Abstract
Sponges are key organisms in the marine benthos where they play essential roles in ecological processes such as creating new niches, competition for resources, and organic matter recycling. Despite the increasing number of taxonomical studies, many sponge species remain hidden, whether unnoticed or cryptic. The occurrence of cryptic species may confound ecological studies by underestimating biodiversity. In this study, we monitored photographically growth, fusions, fissions, and survival of two morphologically cryptic species Hemimycale mediterranea Uriz, Garate & Agell, 2017 and H. columella (Bowerbank, 1874). Additionally, we characterized the main environmental factors of the corresponding species habitats, trying to ascertain whether some abiotic factors were correlated with the distribution of these species. Sponge monitoring was performed monthly. Seawater samples were collected the same monitoring days in the vicinity of the target sponges. Results showed contrasting growth and survival patterns for each species: H. mediterranea totally disappeared after larval release while 64% of individuals of H. columella survived the entire two years we monitored. The species also differed in the number of fissions and fusions. These events were evenly distributed throughout the year in the H. mediterranea population but concentrated in cold months in H. columella. No measured environmental factor correlated with H. mediterranea growth rates, while temperature and dissolved organic nitrogen were negatively correlated with H. columella growth rates. The strong differences in depth distribution, survival, growth, fusions, and fissions found between these two cryptic species, highlights the importance of untangling cryptic species before ecological studies are performed in particular when these species share geographical distribution.
Highlights
Sponges are worldwide-distributed invertebrates, inhabiting shallow to abyssal benthic habitats, at all latitudes (Reiswig, 1973; Uriz, Martin & Rosell, 1992; Hooper & Van Soest, 2002)
When we considered the entire period when both species coexisted, the final mean growth rates were significantly higher for H. mediterranea than for H. columella (Newman– Keuls test, F = 13.94, p < 0.001), which approached 0, as growth and shrinkage were compensated along study months
While no one individual of H. mediterranea survived in shallow environments after seven months, ca. 70% of the monitored individuals of H. columella survived in the deep environments during the same period and 64% survived at the end of two years of monitoring (Fig. 3)
Summary
Sponges are worldwide-distributed invertebrates, inhabiting shallow to abyssal benthic habitats, at all latitudes (Reiswig, 1973; Uriz, Martin & Rosell, 1992; Hooper & Van Soest, 2002). They play a primary role in benthic assemblages by interacting in several ways with vegetal and animal neighbors (Wulff, 2006). Waxing and waning growth rates throughout the year have been documented for several encrusting Mediterranean sponges (Garrabou & Zabala, 2001; Blanquer, Uriz & Agell, 2008; De Caralt, Uriz & Wijffels, 2008), such that their ecological success appears to rely on keeping a colonized niche rather than on getting large
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