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Oceanographic barriers to gene flow promote genetic subdivision of the tunicate Ciona intestinalis in a North Sea archipelago

Journal article
Authors Kerstin Johannesson
Anna-Karin Ring
Klara B. Johannesson
Elin Renborg
Per R. Jonsson
Jonathan N. Havenhand
Published in Marine Biology
Volume 165
Issue 8
ISSN 0025-3162
Publication year 2018
Published at Department of marine sciences
Linnaeus Centre for Marine Evolutionary Biology (CEMEB)
Language en
Keywords marine protected areas, population-structure, vertical-distribution, invertebrate larvae, local adaptation, ascidian larvae, spatial scales, pelagic larvae, dispersal, connectivity, Marine & Freshwater Biology
Subject categories Environmental Sciences, Oceanography, Hydrology, Water Resources


Pelagic larval development has the potential to connect populations over large geographic distances and prevent genetic structuring. The solitary tunicate Ciona intestinalis has pelagic eggs and a swimming larval stage lasting for maximum a few days, with the potential for a homogenizing gene flow over relatively large areas. In the eastern North Sea, it is found in a geomorphologically complex archipelago with a mix of fjords and open costal habitats. Here, the coastal waters are also stratified with a marked pycnocline driven by salinity and temperature differences between shallow and deep waters. We investigated the genetic structure of C. intestinalis in this area and compared it with oceanographic barriers to dispersal that would potentially reduce connectivity among local populations. Genetic data from 240 individuals, sampled in 2 shallow, and 4 deep-water sites, showed varying degrees of differentiation among samples (F (ST) = 0.0-0.11). We found no evidence for genetic isolation by distance, but two distant deep-water sites from the open coast were genetically very similar indicating a potential for long-distance gene flow. However, samples from different depths from the same areas were clearly differentiated, and fjord samples were different from open-coast sites. A biophysical model estimating multi-generation, stepping-stone larval connectivity, and empirical data on fjord water mass retention time showed the presence of oceanographic barriers that explained the genetic structure observed. We conclude that the local pattern of oceanographic connectivity will impact on the genetic structure of C. intestinalis in this region.

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