New records from the southern North Sea and first records from the Baltic Sea of Kornmannia leptoderma

Abstract

Combined genetic, morphological and ontogenetic observations show that the circumarctic boreal green algal macrophyte Kornmannia leptoderma has expanded its distribution range into the Baltic Sea, on a German coastal section of 220 km length. The species is also again (or still) established at its former extreme southern distribution limit in the North Sea, the German island of Helgoland, where it has not been detected during the last four decades. Macroscopic visible sporophytes of K. leptoderma are nowadays present in the Baltic Sea and at Helgoland from February to September, while they were in the past only detected from February to May at Helgoland. This capacity for formation of sporophytes in summer correlates with the circumstance that K. leptoderma from the Baltic Sea can complete its life cycle at 15°C while several studies conducted decades ago with material from Helgoland and from Pacific coasts consistently reported an inhibition of the algal gametogenesis at temperatures that exceed 12°C. Possibly K. leptoderma has undergone adaptations that facilitate its spread into warmer environments, unless the Kornmannia present in the Baltic Sea and on Helgoland today represents a newly introduced cryptic species.

Cryptic, alien and lost species: molecular diversity ofUlva sensu latoalong the German coasts of the North and Baltic Seas

“…This apparent absence or rarity of G. oxysperma is surprising because the species should be present across the entire Baltic Sea (Schories et al, 2009). Descriptions of G. oxysperma (Rothmaler, 1984;Pankow, 1990) are in complete agreement with the morphology of K. leptoderma in our area (Figs 15-21, see also Weinberger et al, 2018). The two species have very different life cycles (Vinogradova, 1969), but ontogenetic observations are time consuming, and for this reason most historical records of G. oxysperma are probably based on the morphological traits of field-collected material.…”

“…Meanwhile, the genetic-based species identities of the three monostromatic taxa corresponded to known phenotypic traits. For example, M. grevillei was only observed during spring, which was not the case for any other entity; K. leptoderma exhibited a characteristic heteromorphic life cycle, as reported elsewhere (Weinberger et al, 2018); and P. undulatum, despite only being observed once, exhibited a typical morphology (see Supplementary Information). Furthermore, specimens that clustered with Ulva intestinalis mostly exhibited the tubular and unbranched morphology considered characteristic of the species (Kornmann & Sahling, 1977;Rothmaler, 1984;Pankow, 1990), but branched specimens were occasionally observed, as reported previously (Reed & Russell, 1978;Blomster et al, 1998), probably promoted by low salinity (Steinhagen et al, 2018b).…”

“…Furthermore, analysis of historical Gayralia oxysperma vouchers from northern Germany and adjacent areas indicated that all the vouchers were genetically identical to Kornmannia leptoderma, which had until now been considered a relatively rare species that was only present on Helgoland (Kornmann & Sahling, 1983) and, therefore, has not been included in identification keys for other parts of Germany (Rothmaler, 1984;Pankow, 1990) or adjacent areas (Brodie et al, 2007). However, K. leptoderma was present in all three main areas of the present study (see also Weinberger et al, 2018). In striking contrast, G. oxysperma was not observed, even at the type locality of its basionym U. oxysperma Kützing (see the Supplementary Information for a description of the relatively complicated nomenclatural history of G. oxysperma).…”