Molecular evidence for the occurrence of ctenophore Mertensia ovum in the northern Baltic Sea and implications for the status of the Mnemiopsis leidyi invasion

Abstract

Nucleotide sequence analysis of 18S ribosomal RNA gene (rRNA), internal transcribed spacer, and 5.8S rRNA was used for taxonomic identification of ctenophores collected in the northern Baltic Sea, where invasive Mnemiopsis leidyi and native Pleurobrachia pileus have been reported to occur. Contrary to previous reports, sequence analysis of 53 randomly selected specimens from seven stations revealed that none of them were M. leidyi or P. pileus. The 18S rRNA and 5.8S rRNA sequences were 100% identical to those of Mertensia ovum, a ctenophore with a broad Arctic and circumboreal distribution, which has never been reported to occur in the Baltic Sea. Polymerase chain reaction screening with primers designed to amplify all three species, and using ctenophores collected by vertically stratified sampling, confirmed that all ctenophores collected in this survey were M. ovum. The ctenophore abundance was high, up to 4500 individuals m 22 , positively correlating with salinity. Our findings emphasize the utility of applying molecular tools to biological surveys and the importance of rigorous species identification. They also indicate that M. leidyi, which is a threat to the southern Baltic ecosystem, does not occur in the northern part of the sea, and call for a pan-Baltic survey to establish current distributions of ctenophores, both native and invasive.

Ctenophore population recruits entirely through larval reproduction in the central Baltic Sea

“…While Arctic specimens measure up to 90 mm, M. ovum in the northern Baltic remain small (less than 6.5 mm) [7,8]. Here, we describe for the first time that a ctenophore population is recruiting solely through larval reproduction (paedogenesis).…”

“…We have no strongly substantiated explanation of this but note that planktivorous fish, potential predators on M. ovum, are abundant in the central Baltic Sea [19], and, more generally, that the relative significance of pelagic versus demersal fish increases from Arctic to temperate and tropical ecosystems [20]. The Baltic relict population of M. ovum [7] has had several thousand years to adapt to the local predation pressure.…”

“…Why does M. ovum population in the central Baltic consists of only larvae that are 1-2 orders of magnitude smaller in length (3 -6 orders in mass) than in the Arctic [7,8]? The temperature -size rule predicts that individual sizes within a species increase with decreasing temperatures, but a temperature difference as observed here of 108C predicts only a 30 per cent difference in individual masses [15].…”

“…In the north, M. ovum individuals are four times larger but their densities are only 1 -8% compared with the central Baltic and they occur primarily at depth greater than 50 m [7]. Since the average residence time of water in the Gulf of Bothnia is 6 years and the southward advection mainly consists of surface water [10], drift recruitment is unlikely to be important.…”

“…Recently, the Arctic cydippid ctenophore Mertensia ovum was discovered in the Baltic Sea possibly as a relict population form the former ice age [7]. While Arctic specimens measure up to 90 mm, M. ovum in the northern Baltic remain small (less than 6.5 mm) [7,8].…”