Ontogenetic and interspecific valve shape variation in the Pinnatae group of the genus Surirella and the description of S. lacrimula sp. nov.

Abstract

The morphology and ontogenetic allometric trends of several taxa in the Pinnatae group of Surirella were studied using traditional morphometrics, landmark-based shape analysis, and light and electron microscopy. We investigated separately two groups of species: the first consisted of S. brebissonii Krammer & Lange-Bertalot, S. brebissonii var. kuetzingii Krammer & Lange-Bertalot and S. ovalis Brébisson, and the second included specimens originally identified as S. minuta Brébisson and S. pinnata W. Smith. Morphological variability within both groups was mainly limited to differences in valve shape and size. Landmark-based shape analysis revealed several shape groups within both species complexes, although these shape groups were not separated by clear gaps. Additional groups of specimens were separated on the basis of different ontogenetic allometric trajectories. In both species complexes, valve shapes converged at later stages of the vegetative life cycle. Within the ‘S. brebissonii–S. ovalis’ species complex, one shape group corresponded to S. ovalis and another to S. brebissonii+S. brebissonii var. kutzingii. The latter two varieties had similar average shape, but differed in their ontogenetic trajectories. In samples from the USA, only representatives of S. ovalis and S. brebissonii var. kutzingii were found. In the ‘S. minuta–S. pinnata’ species complex, three distinct shape groups were found. One of these is described here as a new species, S. lacrimula English and two others corresponded to S. minuta and S. pinnata. We suggest, therefore, maintaining these two previously synonymized species as separate taxa.