Morphological and behavioral antipredatory adaptations of decapod zoeae

Abstract

Summary. Zoeae of some species of estuarine decapods are retained in the estuary throughout development while others are exported into nearshore coastal waters. The horizontal migrations of decapod zoeae to coastal waters may have evolved to reduce the probability of encountering planktivorous fishes which are most abundant in the estuary. If so, then the morphological vulnerability of zoeae to fish predation should be inversely related to the number of predators occurring where they develop. Six species of estuarine decapod zoeae were offered to Menidia menidia and Fundulus heteroclitus. The behavioral interactions were observed to determine the prey's vulnerability to predation, and the mode of operation and relative effectiveness of their defenses. Feeding trials and behavioral observations both demonstrated that M. menidia 6-16 mm long preferred Uca minax and Callinectes sapidus zoeae, which are exported from the estuary, to Rhithropanopeus harrisii, Sesarma reticulatum and Palaemonetes pugio, which are retained within estuaries. Pinnotheres ostreum zoeae develop in the lower estuary and fish demonstrated an intermediate preference for the zoeae. Menidia menidia 20-40 mm long showed similar preferences for R. harrisii, S. reticulatum, P. ostreum and U. minax as did small silversides. Large-mouthed demersal fish, Fundulus heteroclitus 6-10 mm long, also preferred U. minax to R. harrisii, but more readily preyed on zoeae than did M. menidia. The exported species of zoeae have shorter spines and smaller bodies than do retained zoeae, except P. ostreum which is small, spineless and passively sinks when attacked by fish. Other retained species of zoeae also have postcontact behavioral defenses which enhance the effectiveness of their morphological defenses. Zoeae do not evade attacks by fishes, but fishes quickly learned to avoid zoeae, which increases the effectiveness of the zoeae's antipredatory adaptations.

Planktivory as a selective force for reproductive synchrony and larval migration

“…Silversides and killifishes not only occur in high densities in Flax Pond but predominate in marshes all along temperate and tropical shorelines (Morgan 1986, Sogard & Abele 1991, Ayvazian et al 1992, Rountree & Able 1992, Sasekumar et al 1992, Baltz et al 1993. Therefore, crabs living in shallow waters likely experience similar predation pressures throughout their geographic ranges thereby providing strong selection for the evolution of predator avoidance behaviors.…”

“…Planktivorous fishes are especially abundant in productive marshes and other shallow water habitats, and rapid dispersal from these areas may reduce predation on crab larvae (reviewed by Morgan 1986Morgan , 1990. Crab larvae released during NMAHT do appear to avoid predation in time and space because they are rare or absent in the diets of fishes, even though they may be the most abundant zooplankter in shallow water habitats soon after hatching (DeCoursey 1979, Christy & Stancyk 1982, Salmon et al 1986, Morgan 1990).…”

Predation by planktonic and benthic invertebrates on larvae of estuarine crabs

“…harrisii are large and have long spines that apparently defend them against planktivorous fishes better than the short :~pines of small U. minax ( Fig. 1; Morgan, 1987aMorgan, , 1989Morgan, , 1990. These species also were selected because adults inhabit the same area of the estuary and newly hatched larvae are exposed to the same suite of predators.…”

“…Previous evidence suggests that differential vulnerability of crab larvae to planktivorous fishes may determine the dispersal patterns and timing of larval release of estuarine crabs (Morgan, 1987a(Morgan, ,b, 1990Morgan & Christy, 1992a,b). Spine length, body size and visibtlity determine the susceptibility of crab larvae to planktivorous fishes (Morgan 1987a(Morgan , 1989(Morgan , 1990Morgan & Christy, 1992a), which generally appear to be more abundant in estuaries than coastal waters (see Morgan, 1990).…”

“…Because most invertebrates do not rely on vision to capture prey, larval morphologies primarily will determine predation rates on larvae of various species. However, the elongate, numerous spines of crab larvae were ineffective against most invertebrate predators and were very effective against larval and adult fishes (Morgan, 1987a(Morgan, , 1989. This suggests that the primary morphological defenses of crab larvae may have evolved specifically to deter planktivorous fishes rather than predatory invertebrates (Morgan, 1989).…”