Unidentified wild-caught Müller’s larva

On April 17, 2013 I took a plankton tow off a dock in Charleston, OR. While sorting plankton I found a Müller’s larva (pictured here). The Müller’s larva is found in free-living marine flatworms from the order Polycladida (class Turbellaria), both in suborder Cotylea and Acotylea (Smith et al 2002). When a Müller’s larva hatches from the egg it usually has 8 lobes, though larvae of some local species have only 6 lobes (e.g. Pesudoceros canadensis). The entire larval body is covered in cilia, but the lobes usually bear longer cilia. Müller’s larvae may be either transparent or opaque and are often brown, mine was greenish. They normally have three eyes to begin with, two subepidermal and associated with the brain and a third, epidermal eye (Smith et al 2002). The two subepidermal eyes can be seen in this picture near the anterior end (upper right). The eye spots often increase in number after metamorphosis (Martín-Durán et al 2012). The larval lobes are reabsorbed during metamorphosis. 

My larva had at least 4 lobes, more likely 6 (two ventro-lateral, two dorsal-lateral, and two unpaired). One can see clearly two of the lobes in focus in these pictures. The lobes are often retracted or flattened when the larva is under the coverglass. Note the dark green branched gut inside the larva. Polyclads, in general, are characterized by a multilobed gut. Many Müller’s larvae apparently require food in order to reach metamorphosis but only a few species have been observed to feed in the laboratory on microscopic algae (Rawlinson 2010 and references therein).

Martín- Durán, J. M. and Egger, B. 2012. Developmental diversity in free-living flatworms. EvoDevo, 3:7.

Rawlinson, K. 2010. Embryonic and post-embryonic development of the polyclad flatworm Maritigrella crozieri; implications for the evolution of spiralian life history traits. Frontiers in Zoology 7: 12.

Smith NF, Johnson KB and Young, CY. 2002. Platyhelminthes. In: Atlas of Marine Invertebrate Larvae. Edited by C. M. Young. Academic Press. New York.