Mechanisms causing size differences of the land hermit crab Coenobita rugosus among eco-islands in Southern Taiwan

Abstract

Numerous environmental factors can influence body size. Comparing populations in different ecological contexts is one potential approach to elucidating the most critical of such factors. In the current study, we found that the body size of the land hermit crab Coenobita rugosus was significantly larger on Dongsha Island in the South China Sea than on other eco-islands around Southern Taiwan. We hypothesized that this could be due to differences in (1) shell resources, (2) parasite impact, (3) competition, (4) predation, and (5) food. We found no supporting evidence for the first three hypotheses; the shells used by the hermit crabs on Dongsha were in poorer condition than were those used elsewhere, extremely few individuals in the region had ectoparasites, and the density of hermit crabs varied considerably among localities within each island. However, significantly higher percentages of C. rugosus reached age 3 years on Dongsha than at Siziwan bay in Taiwan. Two growth rate indices inferred from size structures suggested faster growth on Dongsha than at Siziwan. The condition index (i.e., the body mass/shield length ratio of C. rugosus) was also greater on Dongsha than at Siziwan. Therefore, Dongsha hermit crabs seem to have superior diet and growth performance. Seagrass debris accumulation at the shore of Dongsha was considerable, whereas none was observed at Siziwan or on the other islands, where dicot leaves were the dominant food item for the vegetarian hermit crabs. We then experimentally evaluated the possible role of seagrass as food for C. rugosus. The crabs on Dongsha preferred seagrass to dicot leaves, and their growth increment was faster when they fed on seagrass than when they fed on dicot leaves; no such differences were found in the Siziwan hermit crabs. The aforementioned results are compatible with the food hypothesis explaining the size differences among the islands. The predator hypothesis could explain the greater life span but not the other findings. Populations of C. rugosus on islands with seagrass debris piles probably contribute more to the gene pool of the species because higher proportions of these populations could achieve high fecundity. The fate of these terrestrial hermit crabs may rely on the health of underwater seagrass ecosystems that are under threat from global change.

What are the sympatric mechanisms for three species of terrestrial hermit crab (Coenobita rugosus, C. brevimanus, and C. cavipes) in coastal forests?

“…They play major roles in ecosystems and substantially contribute to nutrient cycling and seed dispersal in coastal forests [1, 2]. Terrestrial hermit crabs are scavengers of terrestrial debris and several species may occur sympatrically in coastal forests [3–5]. However, knowledge about sympatry of terrestrial hermit crabs, and information to underpin their conservation and management is lacking.…”

“…violascens , and C . purpureus [5, 7–9], but the latter two species are not abundant enough for inclusion in our study. C .…”

“…If no niche differences occur, the crabs must compete for resources. Although distance from shore and elevation may be independent, because further distance from the sea does not necessarily mean higher elevation [10], our study did not have enough data points to test both distance from shore and elevation separately. In our study, distance from shore in most cases also meant higher elevation.…”

“…In the same habitat, different foraging behaviors or olfactory abilities are sufficient to cause ecological niche differences and ensure the joint survival of multiple species [11]. Although they are scavengers and have seemingly unselective food choices, they exhibit clear preferences [3, 10, 12]. They ingest fruit, fallen leaves, seeds, animal excretions, and animal carcasses [3, 13, 14].…”

Shell occupation by the land hermit crab Coenobita violascens (Anomura, Coenobitidae) from Phuket Island, Thailand

“…The foraging period and other activities of C. violascens presumably occur at different times to reduce competition with other species. Consequently, other sampling methods may be more effective for collecting C. violascens in other habitats such as bait pitfall traps (Morrison and Spiller, 2006;Hsu and Soong, 2017).…”