California Sea Grant College Program

The roles of life history and behavior in the dynamics of dispersal, recruitment and population maintenance were investigated for a dense infaunal polychaete assemblage on the Kendall-Frost mudflat in Mission Bay, California. Polychaete life history features provided the framework for investigations of (1) the relevant scales and mechanisms of dispersal, (2) response to disturbance, and (3) the spatial and temporal predictability of species abundances in the plankton, during recruitment, and in the benthos. Field and laboratory studies of the Kendall-Frost polychaete species Pseudopolydora paucibranchiata Okuda, Polydora ligni Webster, Rhynchospio arenicola Hartman, Streblospio benedicti Webster, Exogone lourei Hartman, Fabricia limnicola Hartman and Capitella capitata Fabricius, revealed many shared life history traits which limit the spatial extent of dispersal. These include annual life cycles, small size, brood protection, small brood size and reduced or facultative planktonic larval stages.

Hydrographic studies and larval plankton distributions support the contention that dispersal is limited. Salinity and temperature gra­dients, and dye, drogue and drift tube trajectories demonstrated slug­gish back-bay circulation. Most larvae released on the Kendall-Frost mudflat remain in the back of Mission Bay, where dispersal is con­strained to small-scale oscillations in back waters or movements over the mudflat itself. Long distance transport between bays is minimal; drift tube releases and spionid larval distributions indicate a 3% pro­bability of transport out to sea for planktotrophic larvae originating on the mudflat. However, larvae which do leave the bay may experience longshore transport 100 km or more to the north or south, particularly during winter storms.

Small-scale dispersal was examined by studying patterns of lar­valavailability, recruitment into settling cartons and colonization of defaunated sediments. Polychaete distributions in the plankton and lar­val recruitment exhibited strong spatial and temporal variation includ­ing seasonal, lunar and diel cycles. Small-scale disturbance, commonly generated on the mudflat by ray foraging and human activities, was stu­died by artificially defaunating small (< 1 m2) sediment patches; a severe storm and consequent raw sewage spill created an episodic large-scale perturbation. Analyses of species' responses revealed colonization ability to be distinct from dispersal ability. Rates and mechanisms of colonization were governed by larval development, settlement and mobility patterns and varied with the scale of perturbation. For Streblospio, Exogone and Fabricia, factors such as brood protection, reduced or flex­ible pelagic larval phases, and post-larval movements, particularly by brooding adults, confer small-scale dispersal abilities which permit rapid colonization of disturbed patches and result in maintenance of high infaunal densities (over 200,000/m2). Pseudopolydora and Polydora possess planktotrophic, long-lived larvae whose planktonic abundances are highly seasonal and variable from year to year. The timing of disturbance must coincide with successful colonization by these species. In general, the annual life cycles and flexible small-scale mobilities of most species confer substantial community resilience.

Within the dense assemblage, intra- and interspecific behavioral interactions appear to mediate spatial pattern and influence the relative success of surface feeding species. The presence of dense Pseudopolydora or Fabricia tubes inhibits recruitment of Pseudopolydora lar­vae. Intraspecific territoriality in Pseudopolydora, manifested as uni­form tube spacing and aggressive palp contact, is initiated during settlement and enhanced during subsequent interaction. Interspecific interference by Pseudopolydora causes withdrawal and loss of foraging time in Streblospio and Fabricia.

These findings demonstrate that interference competition, life histories and dispersal influence population and community structure in Mission Bay and are probably of general ecological and evolutionary significance in most infaunal communities.