Pilidial morphotypes

Johannes Müller discovered the pilidium larva in 1847. As it was unrecognizable to him as a nemertean, he assigned the larva the binomen, pilidium gyrans, referring to its hat-like appearance and rotating swimming behavior (Müller 1847). Since then, many different pilidial morphotypes have been described that vary in shape, pigmentation and juvenile morphology and researchers have assigned these unique morphotypes binomena out of tradition. Whether these morphotypes represent species or groups of closely related species has been debated (Dawydoff 1940), but we believe them to represent and characterize groups of closely related species. Below is a list of the common pilidial groups.

A pyramid-like episphere is what defines the morphotype called pilidium pyramidale (above left from Bürger 1895). Often, these pilidia also have spots on their lobes and lappets. Juveniles often develop a caudal cirrus – or tail – and two tiny eyes before they metamorphose.   

As the name suggests, pilidium magnum (Bürger 1895) (above right from Chernyshev 2001) is the morphotype characterized by their large size, particularly that of the episphere, which is large and rounded. Bürger (1895), Dawydoff (1940) and Thorson (1946) indicate larvae of this morphotype bearing a short apical tuft and we have noted that the juvenile often develops off larval center.

You'll note that these morphotypes are quite similar and there is certainly overlap between the two. Using a morphotype designation is simply to help describe the larva and these two morphotypes describe the shape of the episphere whether it is shaped like a pyramid (pilidium pyramidale) or exists as a much larger dome-like vestibule (pilidium magnum).

The amniotic sac surrounding the juvenile nemertean is decorated with polka-dots in the pilidium maculosum morphotype, which was collected by Lacalli in Bamfield, Canada (2005) and described by Hiebert and Maslakova in 2015. The pigment spots are in colors maroon, black and brown and can be seen early in development, surrounding the cephalic discs. The juvenile anterior-posterior axis is perpendicular to the larval axis. 

Pilidium auriculatum, described by Leuckart and Pagenstecher in 1858, characterizes a palaeonemertean genus, Hubrechtella and is characterized by narrow, side-burn like lappets and prominent epidermal cell outlines. These larvae (right from Chernyshev 2001) are shaped like a Roman helmet and the juvenile inside is often situated at an oblique angle to the larval axis. These larvae are relatively long lived in the plankton and the episphere can be rather large, but the lappets are always narrow and small.

Not all pilidia look like hats.  The pilidium recurvatum morphotype looks more like a sock. Atop the heel of the sock is an apical tuft and at the toe, a posterior cirrus. Thus, the anterior-posterior axis of the juvenile nemertean is parallel to that of the larval axis. A funnel surrounded by stiff cirri opens to the larval esophagus leading to the gut.  Although not always present, this larva (left from Fewkes 1883) has a posterior transverse ciliated band or telotroch.

The diverse morphology among pilidiophoran larvae has come full circle as some pilidia more closely resemble direct-developing hoplo- or palaeonemetean larvae than other pilidia. We call these forms modified pilidia and they are characterized by a oblong shape, an apical tuft and lecithotrophy (they do not feed). They do however, still metamorphose catastrophically as other pilidia do and sometimes one can see the developing juvenile nemertean inside the larval body. The axis of the developing juvenile can be parallel to (Schwartz and Norenburg 2005; Schwartz 2009) or opposite (Iwata 1958; Maslakova and Hiebert 2014) (below, right) of the larval axis, where apical tuft marks the anterior. 

Recently, yet another type of modified pilidium, named pilidium nielseni was described (Maslakova and von Dassow 2012). These larvae (below, left) are shaped like barrels with two transverse ciliated bands and they can be recognized by their interrupted swimming behavior. As in other modified pilidia, the developing juvenile can often be seen within the larval body. Pilidium nielseni larvae have a very prominent and blade-like apical tuft and a posterior cirrus either between the ciliated bands or immediately opposite the apical tuft. (see also Maslakova and Hiebert 2014)