The larval morphotype of the genus Riserius are characterized by a large esophageal funnel. The funnel can be outstretched as in the first three images or compressed toward the larval body (right). The margin of the funnel bears long stiff cirri. Scale bars 100µm.
Riserius spp.
The pilidium recurvatum morphotype (above) was described as early as 1883 by Walter Fewkes. These larvae have a very distinctive sock-like shape where the esophageal region is a large funnel that can be collapsed toward the larval body while swimming or outstretched at a near right angle to the rest of the larva. Along the funnel margin are long stiff cirri at regular intervals that, when stiffly outstretched, resemble the hair-like projections of a venus fly trap. The larval anterior has a conspicuous apical tuft and the posterior, a cirrus. The juvenile nemertean develops within the toe of the sock and is situated parallel to the anterior posterior axis of the larva (as opposed to the hat-like pilidium where larval and juvenile axis are perpendicular). Larvae like this have been found all around the world from the Bay of Nha Trang in Vietman (Dawydoff 1940), Sea of Japan (Chernyshev 2001), Gullmarfjord, Sweden (Cantell 1966) and the NE Pacific (Schwartz 2009) and we identified this larva as that of the genus Riserius in 2013 (Hiebert et al. 2013). The genus Riserius is monotypic and its only described species, Riserius pugetensis (Norenburg 1993), is interstitial. This genus was previously unrepresented outside of Puget Sound WA (Riserius pugetensis type locality) in the NE Pacific and we found two larvae that represent two new species in this (currently) monotypic genus in Charleston, OR.
Larva of Riserius sp. "1" : Pilidium recurvatum without juvenile eyes. Scale 100µm.
Months collected: July–Oct, Dec
Larva of Riserius sp. "eyes" : Pilidium recurvatum with juvenile eyes. Scale 100µm.
Months collected: Dec
Riserius sp. 1 & "eyes"
Sequence Data: 16S, COI (sp. 1) 16S, COI (sp. "eyes")
The larvae of these two species can only be differentiated in advanced developmental stages, by the presence or absence of eyes in the developing juvenile. Unfortunately, maintaining these larvae in the laboratory is currently a challenge as their diet is unknown. They survive when raw plankton is filtered to 75 µm suggesting that they eat small flagellates (von Dassow personal communication), but raising young individuals to determine one species or the other is not always easy. However, if you are able to obtain individuals at advanced developmental stages, maintaining the juveniles is actually rather straightforward – they will readily ingest another nemertean, Carcinonemertes errans (Hiebert et al. 2013).