Population Structure
Population characteristics such as size/age class structure, sex ratio, breeding season and fecundity can vary widely across hermit crab species, even amongst those of the same genera (Macphersen and Raventos 2004). Population studies are not uncommon for hermit crabs of the Americas and Europe (Litulo 2005), but are lacking for D. megistos and species of the Great Barrier Reef and Australia in general. Furthermore, population studies have rarely compared populations of a single species from multiple localities, which may vary significantly (Bertness 1981). Such population differences in hermit crabs are suggested to be largely influenced by the community composition and population characteristics of the gastropod molluscs upon which they rely (Bertness 1981). Therefore the following summary of literature in this area should be taken cautiously for application to D. megistos, and particularly so for the Heron Island population specifically. Certainly population analysis of D. megistos and the other hermit crab species occurring at Heron Island would be interesting and novel research.
The sex ratio of a population of hermit crabs may either be bias towards females (Macphersen and Raventos 2004) or close to 1:1 (Macphersen and Raventos 2004, Litulo 2005). However, even where the long term ratio is 1:1, females may be significantly more common in certain months of the year, the case for a recent study concerning a population of Dardanus deformis from Mozambique (Litulo 2005). This recent finding may suggest a misinterpretation of previous research; studies reporting 1:1 ratios may have measured long term averages and likewise studies reporting female bias may have been reporting short term patterns. It may well be that the generalised hermit crab sex structure is typified by monthly variability but long term stability.
Reproduction in hermit crabs may be seasonal or continuous in both tropical and subtropical species (Litulo 2005). In temperate species breeding tends to occur seasonally, over the warmer summer months (Macphersen and Raventos 2004). It is not known whether D. megistos breeds continuously or seasonally, reproduction strategies may even vary over its large range (see Distribution). Reproductive activity may also increase opportunistically following shell availability (Bertnes 1981). In Mozambique, D. deformis breeds continuously, though the number of egg-carrying females varies across months of the year (Litulo 2005). Similar trends have been observed in other species (Mantelatto and Sousa 2000, Turra and Leite 2000, Macpherson and Raventos 2004) and results in a constant supply of larvae (Litulo 2005). This likely plays an important role in the population stabilisation and the determination of structure.
Several studies have observed that juveniles may be relatively uncommon within a hermit crab population (Garcia and Mantelatto 2001, Mantelatto 2001, Macpherson and Raventos 2004, Litulo 2005). This observation has prompted the hypothesis that due to different needs in food and shelter, juveniles are recruited into different habitats than those used by adults. Supporting this theory, populations of juvenile hermit crabs have been found in sea grass beds (Manjon-Cabeza and Garcia-Raso 1998) and likely also occur in algal beds (Fransozo and Mantelatto 1998). However, both juveniles and adults of three sympatric Mediterranean species were recorded from the same habitat by Macphersen and Raventos (2004). Such observations are lacking from the Great Barrier Reef and details of juvenile recruitment for D. megistos are unknown.
D. megistos reportedly occurs from deeper water with smaller individuals not uncommon from the reef flat and crest. It is possible that D. megistos recruits onto the reef flat and then moves into deeper waters as it matures, possibly driven by the search for larger shells. Alternatively it may recruit into the lagoonal areas of reefs like Heron Island or even coastal sea grass and algae beds. Indeed D. megistos has been recorded from both seagrass and algal beds of Fiji, though maturity of specimens was not determined (Alfaro et al. 2009). This sort of information is lacking for Great Barrier Reef hermit crabs in general.
As with many decapod crustaceans (Restrepo and Watson 1991), fecundity in hermit crabs increases exponentially with female size (Liluto 2005). For D. deformis, a single large female with a shield length of 12mm can be expected to carry around 6000 eggs in a single clutch (Litulo 2005). D. megistos reach much greater sizes than D. deformis and so can be predicted to carry more eggs. As well as carrying more eggs, larger females are more likely to be carrying eggs during the breeding season, with the proportion of gravid females of a particular size class approaching 1 as size increases towards the maximum for the given species (Restrepo and Watson 1991). The large range in body size for D. megistos and the (presumed) rarity of the largest individuals within a population makes large and therefore highly fecund females extremely important to the population dynamics.
 |
|
Large shells like this tun being inspected by Dardanus megistos are often the most important limiting resource in a hermit crab population. Photo: Storm Martin, Heron Island, 2012 |
|