Spatial and temporal abundance of juvenile hermit crabs 121

121

salinity gradient is more obvious for example, abundance of D. moosai at river mouth was highest in August and October 2010 which coincided with the highest monthly salinity recorded at the station. Abundance of D. moosai also showed significant positive correlation (p<0.05) with oxygen level (Table 3.2.2) which may suggest that this species is less tolerant to lower oxygen level.

Pooled data (northeast monsoon and southwest moonsoon) of hermit crab abundance between seasons at both mudflat and shoal stations showed that D. moosai and D. lopochir at each respective station where they were dominant was significantly more abundant during southwest monsoon (Table 3.2.5 and 3.2.6), a period characterized by lower rainfall. This observation concerned with monthly abundance data showing that the temporal variation in abundance of both species was obvious with peak abundance during the drier months at both mudflat (May, end of June and July 2010) and shoal (January, July and September 2010) stations (Fig. 3.2.4).

122

February 2011 (Fig. 3.2.6) which was about 7 to 8 months after the highest occurence of ovigerous females observed in both species (Fig. 3.5.3). The high recruitment of juveniles during this period results from the high reproductive activity during the onset of southwest monsoon in June and July 2010. However, no study has been done to observe the larval development of D. moosai and D. lopochir and time taken for the larvae to develop into juvenile stage. Garcia & Mantellato (2001) reported continuous recruitment of Pagurus erythrops in Anchieta Island, Brazil with peak recruitment of juvenile being observed six to nine months after peak occurence of ovigerous.

3.2.7.5 Short term variation in abundance and assemblages of hermit crabs

Although previous studies have shown the association between circatidal or circadian rhythms with distributional and activity patterns of hermit crabs (e.g. Bertness, 1981a; Gherardi & Vannini, 1989, 1993, 1994; Barnes, 2001, 2003; Turra & Denadai, 2003; De Grave & Barnes, 2001), this was less obvious for hermit crabs in the present study (e.g. Table 3.2.9). There was no diel variation in hermit crab abundance.

However, the consistent presence of hermit crabs could be due to homogeneity of habitats, which were characterized by narrow variation in environmental conditions.

Further, diel samplings were carried out at the subtidal region of the mudflat and therefore, not subjected to environmental extremities as in the intertidal zone. An interesting observation is that D. lopochir was more abundant at the mudflat during spring flood tide showing that this species could have been tidally transported from the shoal station where they were more abundant. It has been shown that animals may rely on either the ebb or flood tide current as a passive transport mechanism to move them from one location to the other (Tankersley et al., 2002). According to Gherardi &

Vannini (1991), hermit crabs adopt two strategies in space utilization; first by remaining along a narrow belt where they are undirected by the periodic submergence and

123

emergence caused by tides, and second by their reliance upon tides for their daily movements.

The majority of Clibanarius infraspinatus that were caught in mudflat were males (average relative abundance 91.07%). Apart from the hypothesis that the disproportionate sex composition is a result of the interplay between predation pressure and reproductive strategy, it may also suggest that the male of C. infrapsinatus is predominantly migratory investing more energy on locomotion to reach richer and higher quality foraging ground which compensates for the energy spent. In contrast, the female is less mobile and spent more of its energy in egg production. Migratory tendency of male has also been noted by Hunter & Naylor (1993) in a study on intertidal migration of the shore crab, Carcinus maenas. In addition, body size could play a significant role in habitat segregation of hermit crabs in two ways. Firstly, larger crabs (e.g. C. infraspinatus) have higher locomotory capabilities and are capable of faster movement than their smaller conspecifics (e.g. D. moosai), probably a consequence related to their greater muscle development and lever length. Secondly, smaller individuals have a greater surface to volume ratio and are more susceptible to dessication than larger crabs (Gherardi & Vannini, 1993). Thus, in the context of habitat segregation, the occupation of the lower zone by smaller D. moosai may confer lower risk of dessication during ebb tide.

