The Importance of Crabs!!! (Part 2)

I have received a summary of findings on the Hermit Crab survey (see post of July 16th) from our volunteers Tori and Vicki. The hermit crab plays a very important role in Denis Island’s ecosystem and it had been anticipated that we would see a population explosion following the eradication of rats in 2002. This, however, has not materialised and so further research was required in order to understand the population dynamics of the three species we have on Denis - the Tawny hermit crab (Coenobita rugosus), Red hermit crab (C. perlatus) and the Purple hermit crab (C. brevimanus). It is also important for us to have a better understanding of the habitat preferences of the species before we embark on the more extensive vegetation management foreseen under the islands Environmental Management Plan.
Vicki and Tori used quadrat surveys and pitfall traps to monitor the occurrence of hermit crabs in different vegetation types and “capture-recapture” techniques to derive an understanding of the crabs’ population densities.

Crabs were found to be more abundant on the east and southern coasts (beaches and supralittoral zones) and showed a correlation with areas of coastal vegetation dominated by Veloutye (S. sericea) and Beach Gardenia (G. speciosa) – see post of 27th August.

Though active throughout the day hermit crabs are found in greater numbers in the evening and night time. On Denis the purple hermit crab is the least common and is seldom found on the beach preferring more inland habitats such as the supralittoral zones and interior pathways. The tawny and red species were found in diverse habitats with the tawny hermit crab being the most abundant.

In addition to vegetation the research identified what may be the key factor behind the relative lack of large hermit crabs on the island. Larger hermit crabs are typically found in shells of the Turbo genus, while smaller crabs (defined as those with large chela/pincer less than 10mm) mainly utilise the shells of the mollusc Nerita plicata. The beaches on the southern and east coasts are surrounded by reef flats that support large numbers of Nerita and in these locations large populations of small hermit crabs were found. Larger crabs were however comparatively scarce and many of them were found to be in shells that were unsuitable i.e. cone shells or Turbo shells that were far too small –such that all their limbs remained exposed when they attempted to retreat within their shells. This suggests that there is a lack of suitable, larger shells available on the coasts of the island.

GIF focuses on research that has practical management applications; so what are the ramifications of these findings?. The improved understanding of hermit crab habitat preference will enable us to incorporate their needs into vegetation management ensuring the right conditions prevail for this important component of the ecosystem.
The issue of shell availability will be approached in 2 ways:
• Firstly a large portion of the southern beaches are to be managed as “no-take zones” where notice boards will be put up requesting guests and workers not to collect shells from the beach in the hope that shell numbers on the beach will gradually increase with time.
• Secondly, many old semi-fossilised turbo shells can be found inland in the soil and the gardening team have been asked to collect all the old shells they encounter. Once a good number have been collected – say 50 – 100 – they will be cleaned, measured and numbered before being put out in 1 or 2 specific coastal locations. Ongoing monitoring of the population will then show whether the average size of crabs in these areas increases relative to elsewhere around the island and thereby determine whether shells are indeed a limiting factor in the recovery of hermit crab populations on Denis.

Sounds like more work for future volunteers!