Tuesday, May 14, 2013

Egg capsule and larva of the intertidal snail Littorina scutulata

Pictured here is the adult, egg case, and veliger larva of Littorina scutulata. L. scutulata is a small (up to 1.5 cm shell height) gastropod, common in the mid- to high-intertidal of the west coast  of North America (Kozloff and Price 1996, Reid 2007).  The shape of the penis, tentacle pattern, and characteristics of the egg capsule distinguish L. scutulata from its co-occurring sister species, L. plena.

L. scutulata females typically lay transparent egg capsules shaped like a stack of two unequally sized disks. The egg capsules of L. plena females look similar, except that the upper and lower disk are equal in diameter (Hohenlohe 2002). The upper picture shows a side view of the L. scutulata egg capsule, in which the distinctive shape is evident. The lower picture focuses on the embryos, each of which develops in its own membrane (Buckland-Nicks et al. 1972).

The veligers hatch from the egg capsule about a week after egg laying, and measure about 160 micrometers long. They are competent to settle after spending four weeks in the plankton and reaching about 300-360 micrometers in length. The veliger shown here is about 210 microns long. One can see the velum and its two ciliary bands (the more prominent and more anterior prototroch, and the less prominent metatroch). Note also the larva’s paired eyes and statocysts. The statocyst is a balance organ that looks like a round vesicle with a marble inside. 

Reid, D. 2007. Littorina. In: Intertidal Invertebrates from Central California to Oregon. Carlton, James T. ed. University of California Press. Berkeley and Los Angeles, California. 2007.

Kozloff, E. N. and Price, L. H. 1996. Phylum Mollusca: Class Gastropoda. In: Marine Invertebrates of the Pacific Northwest. Kozloff, E.N. ed. University of Washington Press. Seattle and London.

Buckland-Nicks, J., Chia, F-S, and Behrens, S. 1972. Oviposition and development of two intertidal snails, Littorina sitkana and Littorina scutulata. Canadian Journal of Zoology. 51: 359-365

Hohenlohe, P. A. 2002. Life history of Littorina scutulata and Littorina plena, sibling gastropod species with planktonic larvae. Invertebrate Biology. 12 (1): 25-37.

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