Dwarf jellies

Tuesday 24 Apr 12
by Line Reeh

Recently discovered population of comb jellies in the Baltic consists entirely of dwarf individuals

The comb jelly, Mertensia ovum, is common in Arctic Oceans, where it reaches body sizes of 10 cm. In a recent article published in The Royal Society’s “Biology Letters” Ph.D. student Cornelia Jaspers and her supervisor Thomas Kiørboe, both DTU Aqua, report that a recently discovered population of Mertensia ovum in the Baltic Sea consists entirely of dwarf individuals not exceeding 2 mm in size. The population is presumably a relict from the Ice age

“We were surprised to find high amounts of ctenophore eggs and larvae during our 13 monthly monitoring cruises in the Baltic, but never observed a single adult. We do not know why these animals are so exceptionally small, but natural selection favors early maturation at a small size when mortality rates are high for example due to high predation pressure, and our observations are consistent with this hypothesis,” explains Cornelia Jaspers.

Despite the absence of adults, eggs were abundant. It is known that ctenophores can reproduce in the larval stage (paedogenesis), and experiments revealed that the larvae were indeed reproductively active. The larvae produced sufficient eggs to maintain a self-sustained population.

This is the first time account of a ctenophore population entirely recruiting through larval reproduction.

“In 1892, it was first described that comb jellies are able to reproduce in the larval phase, but this was questioned by other scientists and remained forgotten for a long time. Our experiments and observations are the first account that a ctenophore population is sustained by larval reproduction,” says Cornelia Jaspers who will be defending her Ph.D.-dissertation at DTU Aqua, National Institute of Aquatic Resources, Denmark, on May 14th 2012.

Further reading:

Cornelia Jaspers, Matilda Haraldsson, Søren Bolte, Thorsten B. H. Reusch, Uffe H. Thygesen and Thomas Kiørboe:
“Ctenophore population recruits entirely through larval reproduction in the central Baltic Sea” (pdf)
Biol. Lett. (2012), 1–4 doi:10.1098/rsbl.2012.0163


Ph.D. student Cornalia Jaspers, DTU Aqua
Phone: + 45 35883482
Mobil: + 45 61314579