Snails living the high life

Nigel Chaffey finds a seasonal epiphyte with a head for heights.

We all know – or should know! – how important and impressive trees are as a life form. But all living things can’t aspire to be a tree. In which case the next best thing in life is to associate yourself with one. This ploy has been discovered and exploited by numerous plant species that cling precariously to branches of trees as they perch high above the forest floor. Such a lifestyle is termed epiphytic and is derived from the Greek words ‘epi’ meaning upon, and ‘phyte’ which means plant.

By definition, the term epiphyte says nothing about the organism that has adopted this lifestyle; it merely specifies the surface/substrate that is occupied. However, common usage appears to have reserved this term for plants that live on the outside of other plants. But – and unusually for him – Mr Cuttings is keen to promote an animal dimension to the tree-dwelling epiphytic phenomenon, specifically that of the land-snail Euhadra brandtii sapporo.

A snail
A snail. Image: Jürgen Schoner / Wikipedia

As a land snail one might assume this mollusc’s normal habitat to be the land, such as the forest floor, not up a tree. Yet, these vertically-aspirational individuals appear to prefer to occupy the bark of trees, and not terra firma. Intrigued by this lignophilic behaviour, Ikuyo Saeki et al. duly investigated this snail’s behaviour further. What did they find?

First, tree-dwelling is not this snail’s lifestyle throughout the entire year; E. b. sapporo hibernates in winter in the ground litter, climbs into the canopy – to heights of 10 metres above the ground – in early spring, and returns to the ground in late autumn. This seasonal migration appears to be an effective means of escaping predation by ground-dwelling carabine beetles (which are known to eat the snails), and whose activity was high during the summer. But, stuck up a tree for such a long time, the snails need a food source to survive.

Chemical analyses suggest that the snails use epiphytic lichens and mosses as food resources. Understandably, the researchers conclude that for E. b. sapporo arboreality has a marked advantage in reducing mortality, and that their tree-climbing behaviour is probably supported by food availability as well. Admittedly, the snails aren’t fixed to the tree’s surface like botanical epiphytes such as mosses, orchids and bromeliads. Which is why this animal behaviour is termed arboreality, and not (zoo) epiphytism. And one suspects it’s the ability to move about that means animals are probably excluded from the term epiphytes, so we don’t need to specify phytoepiphytes for plants that live in this way.

There are, however, many animals that are fixed to their substrate (e.g. benthic organisms in the oceans), so the searching question for you all this month is can you name a true tree-dwelling animal epiphyte? Answers (though – let’s be honest – they’re probably more likely to be wildly overly-optimistic suggestions…) to the usual address, please.

[Ed. – and in some attempt to sate your whetted appetite for other arboreal invertebrates, a new species (and a new genus) of tree-dwelling crab has been described by Appukuttannair Biju Kumar et al.. Kani maranjandu is wholly arboreal – living in tree-hollows or the canopy – in the Western Ghats biodiversity hotspot in Kerala (southern India). The crabs have been observed to feed on leaves, seeds, slugs, worms, and insects on the trees; anecdotally, the crabs are in their turn preyed upon by mongooses and owls. In the interests of some sort of botanical balance, one of the tree species frequented by this newly described life-form is Terminalia paniculata, a useful tropical plant. Presumably, usefully guarded by crabs.]

References

Saeki, I., Niwa, S., Osada, N., Hyodo, F., Ohta, T., Oishi, Y., & Hiura, T. (2017). Adaptive significance of arboreality: field evidence from a tree-climbing land snail. Animal Behaviour, 127, 53–66. https://doi.org/10.1016/j.anbehav.2017.02.022

Kumar, A. B., Raj, S., & Ng, P. K. L. (2017). Description of a new genus and new species of a fully arboreal crab (Decapoda: Brachyura: Gecarcinucidae) from the Western Ghats, India, with notes on the ecology of arboreal crabs. Journal of Crustacean Biology, 37(2), 157–167. https://doi.org/10.1093/jcbiol/rux012