07 February 2008

Egg brooding in land snails

ResearchBlogging.orgKuźnik-Kowalska, E., Pokryszko, B.M. (2007). Incipient parental care in Discus - A plesiomorphic state of a truly endodontid character? Journal of Conchology, 39(4), 467-468.

The distribution of the land snail genus Discus, whose shells have wide umbilici, ranges across Asia, Europe and North America. Discus and the North American endemic genus Anguispira are placed in the family Discidae, although some authors may place them in the much larger family Endodontidae of the Pacific Islands, New Zealand and Australia. One interesting behavioral characteristic of the Pacific Island endodontids is the use of the shell's umbilicus as an egg brood chamber (see, for example, Solem & Climo, 1985).

In this paper, Kuźnik-Kowalska and Pokryszko are presenting evidence that 3 European Discus species may also use the umbilici of their shells to transport and brood their eggs. They observed that after a snail lays a batch of 1 to 6 eggs, it crawls over them to cover them with mucus and during that process eggs sometimes enter and stick to the umbilicus of the snail's shell. However, this happened at a rather low rate, about 28% of the time in D. ruderatus and about 7-8% of the time in D. rotundatus and D. perspectivus. Also, in only a few cases were the eggs carried until they hatched. The 2 photographs in the paper are unfortunately of poor quality and do not provide clear details.

The authors suggest that what they observed may be the ancestral (plesiomorphic) state of a character that evolved further in the Pacific endodontids. Until further data become available, it is not clear to me that the described phenomenon in Discus is an adaptive behavior. The outcome rather seems to be an incidental result of the snails' habit of crawling over its own eggs. But then again, that may be how all behavioral adaptations start out.

What would be the adaptive value to a snail the brooding of its eggs in the umbilicus of its shell? The authors suggest that this may protect the eggs against predation, desiccation and cannibalism by their siblings. But egg cannibalism can still take place if more than one egg is present in a snail's umbilicus and the first snail that hatches eats the other eggs. Moreover, egg cannibalism may not be a bad thing at all for the lucky few who get to hatch first. Baur (1992) noted that 67% of the hatchlings of Arianta arbustorum that cannibalized unhatched eggs became adults compared to 38% of those that ate only lettuce.

Forget the salad, I am having eggs for dinner.

Baur, B. 1992. Cannibalism in gastropods. In Elgar, M. A. & Crespi, B. J. (eds.) Cannibalism: Ecology and Evolution among Diverse Taxa. Oxford University Press.
Solem, A. & Climo, F.M. 1985. Malacologia 26:1-30.


David said...

Hi Aydin,

My PhD research includes a couple of pacific endotondids. It seems the general idea (from the few authors who have cared) is that the brooding is an adaptation to protect young from predation. There are now ants on almost every pacific island that still eat the eggs and, as you may know, endontondid extinctions are probably even greater than Partula extinctions in the pacific.

One of the species that's present on the islands I study has taken it even further, eggs hatch in the umbilicus and the young bore their way out through the spire of the shell (the parent's body is sealed off in the body whorl). All the older specimens you find have wholes in the spire.


That's amazing! What if they bore into the parent's shell instead?