Green Fluorescence of Cytaeis Hydroids Living in Association with Nassarius Gastropods in the Red Sea

Abstract

Green Fluorescent Proteins (GFPs) have been reported from a wide diversity of medusae, but only a few observations of green fluorescence have been reported for hydroid colonies. In this study, we report on fluorescence displayed by hydroid polyps of the genus Cytaeis Eschscholtz, 1829 (Hydrozoa: Anthoathecata: Filifera) found at night time in the southern Red Sea (Saudi Arabia) living on shells of the gastropod Nassarius margaritifer (Dunker, 1847) (Neogastropoda: Buccinoidea: Nassariidae). We examined the fluorescence of these polyps and compare with previously reported data. Intensive green fluorescence with a spectral peak at 518 nm was detected in the hypostome of the Cytaeis polyps, unlike in previous reports that reported fluorescence either in the basal parts of polyps or in other locations on hydroid colonies. These results suggest that fluorescence may be widespread not only in medusae, but also in polyps, and also suggests that the patterns of fluorescence localization can vary in closely related species. The fluorescence of polyps may be potentially useful for field identification of cryptic species and study of geographical distributions of such hydroids and their hosts.

Fig 1. The sampling locality and study area in the Farasan Islands (Saudi Arabia), a complex of islands (indicated in green) in the southern Red Sea (inset).

The red star indicates sampling locality within the Farasan Islands group. (The base geographic layer was downloaded from the Landsat 8 satellite database (http://libra.developmentseed.org, Accessed 13 October 2015), via CC by 3.0 (https://creativecommons.org/licenses/by/3.0/)).

Fig 2. Microscopy imagery of fluorescence of hydroids (Cytaeis sp.) collected in the Saudi Arabian Red Sea.

(A) green fluorescence of the polyps, scale bar 0.3 mm; (B) emission spectrum measured directly from fluorescent part of the polyp (red circle in the inset).

Fig 3. Hydroid polyps of Cytaeis sp. from the Saudi Arabian Red Sea, scale bar 2 mm; (A) fluorescence of living polyps on the shell of the gastropod Nassarius margaritifer; (B) polyps on the shell of a N. margaritifer specimen, scale bar 2 mm; (C) close-up of polyps, scale bar 0.5 mm.

Fig 4. Maximum likelihood tree of 16S mitochondrial rRNA gene sequences from Cytaeis, Podocorynoides, and their closest relatives (BLAST similarity >90%).

P. minima from Mediterranean and New Zealand clusters within other representatives of the family Rathkeidae (Rathkea and Lizzia). The Red Sea Cytaeis sp clusters with the sequence of Cytaeis sp. (EU883541) from Japan and Cytaeis capitata from Indonesia. Schematic images of the polyps of Cytaeis sp. from Japan and the Red Sea indicate location of the fluorescence.