Collection of the clam Anodontia edentula in mangrove habitats in Panay and Guimaras,central Philippines

The mangrove clam Anodontia edentula is highly prized in thePhilippines for its flavor and large size. Because this infaunal species isfound down to one meter deep in mangrove areas, harvesting the clam reportedlydamages mangrove stands. To evaluate such reports, a survey of collectionmethods was undertaken in Panay and Guimaras, central Philippines in August1997–December 1999. Host to chemosynthetic bacterial symbionts thatutilize sulfide as energy source, A. edentula are strategically situated insulfide-rich anoxic substrates but also gain access to oxygenated seawaterthrough a ventilation burrow or tube. By locating the opening of this burrow,collectors can detect the presence of a buried clam and harvest itnondestructively with a blade or bare hands. In contrast, the indiscriminatetilling of wide mangrove areas can damage mangrove plants. Most collectors were40–45 years old with 22–30 years collection experience, married with5–7 children, and had low educational attainment. They sold clams directlyin the local markets or through middlemen (to restaurants and beach resorts);sales provided from 10% to 100% of daily family income. Collectors complained ofdecreasing clam sizes and numbers and the physically strenuous work ofcollecting.     

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 study 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 artificial shelters and sand substrate were observed every hour for 25 h. Size classes tested were very small (1–5.9 mm carapace length); small (6–10.9mm CL); medium (11–15.9mm CL); large (16–20.9mm CL); and very large (21–25.9 mm CL). Due to limited juvenile supply, only 3 sizes were tested for each species: P. merguiensis (very small to medium), M. ensis (small to large) and P. latisulcatus (medium to very large) with 5 replicate animals (one per tank) per size. Juvenile M. ensis and P. latisulcatus showed a strong diel periodicity of daytime burial and nocturnal activity. In contrast, P. merguiensis showed active swimming and feeding throughout the day and night. Very small to small P. merguiensis were observed on the shelters, but burying was exhibited only by a few medium‐sized juveniles. This shelter use is consistent with observations of small P. merguiensis entering the mangrove forest (where roots, twigs, etc. contribute to structural complexity) on the flood tide and concentrating in the shallow, turbid waters of adjoining creeks during ebb tide and slack water.     

Diel activity patterns in Metapenaeus and Penaeus juveniles

Small (5–10.9 mm carapace length), medium (11–15.9 mm), and large (16–20.9 mm) juveniles of Metapenaeus anchistus, Metapenaeus sp., Penaeus monodon and P. merguiensis were stocked individually in glass tanks provided with sand substrate, sea water, artificial bamboo shelter, aeration and food. The seven activity types (recorded for each shrimp hourly for 24 h) were classified as below (burrowing) or above substrate (swimming, walking, stationary, in shelter, feeding and cleaning). Shrimp juveniles exhibited a strong diel periodicity — emergence and activity at night and burrowing in the day. The chi-square test showed that type of activity (above/below substrate) was associated with period (light/dark). Diurnal burrowing was greater among Metapenaeus than Penaeus; inversely, above substrate activities were more frequent for Penaeus species compared to Metapenaeus. Feeding was the major above substrate and nocturnal activity for M. anchistus, Metapenaeus sp. and P. monodon. Only P. Monodon used the shelter consistently. Frequency of the 7 activity types was dependent on juvenile size for Penaeus, e.g., the preference for shelters shifted to burrowing with increase in size in P. monodon. Results are discussed in relation to the importance of mangrove habitats in providing shelter to penaeids, in particular the mangrove-associated P. monodon and P. merguiensis.