Predation on postlarvae and juveniles of the shore crab Carcinus maenas:importance of shelter, size and cannibalism

Abstract

Settlement and early juvenile stages are considered a bottleneck in the life history of many epibenthic organisms because of high predation mortality. Nursery habitats may play an irnportant role in mitigating settlement and post-settlement mortality by providing refuge from predation. We examined these relationships in postlarvae and early juvenile stages of the shore crab Carcinus maenas L. in laboratory and field tethering experiments. We studied habitat and size related habitat mortality using postlarvae and young juvenile crabs as prey, and various predators, including juvenile conspecifics, in several habltats common in shallow (0 to 1 m) soft bottom nursery areas on the Swedish west coast. Settling mortality was high in open sand (80 to go%), whereas a significant habitat refuge was obtained in mussel beds, eelgrass and filamentous green algae, the latter yielding the lowest mortality (13 to 14%). Small differences in structural complexity of ephemeral macroalgae dramatically affected predation mortality of tirst instar crabs, with a slgniiicant refuge obtaineci oriiy irl diyde of medium complexity. Predation rate on tethered crabs in the field was high (52 to 67 X) only on the smallest crabs (<S mm carapace width, CW), which obtained a significant refuge in the eelgrass habitat compared to open sand. Mortality for larger crabs (5 to 25 mm CW) was low (<10%) and similar in sand and eelgrass habitats. Our results indicate that predation is an important process that can create a bottleneck for juvenile shore crab populations during settlement and early juvenile stages, mediated by the availability of nursery habitats. Postlarvae obtained refuge from predation in several different habitats, suggesting that the recruitment of juvenile shore crabs will be less affected by temporal and spatial variation of any single habitat type. The strong slze refuge for crabs larger than 4 mm CW indicates that key predators are small. We suggest that cannibalistic juveniles, which caused predation rates similar to or higher than all other investigated predators, are dominant predators on settling postlarvae and young luvenile crabs in nursery areas. We further propose that habitat-and size-specific predation by small epibenthic predators are an important selective force in habitat selection by postlarvae and ontogenetic shifts in habitat use by juveniles.

Experimental studies on predation and cannibalism of the settlers of Chasmagnathus granulata and Cyrtograpsus angulatus (Brachyura: Grapsidae)

“…and Sainte-Marie, 1997;Moksnes et al, 1997Moksnes et al, , 1998 . This is probably due to uniform size within a cohort.…”

The Use of Artificial Benthic Collectors for Assessment of Spatial Patterns of Settlement of Megalopae of Carcinus maenas (L.) (Brachyura: Portunidae) in the Lower Mira Estuary, Portugal

“…Hedvall et al (1998) showed that megalopae of C. maenas avoid unstructured substrates such as sand. In these types of substrate the predation rates are very high, reaching, according to Moksnes et al (1998), more than 80% in 2 days. The synthetic fibbers of the collector surface provide shelter to prevent predation and trap organic matter that may further enhance settlement.…”

“…Other authors showed that megalopae of several species prefer structurally complex substrates, which provide shelter (e.g. O'Connor, 1993;Eggleston & Armstrong, 1995;Hedvall et al, 1998;Moksnes et al, 1998;Stevens & Kittaka, 1998). These substrates include vegetation (Bostro¨m & Bonsdorff, 1997) and shelly gravel .…”

“…Megalopae migrate back to the estuary during spring tides, and transport is accomplished by selective tidal stream transport (Berrill, 1982;Queiroga, 1998;Hedvall et al, 1998;Moksnes et al, 1998). According to Hedvall et al (1998) and Moksnes et al (1998) there are indications that opportunistic behaviour is expressed already at settlement, as several different habitats may provide appropriate settling grounds.…”

Duration of the pre-settlement period of the mangrove crab Ucides cordatus (Decapoda: Ocypodidae) under laboratory conditions

“…Premature releases would include larvae still displaying planktonic behavior, exposing them to greater predation risk (Johns and Mann, 1987;Dittel et al, 1996;Moksnes et al, 1997Moksnes et al, , 1998 as well as reducing the effectiveness of the restocking due to the transport of larvae away from the focal areas. Excessively late releases, on the other hand, entail extensive losses through cannibalism within larviculture tanks (Quinitio and Parada-Estepa 1999), as well as increased rearing costs.…”