Abstract
The tropical red alga Womersleyella setacea (Rhodomelaceae, Rhodophyta) is causing increasing concern in the Mediterranean Sea because of its invasive behavior. After its introduction it has colonized most Mediterranean areas, but the mechanism underlying its acclimatization and invasion process remains unknown. To understand this process, we decided i) to assess in situ the seasonal biomass and phenological patterns of populations inhabiting the Mediterranean Sea in relation to the main environmental factors, and ii) to experimentally determine if the tolerance of W. setacea to different light and temperature conditions can explain its colonization success, as well as its bathymetric distribution range. The bathymetric distribution, biomass, and phenology of W. setacea were studied at two localities, and related to irradiance and temperature values recorded in situ. Laboratory experiments were set up to study survival, growth and reproduction under contrasting light and temperature conditions in the short, mid, and long term.Results showed that, in the studied area, the bathymetric distribution of W. setacea is restricted to a depth belt between 25 and 40 m deep, reaching maximum biomass values (126 g dw m−2) at 30 m depth. In concordance, although in the short term W. setacea survived and grew in a large range of environmental conditions, its life requirements for the mid and long term were dim light levels and low temperatures. Biomass of Womersleyella setacea did not show any clear seasonal pattern, though minimum values were reported in spring. Reproductive structures were always absent. Bearing in mind that no herbivores feed on Womersleyella setacea and that its thermal preferences are more characteristic of temperate than of tropical seaweeds, low light (50 µmol photon m−2 s−1) and low temperature (12°C) levels are critical for W. setacea survival and growth, thus probably determining its spread and bathymetric distribution across the Mediterranean Sea.
Highlights
The spread of non-indigenous species is claimed to cause dramatic ecological impacts and is considered a major threat for biodiversity conservation [1]
Bathymetric distribution Womersleyella setacea displayed the highest quantitative dominance between 30 and 35 m depth, with a maximum coverage at 30 m depth, both at Imbutu and Cala Solana, while it was absent at depths shallower than 20 m (Fig. 2)
We observed high values of W. setacea biomass, which are of the same order of magnitude as those recorded in other Mediterranean areas [5]
Summary
The spread of non-indigenous species is claimed to cause dramatic ecological impacts and is considered a major threat for biodiversity conservation [1]. Womersleyella setacea was described originally from the Hawaiian Islands [16] and later reported for other tropical localities both of the Pacific and Atlantic oceans [17]. It was first observed in Mediterranean coastal waters in the eighties in the Var region, France [18] and in Italy [19], and rapidly increased its distribution throughout Mediterranean waters: Corsica, Mediterranean coasts of Spain, the Balearic Islands, the Adriatic Sea, Malta and Greece [17]. The origin and way of introduction of this species remain unknown, but a suggested vector is ship hull fouling [20]
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