Consumption of turtle grass seeds and seedlings by crabs in the western Gulf of Mexico

Abstract

Seed consumption by animals can limit reproductive success and recruitment of seagrasses. Consumption of seeds by crustaceans has been reported for several temperate seagrass species, but its prevalence for sub-tropical seagrass species remains unknown. Using laboratory and field feeding experiments, we investigated consumption of seeds and seedlings of the subtropical seagrass species turtle grass Thalassia testudinum along the Texas coast. More turtle grass seedlings were removed from uncaged tethers than caged tethers and time-lapse photography captured a spider crab and pinfish near the tethered seedlings. In laboratory experiments, blue crabs and spider crabs readily consumed 35.0 ± 9.3% and 36.9 ± 6.02% (mean ± SE) of offered turtle grass seedling tissue, respectively, but hermit crabs did not consume seed or seedling tissue. Observations indicate that blue crabs broke open turtle grass fruits and ate the seeds within. Seeds contained 250 and 400% more nitrogen and phosphorus, respectively, than the fruits encasing them. The enhanced nutritional value of turtle grass seeds and seedlings relative to fruit and leaf tissue may be the major driver for the observed patterns in consumption. Seedling growth experiments indicate that consumption by blue crabs severely reduces seedling growth and survival. As in terrestrial ecosystems, propagule consumption by benthic animals could potentially limit seedling survival and recruitment of sub-tropical seagrass species, but the significance of this process is not well understood at this time.