Impact of Planktivorous Fishes on Dispersal, Hatching, and Morphology of Estuarine Crab Larvae

Abstract

The effect of planktivory on life history patterns of estuarine crabs was studied by determining preferences of common estuarine fishes for crab larvae in the laboratory and the upper Newport River estuary, North Carolina. Plankton samples (68) and fishes were collected from an upstream and downstream site, on spring and neap low tides, and during the day and night. Over 99.6% of the plankters collected were decapod larvae, copepods, barnacle nauplii, and cyprids. Predominant fishes in the upper estuary were silversides, Menidia menidia, anchovies, Anchoa mitchelli, and killifish, Fundulus heteroclitus, as is typical for other estuaries on the east coast of the United States. Gut contents of 1861 fishes 15—100 mm long were analyzed. Silversides and anchovies preyed upon crab larvae more often than did killifish, and are most likely to influence the life history patterns of crabs inhabiting upper estuaries. Fishes that eat crab larvae are more abundant in estuaries than coastal waters during summer. Fishes in the estuary and the laboratory showed strikingly similar preferences for prey. In order to descending preference, natural populations of fishes preferred copepods, crab larvae that are exported from estuaries (Uca, Sesarma cinereum), and decapod larvae that develop in estuaries (Sesarma reticulatum, Palaemonetes, Rhithropanopeus harrisii). In the laboratory, juvenile and adult silversides and killifish preferred Artemia nauplii to crab larvae, they fed randomly on Uca larvae, and they avoided R. harrissii larvae. These planktivores preferred zoeae that are exported to coastal waters over those that are retained because exported larvae are smaller and have shorter spines. While the large size and spines of retained larvae protect them from their predators in estuaries, vulnerable zoeae may emigrate from estuaries to coastal waters because the rate of encounter with predators offshore is less than in estuaries. The risk of predation also appears to vary spatially and temporally within the estuary. Predation generally was greatest upstream in shallow, narrow areas of the upper estuary on diurnal neap tides. The spatial gradient in predation apparently was due largely to the great abundance of fishes, and particularly small zooplanktivorous fishes, occurring upstream. In contrast, temporal patterns of planktivory were not due to differences in fish size and abundance, but to diurnal foraging of fishes and changes in the availability of prey. Resident zooplankters generally were preyed upon more during neap tides, perhaps because they remained nearer to the substrate on spring tides to prevent being swept downstream. Uca and S. cinereum zoeae were eaten in similar numbers during diurnal neap and spring tides because most zoeae had been transported downstream before dawn when fishes resumed feeding. Estuarine crabs may have responded to predictable trends in planktivory by dispersing newly hatched zoeae downstream on nocturnal ebb tides, regardless of where larvae develop. Small vulnerable zoeae eventually disperse offshore, whereas large well—defended zoeae apparently remain in estuaries. Peak hatching on spring high tides may not have evolved to expedite transport to coastal waters, but instead may facilitate dispersal of larvae of semiterrestrial crabs from the shore where mortality may be high. This study suggests that predation pressure exerted by planktivorous fishes is predictable in time and space, and the timing of larval release, dispersal patterns, and larval morphologies of estuarine crabs have evolved together to reduce the risk of planktivory regardless of whether zoeae develop offshore or in estuaries.