What is natural? The scale of cryptogenesis in the North Atlantic Ocean

Abstract

Aim Cryptogenic species may include those taxa that were historically introduced and are now falsely viewed as native. Investigated here is the scale of cryptogenesis in the North Atlantic Ocean by examining disjunct distributions, defined as temperate species occurring only on both sides of the North Atlantic. Disjunct distributions can be explained by four scenarios: glacial relicts, taxonomic artefacts, natural trans‐oceanic dispersal and human‐mediated introduction. Location North Atlantic Ocean. Methods Model taxa included ascidians, bivalves and hydrozoans. Biogeographic status (native, introduced or cryptogenic) was assigned to all species exhibiting a disjunct distribution, based upon multiple criteria. Results Of 1030 species, 60 have a strictly disjunct distribution. Of these disjunct species, for five species there is no reason to doubt their native status, and 55 species are cryptogenic or introduced. Groups with high relative dispersal capacities do not have disjunct distributions more often. Infaunal bivalves have the lowest relative number of disjunct species; none are cryptogenic or naturally disjunct. This supports the concept that glaciations are unlikely to cause disjunct distributions: there are no studies that provide conclusive evidence for the glacial relict model. Hydrozoa have the highest relative number of disjunct species, which, while historically explained by undocumented rafting, may more likely be the result of dispersal by ships, which travel relatively fast, are independent of currents and provide greater surface area. Main conclusions This reanalysis of the historical biogeography of the North Atlantic marine biota reveals that far more species may have been introduced than previously recognized, potentially significantly altering our fundamental understanding of community evolution and ecology. Species that have been present for centuries and can be important ecological engineers who have shaped contemporary communities are possibly falsely viewed as native: they may in fact be the unrecognized introductions of historical times.

Species are hypotheses: avoid connectivity assessments based on pillars of sand

“…One of the important outcomes of marine biological invasion studies has been to document biotic homogenization at a global scale with the establishment of 'cosmopolitan' species and cryptogenic species (i.e. species that are neither demonstrably native nor demonstrably introduced as defined by Carlton 1996;Haydar 2012). Interestingly, many of these cosmopolitan invasive species actually appear to be composed of morphologically close taxa forming a species complex.…”

Anthropocene invasion of an ecosystem engineer: resolving the history ofCorophium volutator(Amphipoda: Corophiidae) in the North Atlantic

“…Although natural long‐distance dispersal across the Atlantic is hindered by vast expanses of open water, mid‐Atlantic islands such as Iceland and Greenland provided stepping‐stones between Europe and North America for species capable of larval dispersal or rafting via ocean currents (Ingolfsson, ). A disjunct range (present in Europe and America but not in between) is inconsistent with the stepping‐stone pattern expected to result from natural dispersal, and can be explained by one of four models proposed by Haydar (): (1) persistence in glacial refugia on both coasts with no post‐glacial colonization of intermediate islands; (2) unresolved taxonomy with morphologically similar but genetically distinct species on each coast; (3) natural trans‐oceanic dispersal without the colonization of intermediate islands; and (4) human‐mediated introduction. To our knowledge, no studies have yet found evidence for models 1 or 3 in species with a disjunct distribution in the North Atlantic, suggesting that species with similar distributions may be either introduced or phylogenetically unresolved.…”

Sertularia marginata (Cnidaria: Hydrozoa) in the Mediterranean: an alien species in expansion?

“…Other than the observed disjunctive distribution in the Western and Eastern Mediterranean, it presents a disjunctive amphi-Atlantic distribution that could be correlated with a dispersion by anthropogenic vectors (Haydar, 2012). Indeed, it has been recorded as an introduced species in Hawaii through fouling and ballast water (Kirchenpauer, 1864) in the Gulf of Cádiz and Mediterranean Sea.…”

“…Hydroids are one of the animals within the benthic macrofauna most frequently translocated by human-mediation (Haydar, 2012). They are common components of fouling communities in harbours and ship hulls (Morri & Boero, 1986;Megina et al, 2013), and their life cycles, including encysted phases or ability for reverse development (Boero & Bouillon, 1993;Piraino et al, 2004), make some of them perfect invaders.…”

“…Many species are likely considered as native because their human-mediated geographical movements are unknown (Haydar, 2012;Megina et al, 2013). Only a few hydroid species are usually included on lists of non-indigenous or invasive species (Streftaris et al, 2005;Zenetos et al, 2010), such as Cordylophora caspia (Pallas, 1771), Eudendrium carneum Clarke, 1882 or Clytia hummelincki (Leloup, 1935), which are well known to be invasive species (Gravili et al, 2008;Zenetos et al, 2010;Haydar, 2012).…”