Abstract
The widespread decline of canopy-forming macroalgal assemblages has been documented in many regions during the last decades. This pattern is often followed by the replacement of structurally complex algal canopies by more simplified habitats (e.g., turfs or sea urchin barren grounds). Against all odds, the fucoid Treptacantha elegans, a large Mediterranean brown macroalga, broadened its depth range to deeper and exposed environments and displayed an unexpected range expansion along the northern coast of Catalonia over the last two decades. Here, we reconstruct the spread of T. elegans in time and space and unravel ecological and demographic traits such as population dynamics and genetic patterns to provide a comprehensive and integrated view of the current status and geographical expansion for this species. Fast-growing dynamics, early fertile maturity, and high turnover rate are the main competitive advantages that allow the exposed populations of T. elegans to colonize available substrata and maintain dense and patchy populations. We also provided evidence that the deeper and exposed populations of T. elegans constitute a single group across the Catalan coast, with little genetic differentiation among populations. This seems to support the hypothesis of a unique source of spread in the last decades from the Medes Islands No-Take Zone towards both southern and northern waters.
Highlights
How marine populations persist, evolve, and change their geographical distributions as a response to global change is one of the main questions in contemporary e cology[1,2]
The few studies on population genetics suggest that water currents may play a significant role in long-distance dispersal of Cystoseira sensu lato populations[24,25,26]
The seawater temperature in the north Catalan coast is characterized by strong seasonal variability
Summary
Evolve, and change their geographical distributions as a response to global change is one of the main questions in contemporary e cology[1,2]. In the Mediterranean Sea, Cystoseira sensu lato fucoids (here used to designate Atlantic-Mediterranean fucoid species of Cystoseira sensu stricto, Carpodesmia and Treptacantha14) are late successional species and among the major canopy-forming macroalgae They provide three-dimensional structure, food, and shelter to many associated species[9,15]. Personal communication) have expanded their distribution in the Medes Islands No-Take Zone (NTZ) and the nearby areas (North-Western Mediterranean Sea), where the species can be found in wave-exposed habitats located between 5 and 15 m depth, with some sparse thalli at 20 m depth. The reasons for this pattern are yet to be determined. Our goals are: (1) provide insights about the main traits of deeper and exposed populations of T. elegans and the species population ecology; (2) assess its historical and current distribution in the Catalan coast; and (3) provide an estimate of the patterns of genetic diversity and differentiation among populations including ancient and more recently established zones
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