Size and diel differences in activity patterns of Metapenaeus ensis, Penaeus latisulcatus and P. merguiensis

The nursery function of mangroves as shelter has been postulated to explain the positive correlation between shrimp catches and mangrove area. This st udy was undertaken to document shelter use and other activities in mangrove -associated penaeids and to determine diel and size differences relating to these activities. Juvenile Metapenaeus ensis, Penaeus latisulcatus and P. merguiensis collected from mangrove areas in Guimaras, central Philippines and stocked individually in glass tanks (=replicates) provided with artific ial shelters and sand substrate were observed every hour for 25 h. Size cla sses tested were very small (1-5.9 mm carapace length); small (6-10.9 mm CL ); medium (11-15.9 mm CL); large (16-20.9 mm CL); and very large (21-25.9 m m CL). Due to limited juvenile supply, only 3 sizes were tested for each sp ecies: P. merguiensis (very small to medium), M. ensis (small to large) and P. latisulcatus (medium to very large) with 5 replicate animals (one per t ank) per size. Juvenile M. ensis and P. latisulcatus showed a strong diel p eriodicity of daytime burial and nocturnal activity. In contrast, P. mergui ensis showed active swimming and feeding throughout the day and night. Very small to small P. merguiensis were observed on the shelters, but burying w as exhibited only by a few medium-sized juveniles. This shelter use is cons istent with observations of small P. merguiensis entering the mangrove fore st (where roots, twigs, etc. contribute to structural complexity) on the fl ood tide and concentrating in the shallow, turbid waters of adjoining creek s during ebb tide and slack water.