for at least 2 months. Con- trol individuals of S. lacera in shaded crevices damage shaded individuals of S. cubensis, and show no other symptoms of what might be called the "light shocked" condition. Aggressive interactions between reef corals such as S. lacera and S. cubensis occur in at least two other populations of West Indian corals.

Load more

  8

Copy Link