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Annelids constitute a diverse phylum with more than 19,000 species, which exhibit greatly varying morphologies and lifestyles ranging from sessile detritivores to fast swimming active predators. The lifestyle of an animal is closely linked to its sensory systems, not least the visual equipment. Interestingly, many errantian annelid species from different families, such as the scale worms (Polynoidae), have two pairs of eyes on their prostomium. These eyes are typically 100-200 µm in diameter and structurally similar judged from their gross morphology. The polynoids Harmothoe imbricata and Lepidonotus squamatus from the North Atlantic are both benthic predators preying on small invertebrates but only H. imbricata can produce bioluminescence in its scales. Here, we examined the eye morphology, photoreceptor physiology and light-guided behaviour in these two scale worms to assess their visual capacity and visual ecology. The structure and physiology of the two pairs of eyes are remarkably similar within each species, with the only difference being the gaze direction. The photoreceptor physiology, however, differs between species. Both species express a single opsin in their eyes, but in H. imbricata the peak sensitivity is green shifted and the temporal resolution is lower, suggesting that the eyes of H. imbricata are adapted to detect their own bioluminescence. The behavioural experiments showed that both species are strictly night active but yielded no support for the hypothesis that H. imbricata is repelled by its own bioluminescence., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2021. Published by The Company of Biologists Ltd.)

(1): 11 beta,17-Bis(4-bromobenzoyloxy)-9 beta-hydroxy-ent-beyerene, C34H38Br2O5, Mr = 686.5, orthorhombic, P2(1)2(1)2(1), a = 6.1686 (12), b = 14.613 (3), c = 32.935 (7) A, V = 2969 (1) A3, Z = 4, Dx = 1.536 g cm-3, lambda(Mo K alpha) = 0.71069 A, mu = 27.43 cm-1, F(000) = 1408, T = 173 K, R = 0.0347 and wR = 0.0358 for 3064 reflections [Io greater than or equal to 3 sigma(Io)]. (2): 7 beta,17-Bis(4-bromobenzoyloxy)-9 beta-hydroxy-ent-beyerene, C34H38Br2O5, Mr = 686.5, monoclinic, P2(1), a = 12.6424 (15), b = 6.8353 (12), c = 18.496 (2) A, beta = 101.901 (9) degrees, V = 1564.0 (4) A3, Z = 2, D chi = 1.458 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 26.03 cm-1, F(000) = 704, T = 173 A, R = 0.0336 and wR = 0.0361 for 4610 reflections [Io greater than or equal to 3 sigma(Io)]. (3): 17-(4-Bromobenzoyloxy)-9 beta,11 beta-epoxy-7 beta-hydroxy-ent-beyerene, C27H33BrO4, Mr = 501.5, orthorhombic, P2(1)2(1)2(1), a = 6.1315 (7), b = 18.389 (3), c = 21.109 (3) A, V = 2380.0 (6) A3, Z = 4, D chi = 1.399 g cm-3, lambda(Mo K alpha) = 0.71073 A, mu = 17.38 cm-1, F(000) = 1048, T = 198 K, R = 0.0325 and wR = 0.0344 for 2292 reflections [Io greater than or equal to 3 sigma(Io)].(ABSTRACT TRUNCATED AT 250 WORDS)

C18H24O12, Mr = 432.4, monoclinic, Cc, a = 8.996 (3), b = 20.890 (6), c = 11.872 (4) A, beta = 101.11 (2) degrees, V = 2189 (1) A3, Z = 4, Dx = 1.312 g cm-3, Mo K alpha, lambda = 0.71069 A, mu = 1.05 cm-1, F(000) = 912, T = 163 K, R = 0.041, wR = 0.0375 for 2158 reflections (Fo greater than or equal to 6 sigma magnitude of Fo). The ring is in the chair conformation 4C1 with five equatorial groups and one axial group bonded to C(2) as expected. The carbonyl bonds of the acetate groups at positions 2, 4, 5 and 6 are approximately coplanar with their respective ring C-H bonds. However, those at positions 1 and 3 are rotated towards the H(2) atom.

C24H30O4, Mr = 382.5, orthorhombic, P212121, a = 13.091 (2), b = 19.711 (1), c = 8.242 (1) A, V = 2126.7 (5) A3, Z = 4, Dx = 1.195 Mg m-3, lamba (Cu K alpha) = 1.54184 A, mu(Cu K alpha) = 0.56 mm-1, F(000) = 824, T = 295 K. Final R = 0.045 for 2446 unique reflections. The planar A ring is bent relative to the rest of the steroid skeleton. The B ring has a typical chair conformation and the C and D rings assume 13 beta,14 alpha-half-chair and 13 beta-envelope conformations, respectively. The conformation of the progesterone side chain is similar to the conformation observed in other 17 alpha-ester pregnanes: C16-C17-C20-O20 torsion angle -27.4(3) degrees.

C20H20O10, Mr = 420.37, triclinic, P1, a = 8.771 (3), b = 13.508 (3), c = 15.970 (4) A, alpha = 86.04 (2), beta = 81.77 (2), gamma = 84.17 (2) degrees, V = 1860.0 (8) A3, Z = 4 (two molecules per asymmetric unit), Dm (by flotation in ZnCl2/H2O solution at 298 K) = 1.491, Dx = 1.503 g cm-3, Mo K alpha radiation, lambda = 0.71069 A, mu = 1.14 cm-1, F(000) = 880, T = 163 K. Full-matrix least-squares refinement converged to R = 0.039 and wR = 0.037 using 5198 reflections with I greater than 3 sigma (I). The two molecules in the asymmetric unit differ in the torsion angles involving the methoxy groups and in the dihedral angle between the planes of rings B and C, 33.31 (7) and 40.83 (7) degrees in molecules (I) and (II), respectively. This compound belongs to a class of 3,2'-dioxygenated flavonoids whose ultraviolet (UV), mass spectroscopic (MS) and chromatographic properties indicate that in solution these compounds have reduced planarity (and thus reduced conjugation) of the B-ring chromophore with the A, C-ring chromophore.