Nemertean Larval ID

Nemerteans exhibit a wide range of developmental modes from indirect developing pilidium larvae to direct developing larvae that resemble the adult nemertean.  These larval types and associated features are described below.

The name pilidium comes from the Greek word meaning “cap” because the typical pilidium larva is shaped like a hat with earflaps that are pulled down over the ears. This larval form is the most indirect-developing form within the Nemertea. Characteristics of the larval body include a blade like apical tuft which sits atop a dome shaped episphere, two lobes (anterior and posterior) and two lateral lappets (the earflaps). Spanning the lobes and lappets is a conspicuous ciliated band. This band, coupled with muscular contractions of the lobes and lappets, aids in larval feeding (von Dassow et al. 2013). Pilidia are recognizable in the plankton by their unique shape, but also by the way in which they swim. Using their ciliated band, they swim with apical tuft first, usually rotating around their anterior-posterior axis. The movement is distinct in that they swim smoothly, almost drifting, and one often recognizes this first before focusing on their hat-like appearance. What makes this developmental mode indirect is that the juvenile worm has a distinct body plan (similar to that of the adult) and develops within the larval body from a series of isolated rudiments (Salensky 1912; Schmidt 1930; Maslakova 2010a; Hiebert and Maslakova 2015).  Eventually, these rudiments fuse around the larval gut to form a complete worm. The metamorphosis from planktonic larva to benthic juvenile is dramatic as the juvenile worm breaks free and, typically, ingests its larval body (Cantell 1966; Lacalli 2005; Maslakova 2010a). The diagram above shows the development of a typical pilidium larva, but keep in mind they can and do differ dramatically in their morphology and some even resemble direct-developing larvae of hoplo- or palaeonemerteans.

Hoplonemertean and palaeonemertean larvae develop more directly. Although they often have a planktonic larval stage, their larvae are uniformly ciliated, resemble the juvenile, and settle into a benthic habitat without overt conspicuous metamorphosis (unlike in the pilidium larva). Many hoplonemertean larvae are described to resorb or shed their larval epidermis, but this is a subtle transition (e.g. see Hiebert et al. 2010, Maslakova and von Döhren 2009). Hoplonemertean larvae (right) are characterized by an apical tuft at the anterior and a less prominent posterior cirrus. When present, their eyes are sub-epidermal and often occur in several pairs. One can often note the proboscis armed with stylets and conspicuous cerebral ganglia in hoplonemertean larvae. Although hoplonemertean larvae are traditionally considered lecithotrophic (i.e. non-feeding), recent evidence suggests that some species may feed in the plankton (Maslakova and Hiebert 2014) 

Palaeonemertean larvae (right) exhibit a similar shape to hoplonemertean larvae, but when eyes are present - they are within the epidermis. Some palaeonemertean larvae have prominent lateral cirri while, usually, lacking a prominent apical tuft. Palaeonemertean larvae tend to swim in a rotating pattern beginning near the bottom of a sorting dish upward where hoplonemertean larvae are found swimming throughout the dish. Palaeonemertean larvae are known to feed in the plankton on large prey (e.g. larvae of other marine invertebrates), and they have a conspicuous mouth.