Coral tissue microstructure and necrosis: relation to catastrophic coral mortality in Panama

During 1983, patchy but extensive mortality of several stony coral species occurred off the Pacific coast of PanamB, associated with loss of their symbiotic zooxanthellae. This disturbance was coincident with the prolonged 1983 E1 Nino warming event. Normally colored colonies receiving bleached colony portions in iso-, alloand xenografts remained in a healthy state during a 7 mo period. Neither these transplantation experiments, nor histopathological examinations, revealed the presence of an infectious agent which might be responsible for the widespread bleaching, although suspected bacteria were found in electron microscopy preparations of 2 species of bleached corals. The condition of affected coral tissues ranged from slight to severe atrophy and necrosis, but normal-appearing zooxanthellae remained in all but the most necrotic specimens, although reduced in numbers. These observations suggest that environmental (particularly high thermal) stress may have been responsible for the coral deaths. INTRODUCTION to fully colored colonies) and the condition of affected coral tissues as examined by electron microscopy and Reef-building corals suffered catastrophic mortality histopathological methods. off the Pacific coast of Panama from early to late 1983 (Glynn 1983, 1984). Most coral species lost symbiotic zooxanthellae ('bleached') during this period and some METHODS species' populations were severely reduced in size or even extirpated locally. Certain coral species were This study was conducted chiefly on the Uva Island only slightly discolored, showing a partial loss of zooreef (7"48'48" N; 81'45'35" W), one of numerous coral xanthellae, and later recovered. Those species most reefs affected in the Gulf of Chiriqui, Pacific coast of seriously affected typically showed total colony Panama, in 1983. The reef was visited 4 times during bleaching and signs of morbidity about 5 to 6 wk the approximately 9 mo disturbance, on 18 to 21 March following bleaching. This event lasted about 9 mo 1983, when coral bleaching and death were first noted, (Feb-Nov), with bouts of bleaching and mortality 26 to 28 April, 23 to 24 June, and on 29 to 30 October, occurring over this period. Patches of coral ranging when only a few of the remaining live dorals showed from several to hundreds of m2 bleached and died, to signs of bleaching and morbidity. Observations were be followed 2 to 3 mo later by equal-sized patches on also made approximately monthly in the Taboga other parts of reefs. This epizootic-like disturbance Islands (8O47'40" N; 79'32'30" W), Gulf of Panama. suggested that an infectious agent might be involved. In the transplantation experiments, 4 to 5 cm long We report here the timing of events, the results of pieces of coral were broken from bleached or normal transplantation experiments (grafting bleached corals colonies and grafted to normal or bleached colonies, respectively. Isografts were effected through intracolony transplantations, allografts through inter' Present address: Division of Biology and Living Resources, Rosenstiel School of Marine and Atmospheric Science, Unicolony transplantations in a given species, and xenoverjity of ~ i ~ ~ i , 4600 Rickenbacker Causeway, Miami, grafts through interspecific transplantations. The Florida 33149, USA grafts were fitted snugly into host colonies (between O Inter-Research/Printed in F. R. Germany 30 Dis. aquat. Org. 1: 29-37, 1985 Table 1. Coral species collected for microscopic examinations Species Condition Microscopic Collection technique information Millepora intricata Milne-Edwards Bleached Light Uva Island reef 23 June 1983 3-12 m depth Gardineroseris planulata (Dana)