The influence of burrowing thalassinid shrimps on the distribution of intertidal seagrasses in Willapa Bay, Washington, USA

Abstract

Two species of seagrasses frequently co-occur with extensive thalassinid shrimp populations and aquaculture operations in the intertidal zone of estuaries along the west coast of North America. Although thalassinid shrimp are known to be strong bioturbators and affect both aquaculture operations and benthic intertidal community structure, few studies have investigated shrimp–seagrass interactions. Application of the pesticide carbaryl to control shrimp populations for oyster aquaculture in Willapa Bay, Washington provided us with an experimental tool to investigate one such interaction. We found that the seagrass Zostera japonica colonized areas where ghost shrimp ( Neotrypaea californiensis) had been removed via carbaryl application. We applied carbaryl to small (900 m 2) experimental plots and compared seagrass colonization on these to that on control plots where shrimp remained abundant (100 m −2). The cumulative proportion of Z. japonica seeds and sprouts was slightly higher in the surface layer of treated plots (presumably due to the lack of shrimp bioturbation distributing them to depth), but seedling abundance was not significantly different between treated (no shrimp) and untreated control plots when they first emerged in early spring. As the season progressed however, and shrimp became more active, fewer seedlings survived in the untreated areas, and those that did survive grew much more slowly than those in the treated plots. We suspect that this was due to the effects of shrimp bioturbation and either light limitation (shoots that survived were much smaller) or direct burial and loss. Although it is an introduced plant, the natural distribution of Z. japonica is high in the intertidal zone and it is often separated from its congener Zostera marina by an expansive sandflat that is dominated by the ghost shrimp N. californiensis in west coast estuaries. The treatment of intertidal oysterbeds with carbaryl clearly reduces abundance of shrimp in this zone and we documented the same pattern of seagrass colonization on a commercial oyster bed and lack of seagrass in an adjacent unsprayed area. Density of native seagrass Z. marina shoots was also enhanced in plots treated with carbaryl, but only at lower tidal elevations or in intertidal pools where it could survive. We believe the removal of shrimp will continue to broaden the distribution of Z. japonica in Washington coastal estuaries where carbaryl use is permitted and add an interesting perspective to this controversial management issue.