Here are the polyps: in situ observations of jellyfish polyps and podocysts on bivalve shells.

Most Scyphozoan jellyfish species have a metagenic life cycle involving a benthic, asexually reproducing polyp stage and a sexually reproducing medusa stage. Medusae can be large and conspicuous and most can be identified using morphological characteristics. Polyps on the other hand are small, live a cryptic life attached to hard substrates and often are difficult or impossible to distinguish based on morphology alone. Consequently, for many species the polyp stage has not been identified in the natural environment. We inspected hard substrates in various habitats for the presence of Scyphozoan polyps. Three polyps were found on Dogger Bank, Central North Sea, attached to the inside of the umbo of empty valves of the bivalves Mactra stultorum and Spisula subtruncata. One polyp was accompanied by four podocysts. With this knowledge, the inside of bivalve shells washed ashore in Oostende (Belgium) was inspected and supposed podocysts on the inside of empty valves of Cerastoderma edule and Spisula elliptica were found. Polyps and podocysts were identified to species level by 18S rDNA and mitochondrial COI sequencing. The three polyps found on Dogger Bank all belonged to the compass jellyfish Chrysaora hysoscella. One podocyst from the Oostende beach also belonged to this species but another podocyst belonged to Cyanea lamarkii. These are the first in situ observations of C. hysoscella and C. lamarckii polyps and podocysts in the natural environment. Mactra, Cerastoderma and Spisula species are abundant in many North Sea regions and empty bivalve shells could provide an abundant settling substrate for jellyfish polyps in the North Sea and other areas. Several new strategies to increase the detection of polyps on bivalve shells are presented.

Where are the polyps? Molecular identification, distribution and population differentiation of Aurelia aurita jellyfish polyps in the southern North Sea area.

For many species of metagenic jellyfish the location of the benthic polyps is unknown. To gain insight in the distribution, species composition and population structure of scyphozoan jellyfish polyps in the southern North Sea area, polyp samples were collected from natural and artificial substrates (settling plates, marina floats and wrecks) at ten inshore locations in the Netherlands, seven offshore locations in the North Sea and in the Gullmar Fjord in Sweden. Polyps were identified to species level by sequencing both a fragment of 18S rDNA and a fragment of mitochondrial COI, and comparing these sequences to reference sequences available in GenBank and to newly obtained sequences from medusae collected in the area. All polyps sequenced did belong to Aurelia aurita. For this species, molecular diversity in mitochondrial COI was high, with 50 haplotypes among 183 polyps. Population differentiation was detected between the Dogger Bank and other-more coastal-locations, indicating extremely low connectivity. No significant differences were found between coastal samples. The location of polyps of Cyanea capillata, Cyanea lamarckii, Chrysaora hysoscella and Rhizostoma octopus in the study area remains unresolved.