Micro-habitats and Associations

Microhabitat preference of Eriphia sebana: are local distributions driven by microhabitat characteristics?

Introduction

E. sebana are a widespread species of marine crab, and are understood to inhabit coral reef flats and the rocky shores of reefs. This study was conducted to better understand the relationship between microhabitat characteristics of the rocky shore and the distribution of local populations of E. sebana relative to these characteristics. Developing a better knowledge of this relationship will provide insight into the microhabitat ecology of a fairly typical marine crab, leading to a better understanding of ecosystem processes that drive local distributions. This type of understanding will aid in recognising habitats of E. sebana, and predicting abundances and distributions based on characteristics of these habitats. In potential future conservation situations, this type of knowledge regarding habitat-species interactions could prove vital.

Methods

Rocky shoreline habitat was characterised during daylight hours, distinguished into sections recorded as flat rock (F), elevated rock (E), and extra elevated rock (EE). These categories described the texture of the substrate, and could easily be recognised in situ. Flat rock (F) sections described sections of the substrate that were flat, with no jutting rocks or elevated sections. Elevated rock (E) described sections with uneven surfaces, jagged rocks and elevation of the substrate. Extra elevated (EE) rock comprised flat areas that had extra surfaces of rock doubled on top, often creating overhanging sections of rock between the foundation flat layer and the extended elevated section sitting on top. Transects of 20x2metres were conducted at random intervals along the rocky substrate section; 75% of each transect had to be dominated by a substrate type to be categorized. Transects were conducted in sets of three at random intervals along the habitat; at each interval a transect was conducted adjacent to the water line, halfway between the water line and the top of the rocky substrate, and at the uppermost section of the rocky substrate. Abundance of E. sebana individuals were counted along each transect. During each transect, an assessment was made of the level of cracks or potential hiding places for E. sebana within the substrate. Levels ranged from 1-3, with 1 being defined as having one or no cracks or hiding places per 2m² within the transect; 3 was defined as  containing multiple hiding places per 2m², often within larger jagged rocks or deep cracklines; 2 was intermediate level between 1 and 3. 75% of each transect had to be dominated by a hiding place level to be categorized.

Results

Results indicate that the greatest abundance of E. sebana inhabit the area of the rocky shore zone closest to the water line (Table 1).

Table 1: Average abundance of E. sebana per 20x2m transect with respect to perpendicular distance from water line.

In relation to microhabitat characteristics, the greatest abundance of E. sebana was recorded along sections of flat rock (Table 2).

Discussion

Results indicate a trend of greater abundance towards the shore line within the local population of E. sebana. This result suggests a preference of microhabitat that interacts heavily with the tide, being submerged for extended periods of time. This result was largely expected, as the type of microhabitat immediately represents a more appropriate niche for crabs, which are heavily dependent on water for survival. A more surprising result was the interaction between crab abundance and the substrate characteristics. Both defining characters of the substrate, the micro-topography of the rock and the level of cracks/hiding places, exhibited greatest abundances of E. sebana at areas the most flat and areas of the least level of hiding places. This result is contrary to expectation, as the belief was that crabs would prefer elevated rocks that generally provide more cracks and hiding areas that would accommodate for the cryptic hiding strategies of crabs. The preference for flat rocks exhibited by the local population on all three nights could be a result of the nocturnal life strategy of E. sebana. They are most active at night, generally using this time to find sources of food; flat rocks may represent better hunting grounds for the crabs. In situ observations of the faunal abundance throughout the habitat suggests this is not the case, as the only other organisms were recorded in the more elevated sections of rock and within cracks. The greater abundance of crabs in areas with fewer cracks could also be a result of the timing of the search. Again, the more flat areas with less natural obstacles such as cracks and jagged hiding places may represent a better hunting area for E. sebana, of which would likely have been active during the search period. More research on the feeding strategies of E. sebana is needed to determine if this activity is reason for the microhabitat preferences.