- Researchers first found the mud snails, and the tiny polyps that were embellishing their shells, during a night dive at Hindiya Reef on Farasan Islands, Saudi Arabia, in October 2014.
- The green glow of these polyps is localized around their “mouth”, the team found, making them different from the previously known fluorescent sea hydroids.
- The ecological role of the fluorescence in these polyps however remains unclear, study says.
In the southern Red Sea, scientists have discovered what could be a new species of luminous polyps that light up the mud snails they piggyback on. These polyps or sea hydroids most likely belong to the genus Cytaeis, a team of researchers from Russia and Japan write in a recent study published in PLoS ONE.
The newly discovered sea hydroids are tiny — about 1.5 millimeters in length — and live in colonies on the shells of the miniature Nassa mud snails (Nassarius margaritifer), forming what looks like green “fluorescent flashlights”, scientists say. Like the distantly related freshwater hydra, sea hydroid or polyp is a life stage of a group of small marine predators called Hydrozoa. So far, fluorescence has only been reported from the hydroids of six species, scientists write.
During the day, the Nassa mud snail buries into the sand and comes out only at night to hunt other marine creatures. It was during a night dive at Hindiya Reef on Farasan Islands, Saudi Arabia, in October 2014, that the researchers managed to collect 32 specimens of the mud snails. The tiny polyps were embellishing the shells of the snails.
“The gastropods were on the surface of the sandy bottom, actively burying into the sand in response to the blue light of an underwater flashlight,” the authors write. “The colonies of hydroid polyps were attached to the outer shell surfaces of actively moving N. margaritifer and were observed underwater and aboard the research vessel immediately after collection in petri dishes.”
The green glow of these polyps is localized around their “mouth”, the team found. Other known species of luminous sea hydroids have fluorescence localized on their tentacles or stalk or elsewhere. While species identification of hydroid colonies remains problematic, scientists say that the “species-specific fluorescence patterns may be a useful diagnostic tool in field identification and taxonomy.”
“The fluorescence can be useful for quick identification of hardly recognizable species and for the studies of ecological peculiarities and distribution of hydroids and their hosts – molluscs,” co-author Vyacheslav of the Lomonosov Moscow State University, said in a statement.
The ecological role of the fluorescence in Cytaeis however remains unclear, the researchers say. They speculate, though, that the fluorescence which is localized around the “mouth” of these polyps could be because prey are likely to be attracted to the tentacles and the mouth.
“If this is the case, the fluorescence of polyps could be utilized in sampling and study of nocturnal activity as well as in elucidating the distribution of the hydroids and their hosts,” they write.
Many questions still remain unanswered though. For example, many aspects of the Nassarius snails and Cytaeis hydroids association remains murky, the team says, including the “diurnal/nocturnal activity of the Nassarius snails and Cytaeis hydroids, host specificity, and intra- and interspecies variation of fluorescence of hydroids at different developmental stages.”
“Finally, the physiological state of Cytaeis hydroids and environmental conditions during which fluorescence is observed also require additional study,” they write.
Ivanenko added that many such species with unusual features could be lurking around in the shallow waters of seas, waiting to be discovered.
- Prudkovsky AA, Ivanenko VN, Nikitin MA, Lukyanov KA, Belousova A, Reimer JD, et al. (2016) Green Fluorescence of Cytaeis Hydroids Living in Association with Nassarius Gastropods in the Red Sea. PLoS ONE 11(2): e0146861. doi:10.1371/journal.pone.0146861