SIZE AND TIME OF DAY AFFECT THE RESPONSE OF POSTLARVAE AND EARLY JUVENILE GROOVED TIGER PRAWNS PENAEUS-SEMISULCATUS DE-HAAN (DECAPODA, PENAEIDAE) TO NATURAL AND ARTIFICIAL SEAGRASS IN THE LABORATORY

Juvenile tiger prawns (Penaeus semisulcatus De Haan and P. esculentus Haswell) show a strong association with vegetated habitats and are rar ely caught on non-vegetated areas. This pattern of distribution may be caused by postlarvae selecting vegetation when they settle, or to dif ferences in post-settlement mortality in different habitats. In this s tudy, we examined whether the postlarvae and early juvenile stages of P. semisulcatus would distinguish between seagrass (Zostera capricorni Aschers) without epiphytes, artificial seagrass and bare substratum i n the laboratory. The responses of prawns reared from the egg to diffe rent stages of postlarval and juvenile development were tested to dete rmine whether, and when, each size class showed a response to a partic ular habitat. Five size classes of postlarvae (average carapace length s [CL] of 1.2, 1.4, 1.6, 1.7 and 2.0 mm) were offered a choice between Z. capricorni and bare sand. Small size classes of postlarvae either did not respond to Z. capricorni (1.2 and 1.6 mm CL), or were more abu ndant on bare substratum than Z. capricorni. In contrast, the largest size classes of postlarvae (1.7 and 2.0 mm CL) were more abundant on Z . capricorni during the day but not at night. The behaviour of postlar vae changed markedly at a size of 1.7 mm CL (22 days from the first na uplius): smaller postlarvae frequently swam in the water column; 1.7 a nd 2.0 mm CL postlarvae spent much more. time resting on the substrate and perched on seagrass leaves. This size at which postlarvae first r espond to seagrass during the day, and show mainly benthic behaviour, is similar to the size at which they are found on shallow seagrass bed s in northern Australia. Large postlarvae (2.7 mm CL) and juveniles (4 .1 mm CL) both were more abundant on artificial seagrass than bare san d during the day but not at night, indicating that they respond to str uctured habitats. When large postlarvae (2.4 mm CL) and juveniles (3.5 mm CL) were offered a choice between Z. capricorni without epiphytes and artificial seagrass, they were more abundant on the Z. capricorni, which suggests that chemical cues from seagrass may explain some of t he responses of P. semisulcatus to seagrass. (C) 1997 Elsevier Science B.V.