Pre‐ And Post‐Settlement Determinants of Estuarine Dungeness Crab Recruitment

Abstract

Abundance of early juvenile Dungeness crab (Cancer magister) is dramatically higher in intertidal shell habitats compared to mud habitats in several coastal estuaries of the Pacific Northwest. To define the mechanisms underlying this habitat—specific pattern in abundance, we concurrently examined four components of recruitment to intertidal shell and mud habitats at two locations within the Grays Harbor estuary (Washington, USA): (1) water column supply of crab megalopae (postlarvae); (2) settlement patterns of crab megalopae 8 h after settlement substrates were deployed; (3) density of first benthic juvenile instars (J1) 48 h after deployment of such substrates; and (4) density of early juvenile crab in shell and mud habitats over a 4—mo period. We also describe the physical processes likely to be influencing postlarval supply within Grays Harbor, and take advantage of natural variation in postlarval supply between two locations, in combination with a predator exclusion experiment, to define the relative importance of postlarval supply vs. post—settlement survival in regulating population size of juvenile crab in certain intertidal habitats. Water column postlarval supply (measured with plankton and neuston nets, and artificial settlement substrates) in terms of both megalopal density (number per cubic metre) and flux (number per hour) was significantly higher in the southern part of the estuary vs. the northern part during a week—long settlement pulse. Our field observations and measurements suggest that spatial variation in postlarval supply was due to local differneces in wind—driven surface currents, since tidal current speeds in the two locations were similar. More—over, there was no correlation between current speed and flux of megalopae over the bottom. There was generally no difference in postlarval supply between shell and mud habitats. Our experimental results further indicate that: (1) the abundance of recently settled crab megalopae in 0.25 m2 settlement trays was significantly higher in shell than in mud habitats, irrespective of whether the trays were placed in 3—5 ha of shell vs. mud; (2) there was a positive and significant correlation between postlarval supply and density of megalopae in shell and mud habitats; and (3) there was a positive and significant correlation between postlarval supply and density of J1 instars only in habitats where specific predators were excluded. Once the number of J1 instars at both geographical locations was reduced to similar levels, equivalent but steadily decreasing densities persisted throughout the summer growing season. The decoupling of settlement patterns and density of J1 instars took place within our 48—h sampling interval. Thus, future attempts to examine the correspondence between larval supply and post—settlement abundance of marine benthic species with planktonic larvae should do so at extremely small temporal scales or a critical life history phase may be overlooked. The results from this study demonstrate that substrate selection can affect distributi...

The cause and consequence of ontogenetic changes in social aggregation in New Zealand spiny lobsters

“…On rocky shores, for example, crevices shield animals from the crush of waves, resulting in aggregations of bivalves, gastropods, and barnacles. Patches are created on a smaller scale wherever small holes or depressions on rocky substrates offer species such as bryozoans, barnacles, or crabs refugia from fish predators (Keough & Downes 1982, Eggleston & Armstrong 1995). Yet this type of aggregation, dictated by the distribution of habitat and so common among marine benthic invertebrates, is not a consequence of ontogenetic changes in social behavior such as that observed in spiny lobsters.…”

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

“…et al, 1987;Ruiz et al, 1993;Hines and Ruiz, 1995;Smith, 1995, Cancer magister Ž . Fernandez et al, 1993aBotsford and Hobbs, 1995;Eggleston and Armstrong, 1995 , Ž . Ž Hemigrapsus penicillatus Kurihara and Okamoto, 1987 , Chionoecetes opilio Lovrich .…”

“…Roughgarden, 1985;Bertness et al, 1992;Eggleston and Armstrong, 1995 . Among post-settlement processes, predation is a key factor explaining settler or early juvenile Ž .…”

Marine community ecology: importance of trait-mediated effects propagating through complex life cycles

“…In this case there is evidence for an effect of settlement rate on recruitment or on the benthic population. For instance, Eggleston & Armstrong (1995) and Luppi et al (2002) found evidence for strong cannibalism during the early juvenile stages of estuarine crabs. In the crab Cancer magister (Eggleston & Armstrong 1995) initial differences in abundance of megalopae were not reflected in the abundance of juveniles.…”