POPULATION-STRUCTURE AND SWARM FORMATION OF THE CYCLOPOID COPEPOD DIOITHONA-OCULATA NEAR MANGROVE CAYS

The cyclopoid copepod Dioithona oculata forms swarms in water < 30 cm deep among prop roots of red mangroves (Rhizophora mangle) which fringe protected areas of two lagoonal cays, Twin Cays, Belize. During 7 of 8 months surveyed by in situ observation, swarms were present but differed in size from small cylindrical swarms (5-10 cm diameter) to bands extending up to 1200 m. Swarms were never observed at night. Swarms formed at dawn when light intensities reached an average value of 13.82 (log10 quanta cm-2 s-1) and dispersed at dusk at similar intensities. Swarms observed in June formed earlier and dispersed later in the day than swarms observed in January; their swarming behavior followed seasonal changes in light intensity. Mean dioithonan density in swarms (10 ml-1) was much higher than the mean density (0.15 ml-1) of non-swarming dioithonans around mangrove prop roots. In open water 3-5 m away from the mangroves, mean dioithonan density was 7.9 x 10(-5) ml-1 during the day, and 2.68 x 10(-3) ml-1 at night. Swarms were composed predominantly of adults and copepodid stages IV and V, although younger copepodid stages could be present. Nauplii were never present. The 'average copepodid stage' for all 95 swarms sampled was 5.3, where 6.0 represents a swarm with only adults. In open water 3-5 m away from the mangroves, the youngest copepodids (stage one) dominated the dioithonan population during the day. At night when swarms dispersed to open waters, average copepodid stage was higher (3.5) compared with the day value (1.2) in open waters. Although densities in swarms were higher in June than January, average copepodid stage in June was higher (5.6) than that in January (4.9). A higher percentage of adults were females during June than January. Therefore higher densities did not result from increases of smaller stages in swarms, but perhaps changes in behavior or population structure.