Survival in the marine environment is a
daily struggle for a large range of prey item species. The pressures placed
against sea creatures drive them to evolve and adapt skills and various methods
of survival. A species of hermit crab (Eupagurus bernhardus) has formed
a symbiotic relationship with a species of sea anemone (Calliactis
parasitica) as a strategy of perception and protection against
predators. These omnivorous detritivores range from the arctic water of
Iceland, extending south as far as Portugal occupying small rock pools and oceanic waters to
depths of 140 meters; larger individuals tend to be located in deeper waters (Hazlett, 1981).
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| Figure1. Hermit crab Eupagurus bernhardus paired with Calliactis parasitica. Photographer: Anonymous (2004) |
Hermit crabs are a tasty treat to hungry predators such as the
octopus which exhibits a large bird like beak with extreme crushing force that
can pierce the shell of a hermit crabs with ease. Hermit crabs have formed an
unsuspecting mutualistic relationship with the anemones which was first distinguished
by scientists as parasitism, but later corrected to mutualism. The reason this
relationship is successful is the sting of the anemone which deters the octopus
from further attempting to eat the crab. Once the tentacles trap the hermit crab and anemones,
they receive a nasty deterring sting, and the octopus lets go and the crab remains unharmed. These two species are considered
permanent partners once paired, and is uncommon to see them apart from one another (Ross, 1960).
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| Figure2. Hermit crab camouflaged by anemones.Photographer: Anonymous (2006) |
These cunning crabs poses perfected
methods of persuasion; a coaxing tickle is used to release the anemone’s pedal
disc which is used to lock onto substrates and surfaces, (McFarlane, 1969). The crab then strategically places the anemone
onto its shell until it re-adheres ensuring an evenly distributed and balanced
weight is maintained on its shell; it is common for the exterior of the shell
to be covered in multiple anemones for maximised protection and camouflage. As
the hermit crabs increase in size they require larger accommodating shells and
transfer these anemones across to new homes. (Brooks, 1989)
The anemones greatly benefit from this
relationship from the mobility factor allowing access to a larger range of food
sources. The anemones main source of food are scraps from the hermit crab as
they float past. The below video demonstrates the crabs ability to acquire
anemones and its defensive benefits.
References:
BROOKS, W. R. 1989.
Hermit crabs alter sea anemone placement patterns for shell balance and reduced
predation. Journal of Experimental Marine
Biology and Ecology, 132,
109-121.
HAZLETT, B. A. 1981. The behavioral ecology of hermit crabs. Annual Review of Ecology and Systematics, 1-22.
MCFARLANE, I. 1969. Co-ordination of pedal-disk detachment in the
sea anemone Calliactis parasitica. Journal
of Experimental Biology, 51,
387-396.
ROSS, D. 1960. The association between the hermit crab Eupagurus
bernhardus (L.) and the sea anemone Calliactis parasitica (Couch). Proceedings of the Zoological Society of
London. Wiley Online Library, 43 57.
Figure 1. Anonymous (2004). Hermit crab Eupagurus bernhardus paired with Calliactis parasitica. http://wildlife-archipelago.gr/wordpress/marine-inverts/common-hermit-crab-pagurus-bernhardus/, retrieved 10/03/2015.
Figure 2. Anonymous (2006). Hermit crab camouflaged by anemones. www.alamy.com, retreived 10/03/2015.
Figure 1. Anonymous (2004). Hermit crab Eupagurus bernhardus paired with Calliactis parasitica. http://wildlife-archipelago.gr/wordpress/marine-inverts/common-hermit-crab-pagurus-bernhardus/, retrieved 10/03/2015.
Figure 2. Anonymous (2006). Hermit crab camouflaged by anemones. www.alamy.com, retreived 10/03/2015.
A very interesting story to start off your blog. I find mutualisms absolutely fascinating. Do you know if the crab suffers a cost (maybe an energetic one) from carrying too many anemones around? Is there a limit on the number of anemones carried? I am really eager to learn more.
ReplyDeleteHi Tasmin, an article I found stated that the hermit crabs strategically place the anemones for an even distribution and balance of weight and will continue to do so until the exterior of the shell is covered. The hermit crab does suffer a small energetic cost even though they are relatively slow moving to begin with. The energetic trade off has its advantages as it now has protection from large, fast moving predators. The crabs sifts through organic matter on the sea floor to obtain nutrients and does not require a large amount of energy to find its food source. So I think the trade off is largely beneficial for the crab and anemone.
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