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Some barnacles ‘deform’ to protect themselves from predatory warm-water sea snails, which are expanding into their territories due to climate change.
Research led by the University of Southampton and published in the Journal of Biogeography shows how temperate prey species are adapting to changing water temperatures, which bring the threat of warm-water predators encroaching on their territories.
As global sea surface temperatures rise and the number of marine heat waves increases due to global warming, coastal marine communities are changing. Warm-water predators that were restricted to subtropical and tropical areas are now settling in more temperate waters. This phenomenon is known as ‘tropicalization’.
As these predators move from the equator to the poles, temperate prey species, such as the barnacle Tetraclita rubescens, encounter new, larger predators. Researchers wanted to know how this barnacle species might adapt its defenses in response to tropicalization.
In particular, they studied the prevalence of ‘curved morphs’: individual barnacles that morph into a curved shape, concealing an opening in their shells that can be attacked by predatory sea snails (muricides). Curved morphs have slower growth and lower reproduction than the standard conical barnacles, but this may be a necessary trade-off in light of increased predation.
“We found that curved forms of T. rubescens are more common in the southernmost part of its geographic range on Mexico’s Baja California Peninsula, a region that is becoming tropical,” said Dr. Phillip Fenberg, associate professor of ocean and earth sciences. at the University of Southampton and lead author of the study. “In these waters, T. rubescens barnacles are preyed upon by at least three species of warm-water sea snails that are expanding their geographic range as a result of rising sea temperatures. These sea snails have large bodies and some can secrete toxins, making them very effective predators.”
The research team studied 30 locations along the Pacific coast of North America and examined more than a thousand photographs taken between 2017 and 2022. Using these photos, they determined the prevalence of curved shapes, as well as their relative size compared to conical shapes. The team also used published research and field research to figure out how barnacle populations overlap with the range expansion of predatory sea snails in warm waters.
Curved shapes were common only on Mexico’s Baja Peninsula, where tropicalization occurs. Here they constituted an average of 29% of the population. Curved shapes were also significantly smaller than conical barnacles.
In more distant areas (in California, USA) curved morphs were absent, indicating that cold-water predatory sea snails do not provide the same morphing defenses.
Karolina Zarzyczny, co-author of the paper, also from the University of Southampton, explains that there could be several reasons for this: “The curved morph defense could be unnecessary, ineffective or not possible for barnacles in the cooler regions of their geographical distribution area. California We know that T. rubescens is preyed upon by cold-water sea snails, but these snails are smaller than their warm-water relatives and may prefer other species of barnacles that pose less of a challenge.
“The curved shape could also be a less effective defense against cold-water predators, which attack their prey differently. Finally, more northern populations in California may not have the genetic ability to produce curved shapes, but this remains to be determined.”
The research shows that some species may adapt to tropicalization in surprising ways, which is likely to have a significant impact on marine ecosystems, in addition to the direct effects of ocean warming. The researchers say more research is needed to understand the drivers and impacts of tropicalization on marine ecosystems.
Predator-induced defense under tropicalization: a biogeographical approach is published in the Journal of Biogeography and is available online.
The research was funded by The Royal Society, the Natural Environment Research Council (NERC), the Malacological Society of London, the Genetics Society and the Multi-Agency Rocky Intertidal Network (MARINe).
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