What is driving the range extension of Gibbula umbilicalis (Gastropda, Trochidae) in the eastern English Channel?.

Until around 2000, the marine gastropod Gibbula umbilicalis (Da Costa 1778) was previously only present as far east as the Isle of Wight on the south coast of England. Since 2000 the range of Gibbula umbilicalis has extended 140km eastwards with records now present as far along the coast as Kent. Since the 1990s temperatures in the English Channel have increased faster (and to higher levels) than during any other period recorded. The importance of sea temperature driving the range extension of Gibbula umbilicalis has not been fully evaluated. Field and laboratory studies were carried out between March and September 2015 on populations at two long-established sites in the west of the Channel (Lyme Regis and Osmington) and two recently colonised (since 2000) sites in the east (Cuckmere and Eastbourne) to determine whether: I. reproductive development and spawning was synchronised between different populations, and whether it was correlated to sea temperature; II. there was evidence of recent recruitment at the ―new‖ sites and if there was a difference in population structure at each location;. III. new populations east of the Isle of Wight can now be considered established. Reproductive development was positively correlated to sea temperature, however this was not synchronised between sites; populations at Lyme Regis and Osmington matured before Cuckmere and Eastbourne, although the differences were slight. By September, spawning had begun at the long-established (western) sites but not the recently colonised (eastern) sites. There was evidence of recruitment of a 2014 cohort at all sites. However, the number of juveniles observed was lower in the eastern Channel during spring (March- May) in comparison with western sites, but increased each month from June onwards. The later detection of juveniles in the east is attributed to delayed spawning and settlement. On the evidence of continuing reproductive development and recent recruitment, populations at Cuckmere and Eastbourne can now be considered established. Their proximity to the continent exposes them to colder winters which may intermittently increase mortality. However, it is considered that these populations are now sufficiently large to withstand all but extreme events. The range extension of Gibbula umbilicalis in the eastern English Channel is attributed to multiple factors related to increasing temperatures. Higher summer sea temperatures are likely to have accelerated gonad development and spawning at the previous range limit. This provided juveniles more time to mature before temperatures fell during winter. Increased size/fitness limits winter mortality and promotes recruitment, resulting in growth of the range-edge populations. The larger breeding population resulted in an increase in larval supply, allowing the barrier at the range-limit to be penetrated and new sites seeded. The annual sea temperature increases will have had the same effect on the new arrivals settling in the eastern Channel; recruitment at levels exceeding mortality, allowing the populations to increase and become established.