Sparse distribution of hermit crabs at mudflat may present bias due to patchiness and zonation such that the abundance of C. infraspinatus may be underestimated due to its solitary nature especially males (commonly one or two individuals present in a single trawl). C. infraspinatus were sparsely distributed as compared to D. moosai which may have formed aggregated clusters confined to the lower zone of the mudflat and thus,

124

caught in high abundance. Barnes & Arnold (2001) raised such questions of density estimation due to the sparse and clustering nature among different species of hermit crabs. They noted that many species of hermit crabs aggregated, their estimated abundance estimation might be low if species are sparsely distributed over wide subtidal shelf areas.

Literature reviews on the diel activities of hermit crabs have reiterated that activities of hermit crabs are often initiated during submergence where feeding and other social activities ensued (Gherardi & Vannini, 1993). Although consistent patterns of hermit crab density were observed throughout samplings, some variations are distinguishable in the present study. Density peaks were recorded during neap tides (first quarter moon and third quarter moon) during the northeast monsoon, but at the southwest monsoon, density peaks were noted at spring tides (full moon and new moon) (Fig. 3.2.7).

These combined patterns of hermit crabs ingression by season and lunar phase are not clear but could be related to interplay of predation pressure, feeding intensity, reproduction strategy, availability of shells and rainfall. The peaks at northeast monsoon could correspond to the influx of nutrients due to higher rainfall which subsequently increasing concentration of phytoplankton. This condition could have triggered reproduction in hermit crabs including aggregations for mating and release of larvae which is energetically beneficial as food source is abundant. Chew (2011) recorded phytoplankton bloom in the study area in January 2003 during the period of the northeast monsoon. Hermit crab density peaks at night flood of the last quarter moon during northeast monsoon and new moon during southwest monsoon may be related to the decrease in predation pressure as the peaks coincided with low densities of both

125

ariid and sciaenid fishes. Presence of predators can affect the distribution of mobile intertidal prey invertebrates as they seek the safer zone in intertidal areas (Rochette &

Dill, 2000).

The high standard deviations of crab density data in all stations in the present study suggests aggregations and patchy distribution of hermit crabs. Since empty shells are a limiting resource at the study site, shell exchange between similar-sized individual hermit crabs would enhance the probability of achieving appropriate shell fit (see Barnes & Arnold, 2001). However, the frequent shell exchanges among hermit crabs would put them at higher risk of predation (Rotjan et al., 2010). This is plausible since

‘naked’ hermit crabs were observed in the stomach of sciaenid fishes.

Although ovigerous females were present in all moon phases and tidal conditions in both seasons, there are two interesting patterns. Firstly, the percentage of ovigerous females was relatively higher during spring tides (full moon and new moon) compared to neap tides (first quarter moon and third quarter moon) at both seasons.

Secondly, the percentage of ovigerous females was regularly higher at ebb tide. Both patterns could be explained by crab behaviour to release and disperse larvae at the highest and swiftest tide. Hence, D. moosai likely uses the strong ebb tidal current during spring tide to disperse their larvae offshore. It is been known that megalopae of some decapod crustaceans use tidal stream as a mode of passive transport (Gonzales-Gordillo et al., 2003). To do so, females synchronise their larval release to the changing tidal phase; high ebb tide for larvae to drift out and low flood tide for them to be retained (Drake et al., 1998).

126

Estuarine crabs commonly employ two patterns of larval dispersal in which the larvae are either retained in estuary or dispersed elsewhere (Morgan, 1987). According to Chew (2012), decapod larvae (mainly Sergestidae, Brachyura, Diogenidae and Luciferidae) were found to be 3 to 8 times greater at shoal water compared to mangrove waters. This observation suggests that hermit crabs are similar to other common decapods releasing their larvae offshore instead of retaining them inside the estuary.

The advantages of dispersive larval stages in marine benthic invertebrates are the avoidance of competition with adults for resources and higher ability to withstand local extinction (Pechenik, 1999). Therefore, the dispersive ability of hermit crabs is presumed to allow them to colonise environment within the limits of physiological tolerance rendering them ubiquitous along mangrove coastline.