Rumarcanella Hirose & Mawatari 2011

Rumarcanella Hirose & Mawatari, 2011

The genus Rumarcanella is the first ever to be based primarily on a single feature of the statoblast: the presence of hypertubercles. A hypertubercle may be defined as a small tubercle that sits atop a larger tubercle, generally seen on the floatoblast fenestra, but occasionally also found on the sessoblast frontal valve. Other characteristics of the genus include a colony “ entirely adherent, composed of narrow, branching tubes, weakly chitinized; ectocyst thin, transparent, with almost no encrustation” (Hirose & Mawatari 2011b).

Hypertubercles were first noticed in R. vorstmani (Toriumi, 1952) and R. minuta (Toriumi, 1941), now synonymized under R. himalayana. The list was later expanded to include two new species, R. gusuku Hirose & Mawatari, 2011b and R. yanbaruensis Hirose & Mawatari, 2011b.

This genus is considered problematic due to the likelihood that hypertubercles may not be a reliable unifying feature. For example, the floatoblasts of Plumatella raoi also have hypertubercles, but the colony features free branches, not the “entirely adherent branches specified in the genus description. Therefore, we must either broaden the genus definition, or else recognize that hypertubercles do not occur exclusively in this genus. In fact, they may commonly occur in any species where the statoblast tubercles are especially prominent.

Hirose & Mawatari (2011b) provided genetic evidence that four Rumarcanella species with hypertubercles can be grouped in a single clade within a field that includes common plumatellids and fredericellids. This is useful information. However, it is possible that other species, with similar features but lacking statoblast hypertubercles, may also fall within the clade. Here again molecular studies could prove very useful.

In time the definition of genus Rumarcanella may be modified in ways that place less reliance on hypertubercles. Until then it should remain as an interesting concept, but one that allows certain species with hypertubercles not to be included.