Milcho Todorov

Shell ultra-structure and morphometrical variability of Nebela tenella were investigated using scanning electron microscopy (SEM). N. tenella was isolated from two widely separated populations, one from Switzerland and another from Canada. The shell’s structural elements were similar to those of the other nebelids, but N. tenella has a characteristic peculiarity – always present depressions on the shell surface, which makes an uneven outline of the shell. Moreover, light microscopy and SEM study showed that the collar of the N. tenella represents a turned-over continuation of the neck, which is covered by the same idiosomes as on the shell body. The biometrical analysis showed that the majority of the basic characters of N. tenella vary moderately and give continuous series of transition forms. According to the shell depth and the ratio depth/width both populations were significantly different of each other. Variation coefficients showed that the variability of the characters differs in both populations and the Swiss population is more stable than the Canadian one. All new obtained data for N. tenella raise the question whether the shell’s size, cross section and ultramorphology are reliable enough as characters for the differentiation of N. tenella and N. griseola, and whether they are two distinct species or should just be considered as ecophenotypic variation within one species?

A new sphagnicolous species of the genus Nebela isolated from wet Sphagnum mosses gathered at Vitosha Mountain (Bulgaria) is described. Shell ultrastructure and morphological variability of Nebela golemanskyi sp. nov. are investigated using scanning electron microscopy. The newly described species is characterized by its almost uncompressed shell and by its large, rounded, voluminous and hollow structural elements (idiosomes). The analysis of the variation coefficients shows that the studied population is too homogeneous and all measured morphological characters are feebly variable (CV range from 3.28% to 5.02%). Size frequency distribution analysis indicates that N. golemanskyi is size-polymorphic species, characterized by not well-expressed main-size class in favour of many subsidiary classes. A biometrical and morphological comparison between N. golemanskyi and N. speciosa is made.