Alderia modesta (Loven, 1844)
Krug, P.J. 2007. Poecilogony and larval ecology in the gastropod genus Alderia. American Malacological Bulletin, 23: 99-111.
Krug, P.J., Ellingson, R.A., Burton, R.A. and A. Valdés. 2007. A new poecilogonous species of sea slug (Opisthobranchia: Sacoglossa) from California: Comparison with the planktotrophic congener Alderia modesta (Lovén, 1844). Journal of Molluscan Studies, 73: 29-38.
Ellingson, R.A. and Krug, P.J. 2006. Evolution of poecilogony from planktotrophy: Cryptic speciation, phylogeography and larval development in the gastropod genus Alderia. Evolution 60: 2293-2310.
Krug, P.J. and Zimmer, R.K. 2004. Developmental dimorphism: Consequences for larval behavior and dispersal potential in a marine gastropod. Biological Bulletin 207: 233-246.
Krug, P.J. 2001. Bet-hedging dispersal strategy of a specialist marine herbivore: A settlement dimorphism among sibling larvae of Alderia modesta. Marine Ecology Progress Series 213: 177-192.
Krug, P.J. and Zimmer, R.K. 2000. Developmental dimorphism and expression of chemosensory-mediated behavior: Habitat selection by a specialist marine herbivore. Journal of Experimental Biology 203: 1741-1754.
The species remarkably lacks a pericardium and a heart (Evans, 1953). The cerata, fingerlike dorsal appendages, pulsate on alternating sides of the body to circulate body fluids (Marcus, 1972).
"The flattened body may reach 10 mm in extended length and is pale fawn in colour with blotches of green, brown and white, the whole effect being rather dark and drab. The cerata are arranged dorso-laterally and may total up to 32 per individual, arranged in up to seven rows, each containing up to six cerata per half-row. Regular pulsations of these cerata (visible with a lens) bring about blood-circulation; no heart is present. Through the skin of the cerata and of the foot, the ramifying diverticula of the digestive can be seen. The anal opening is posterodorsal."
Ecology and Distribution
Long-distance dispersal by planktonic larvae keeps distant populations connected by the regular exchange of immigrants (Ellingson and Krug 2006).
One of only two known species in the genus Alderia, the estuarine slug A. modesta has one of the widest geographic ranges of any coastal invertebrate, and a true circumpolar distribution in the Northern Hemisphere. The species occurs on both sides of the Atlantic and Pacific oceans. Eastern Atlantic Populations are known from Norway south to the Atlantic coast of France, while western Atlantic populations occur at least as far south as Massachusetts. The species is abundant in estuaries along the Pacific coast of north America from Alaska to San Francisco Bay, California, and also occurs in Russia and Japan in the western Pacific.
A. modesta lives in the high intertidal zone of salt marshes and brackish estuaries, found feeding on the yellow-green alga Vaucheria. Slugs have exceptional tolerance to osmotic stress and despite normally living in full-strength seawater, can also survive days in nearly freshwater.
Like all sacoglossans, this species is a simultaneous hermaphrodite. Copulation occurs by random hypodermic insemination. Unlike its congener A. willowi, A. modesta is exclusively planktotrophic, producing about 400 eggs per day that develop in 4 days and hatch as feeding veliger larvae with a roughly one-month feeding period prior to metamorphosis.
Evolution and Systematics
This is one of only two known species in the genus Alderia. Molecular data suggests the two Alderia spp. diverged over 4 million years ago, the time of a major marine radiation along the California coast; genetic data and a molecular clock for the Sacoglossa also indicate that eastern Atlantic populations were isolated from Pacific populations about 1.7 million years ago, at the beginning of the Pleistocene ice ages, when lower sea levels and ice cover likely interrupted gene flow via the Arctic ocean.