The habitats of the Western Australian soldier crab Myctyris accidentalis Unno 2008 (Brachyra: Myctyridae) across its biogeographical range
Perth Royal Society of W.A
Computing, Health and Science
Natural Sciences, Centre for Ecosystem Management
The habitats of the Western Australian soldier crab, Mictyris occidentalis Unno 2008, were examined across its entire biogeographic range. The species transcends a range of environments from tropical, semi-arid to sub-tropical, arid climates, microtidal, macrotidal, to extremely macrotidal ranges, in different regional coastal sectors, and in various large-scale coastal geomorphic units such as barred lagoons, semi-protected beach/dune shores, and tidal creeks and lagoons in deltaic complexes and limestone barrier coasts. Locally, the species inhabits a variety of smaller-scale coastal geomorphic units (or habitats), including point bars, sloping beaches, shore parallel shoals, tidal-delta shoals, tidal-creek shoals, tidal-creek banks, sand flats at low neap-tidal levels to mean sea level, mid- to high-tidal sand flats, and in discrete areas it may occupy sand flats up to the level of high-water spring tide and near the highest astronomical tide. It also inhabits sand flats behind mangroves, and locally may occur on sand flats within mangrove vegetation. The most significant factors determining soldier crab habitats are relatively stable, low to moderate wave-energy environments (so that their subsurface air cavities are not continually disrupted), exposure during low tides so that they can swarm and carry out shallow horizontal tunnelling without tunnel collapse, and sandy substrates comprising fine to medium sand (125 μm to 250 μm modal grain size) that can be pelletised, with generally < 8% mud content, 0.4–3.4% organic carbon content, a water table generally 10–15 (–20) cm below the surface at low tide, sand damp enough during low tide so that the surface can be tunnelled without collapsing, and/or pelletised, i.e., a pellicular water content generally > 16%, groundwater and pellicular water that is less than hypersaline (groundwater salinity of 31–45 ppt, and a pellicular water salinity of 27–44 ppt) and, if located above MSL, a general absence of mangrove or salt marsh roots that would otherwise hinder horizontal tunnelling. Above levels of mean low water neap tide, tidal level is not a major factor – though mostly found in low-tidal environments and above the level of mean low-water neap tide, if there are appropriate damp surfaces and dilution of hypersaline groundwater and pellicular waters by marine water or fresh-water discharge, soldier crabs can inhabit sandy environments from mean low-water neap tide to above mean high-water spring tide, even to levels near the highest astronomical tide. Local stratigraphy under the tidal-flat and coastal zones can determine soldier crab habitat in that it can influence delivery of seawater or fresh water to maintain the habitat moisture levels and salinity. Microtopography within the habitat is important for determining the distribution of the crab at finer scales by providing suitable moisture conditions for feeding, tunnelling and other ichnological constructions.