Larvae of five species with pilidium nielseni morphotype. The adults have been identified for three of the five species including Micrura sp. "dark" (left), Micrura sp. "albocephala" (third from left), and Cerebratulus longiceps (fourth from left). The remaining species are currently represented by orphan larvae (Micrura sp. 3, 4). Scale bars 100 µm.
Micrura sp. "dark" species complex
The pilidium nielseni morphotype can be separated into groups based on the position of the transverse ciliated bands. There are two species which produce larvae where the anterior-most ciliated band is positioned equatorially (e.g., Micrura sp. "dark") and those where the anterior-most ciliated band is positioned posterior (e.g., Micrura sp. "albocephala").
Micrura sp. "dark"
Observed in Coos Bay: Oct-Jan
The larvae of Micrura sp. "dark" were identified in 2012 (Maslakova and von Dassow) and are recognizable by a thick and blade-like apical tuft. They have two transverse ciliated bands, the anterior band is equatorial and the posterior band is at the larval posterior. Between the two bands is a stiff 'posterior' cirrus. The larval epidermis is covered in spherical lipid granules and a worm can often be observed within the larval body (right). Development from fertilization to metamorphosis is described by Hunt and Maslakova (2017).
The larva of Micrura sp. "dark" exhibits the pilidium nielseni morphotype. Shown here is a larva raised in the lab from an artificially fertilized culture. Note the 'posterior' cirrus at 4 o'clock. Scale bar 100 µm.
The swimming behavior of the larvae of this species is unusual. The cilia beat regularly, but stop intermittently during which time the larva is suspended and somewhat motionless. The larva also constricts along the ciliated bands. Adult Micrura sp. "dark" are collected from amongst Phyllospadix sp. root masses in the rocky intertidal and are ripe in winter months.
This larva was collected from plankton in Dec shown here at two focal planes. Note the blade-like apical tuft (left) and the epidermal lipid granules (right). Scale 100 µm.
Micrura sp. 3
Sequence Data: 16S, COI
Observed in Coos Bay: Dec
We have only found one larva from this species in the plankton and its morphology was similar to Micrura sp. "dark" (above). The anterior ciliated band was positioned equatorially, the larva had a prominent apical tuft but the position of the posterior cirrus is currently not known. We have yet to find the adults.
The oocyte (left), collected from a ripe female in January, and larva (right), collected in December of Micrura sp. "albocephala". Note the position of the posterior cirrus in the larva at 6 o'clock. Scale bars 100 µm.
Micrura sp. "albocephala"
Sequence Data: 16S, COI
Observed in Coos Bay: Dec-Feb
Of the species which produce larvae where the anterior ciliated band is positioned posterior, Micrura sp. "albocephala" is the only larvae for which we've found corresponding adults. Micrura sp. "albocephala" is another currently undescribed nemertean species that produces a larva of the pilidium nielseni morphotype. Lecithotrophic larvae usually develop from larger oocytes and this species is no exception. The eggs of Micrura sp. "albocephala" were collected from a ripe adult in December and they are 200 µm in diameter and surrounded by both a conspicuous chorion and an egg jelly (above left).
Wild-caught larvae of Micrurasp. "albocephala" collected in December. Two focal planes are shown to emphasize the apical tuft, ciliated bands and posterior cirrus (left) and the epidermal yolk granules (right). Scale 100 µm.
Besides the position of the ciliated bands, these larvae are recognizable by a posterior cirrus that is positioned posteriorly (rather than between the two bands). In some cases, the larvae of this species are smaller that the morphologically similar Micrura sp. 4 (below), but this characteristic has yet to be quantified.
Larva of Micrura sp. 4 belongs to an undescribed species for which we have yet to find the adults. Note posterior cirrus at 6 o'clock. Scale 100 µm.
Micrura sp. 4
Sequence Data: 16S, COI
Observed in Coos Bay: Dec-Jan
Like the larvae of Micrura sp. "albocephala", the larval morphology of Micrura sp. 4 exhibits ciliated bands which are posterior and a posterior cirrus that is positioned at the larval posterior (immediately opposite the apical tuft). We have observed the larvae of this species several times in the plankton, however we have yet to find their adults. These larvae are, for the most part, indistinguishable from the larvae of Micrura sp. "albocephala" although they tend to be a bit larger (but this character has not yet been quantified).
The larva of Cerebratulus longiceps, collected from plankton in January shown at two focal places. Note slight red hue near ciliated bands (right). Scale 100 µm.
Cerebratulus longiceps
Sequence Data: 16S, COI
Observed in Coos Bay: Jan
Another species to produce larvae of the pilidium nielseni morphotype with posterior ciliated band has been identified as Cerebratulus longiceps. This larva has only been collected once and it was noted for its reddish color, particularly along the ciliated bands. Although all other species which produce the pilidium nielseni morphotype form a well-supported clade, Cerebratulus longiceps is only distantly related to the other species. Is it possible that this unique morphotype evolved independently in several pilidiophoran lineages?