Experimental studies on selection for vegetative structure by Penaeid shrimp
Variability in selection for vegetated habitats by juvenile brown shrimp, Penaeus aztecus, and white shrimp, P. setiferus, as evidenced by distributions in estuaries, suggests that the value of these habitats is not constant. Previous laboratory work indicates that selection for structure itself is one component of habitat selection, but environmental conditions and other habitat characteristics undoubtedly affect the utilization of vegetated estuarine habitats. This study was designed to examine the effect of environmental variables on selection for structure in the laboratory in an effort to increase our understanding of the way habitats are utilized by penaeid shrimp. Brown shrimp are generally found in association with estuarine vegetation, and they selected for vegetative structure in the laboratory. An average of 81% of brown shirmp were distributed in the vegetated half of control tanks. Reductions in salinity to oligohaline levels, used to simulate flood events in estuaries, significantly reduced selection for structure. The reduction of light, either through the manipulation of lighting or through turbidity, had a similar effect on brown shrimp distributions. Neither reduced salinity or light, however, reduced the mean percentage of shrimp in the grass below 50%. The overall presence or absence of food or of an appropriate substrate for burrowing, did not alter selection for structure, but the distribution of these habitat characteristics had a dramatic effect on shrimp distributions. Attraction to food or to a substrate for burrowing can override the inherent selection for structure normally exhibited by brown shrimp. Other variables examined including day length and shrimp size did not significantly affect selection. White shrimp distributions in relation to estuarine vegetation are more variable. In our experiments, white shrimp also showed an inherent selection for the vegetated half of the control tanks (75% of shrimp in the vegetation), but none of our experimental variables appeared to influence this selection to any great extent. There was a strong correlation between white shrmp activity and selection for structure, and this relationship may have contributed to the relatively large variability in selection by this species.