Abstract
The initial discovery in May 2009 of eelgrass (Zostera marina) seeds in fecal samples of wild-caught northern diamondback terrapins (Malaclemys terrapin terrapin) was the first field evidence of eelgrass seed ingestion in this species. This finding suggested the potential of terrapins as seed dispersers in eelgrass beds, which we sampled for two additional years (2010 and 2011). Seeds were only found in feces of terrapins captured prior to June 8 in all three years, coinciding with eelgrass seed maturation and release. Numbers of seeds in terrapin feces varied annually and decreased greatly in 2011 after an eelgrass die off in late 2010. The condition of seeds in terrapin feces was viable-mature, germinated, damaged, or immature. Of terrapins captured during time of seed release, 97% were males and juvenile females, both of which had head widths <30 mm. The fraction of individuals with ingested seeds was 33% for males, 35% for small females, and only 6% for large (mature) females. Probability of seed ingestion decreased exponentially with increasing terrapin head width; only males and small females (head width <30 mm) were likely to be vectors of seed dispersal. The characteristic that diamondback terrapins have well-defined home ranges allowed us to estimate the number of terrapins potentially dispersing eelgrass seeds annually. In seagrass beds of the Goodwin Islands region (lower York River, Virginia), there were 559 to 799 terrapins, which could disperse between 1,341 and 1,677 eelgrass seeds annually. These would represent a small proportion of total seed production within a single seagrass bed. However, based on probable home range distances, terrapins can easily traverse eelgrass meadow boundaries, thereby dispersing seeds beyond the bed of origin. Given the relatively short dispersion distance of eelgrass seeds, the diamondback terrapin may be a major source of inter-bed seed dispersal and genetic diversity.
Highlights
Plants rely on abiotic and biotic processes by which to transport their seeds to suitable habitat [1]
Captured terrapins were marked with a unique turtle identification number (TID) etched into marginal scutes along the right side of the carapace [23] plus one additional notch made in the second to the left, rear marginal scute to distinguish these captures from previous collections
The highest occurrence of ingested eelgrass seeds was in small terrapins (HW,30 mm) of both sexes: 33% of small males and 35% of small females with little interannual variation (Table 1)
Summary
Plants rely on abiotic and biotic processes by which to transport their seeds to suitable habitat [1]. Biotic dispersal relies on other organisms to move seeds to new locations, often further than abiotic processes can achieve [1]. Seed acquisition and transport can be realized through active or passive involvement of the organism [1]. Seed ingestion is a passive mode of dispersal and can result from mutualism between plants and animals [1]. Effective biotic dispersal can be critical to a plant’s reproductive success [2] and depends on the number of seeds consumed and egested, as well as the probability that a dispersed seed will germinate in the new habitat [2]
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