New species in Stockholm red mysid shrimp, Hemimysis anomala
This beautiful little red shrimp, Hemamysis anomala, was discovered here at Aquaria in the summer of 2003. A very small species, adults reach between 5,5 mm and 12,5 mm, with females growing slightly larger than males. They are protandrous hermaphrodites (change sex from male to female). Their colour varies from deep red to ivory-yellowish or even transparent, depending on background colour, light and temperature variations.
The species is very adaptable and can tolerate a wide range of salinities, from 18 ppm to freshwater. During the day they swarm close to the bottom or under large rocks and come out at night to disperse in the water column. They are omnivorous, primarily feeding on zooplankton, but will also eat phytoplankton. As juveniles they feed manily on algae. Females carry surprisingly few eggs, on average 10-15, with around 30 being the maximum.
The species orginates in the Black and Caspian Seas and their tributaries. It has probably spread to the Baltic either through the ballast tanks of ships or from Lithuania, where Hemamysis anomala was introduced as a supplementary food source for commercial fish species in the 1960s. At present their effect on the Baltic sea has not been thoroughly studied, but recent reserch in Holland of Hemimysis anomala in freshwater reservoirs have shown that they do have dramatic effect on both zooplankton and phytoplankton variation and abundance.