Subfamily
Eusyllinae
Eusyllinae presents a great variety of morphologies as it is the second most diverse group within Syllidae (22 genera and approximately 158 species).
DIAGNOSIS
Eusyllis assimilis is a relatively large species, up to 20 mm (60–70 segments). Its head is very characteristic, since it has a very wide prostomium, with frequently divergent palps, without fusing. It has four thick eyes, and its appendages are remarkably long, partially segmented (Fig. 1A). Its chaetae are compound, of bidentate blades, with both teeth of similar size in the anterior chaetae, but with a longer proximal tooth towards the posterior part of the body (Figs. 1B, 1C, 1D). The spines are short, slightly patent. It has two easily identifiable aciculae in the middle area, with a rounded and curved tip (Fig. 1E).
Figure 1. Eusyllis assimilis: A) Dorsal anterior view; B) Medium compound chaetae with a long blade; C) Medium compound chaetae with a short blade; D) Anterior compound chaetae; E) Aciculae. Scales: A, 0.375 mm; B–E, 20 µm. Original work
VARIABILITY
The bodies vary in length and width, as well as their appendages, which can be smooth and long, partly segmented and long, or smooth and short. Coloring can be both uniform or with spots. Its compound chaetae can be setigerous (Fig. 2A), unidentate or bidentate, with short or medium spines, and long or short blades, sometimes very thick. The teeth can have the same or different length, and be more or less open. In some species the proximal tooth is patently curved (Figs. 2B, 2C). Simple chaetae can be uni or bidentate, pointed or truncated, with or without spines (Figs. 2D, 2E). The aciculae are also highly variable, ranging from rounded to low-cut, with a curved tip, tricuspid, or ending in a crown of thorns (Figs. 2F, 2G, 2H, 2I). To access more figures on these morphologies, review other subfamilies.
Figure 2. Detail of the variability of the Eusyllinae family: A) Setigerous compound chaetae; B) Compound chaetae with strongly curved proximal tooth; C) Compound chaetae with a thickened blade; D) Simple bidentate chaetae with spines and the proximal tooth more prominent than the distal; E) Simple truncated chaetae with thorns; F) Tricuspid aciculae; G) Widened aciculae ending in a point; H) I) Aciculae crowned by spines. (1)
Eusyllinae colour image © Arne Nygren, Institutionen för marina vetenskaper, 2022.
(1) San Martín, G. (2003). Museo Nacional de Ciencias Naturales, CSIC, 21.