Barnacles are learning to morph their shape to evade sea snails

In the face of rising global temperatures and the shifting borders of marine habitats, a study from the University of Southampton has uncovered a remarkable tale of adaptation within the intertidal zone. The research documents how a species of barnacle, Tetraclita rubescens, commonly found along the Pacific coast of North America, is morphing its physical structure to fend off an emerging threat.

Predatory warm-water sea snails, which are encroaching into their territory as a direct consequence of climate change.

‘Tropicalisation’ and barnacle morphing

As sea-surface temperatures climb, a process described as ‘tropicalisation’ is unfolding, where organisms from subtropical and tropical waters are migrating to regions that were previously temperate. This migration is not just a relocation of species but a potential reconfiguration of the existing ecological hierarchies and food webs.

Barnacles, those hardy, sessile crustaceans that seem to steadfastly anchor themselves against the ebb and flow of the ocean’s whim, are showing they are not as immutable as once thought.

The study zooms in on ‘bent morphs’ of T. rubescens, an adaptation strategy where the barnacles assume a bent shape, camouflaging an opening in their shells that is vulnerable to attack by predatory sea snails. This defensive strategy, while innovative, comes at a cost. Bent morphs experience slower growth and reduced reproductive output compared to their cone-shaped counterparts.

Geographical patterns of adaptation

The research, led by Dr. Phillip Fenberg, reveals a striking geographical pattern in the occurrence of these bent morphs. Predominantly found in the Baja California peninsula of Mexico, a hotspot of tropicalisation, these barnacles display an increased prevalence of the bent shape, presumably in response to the predation pressure exerted by at least three species of warm-water sea snails.

The team’s methodology was comprehensive. They scoured through over a thousand photographs from 30 sites, spanning five years, to analyze the frequency and physical attributes of the bent morphs. The comparative analysis extended to examining how the shifting ranges of predatory sea snails overlapped with the distribution of the barnacles.

Surprise findings from the research

A key finding from the study is the significant difference in the presence of bent morphs depending on the region.

In the Baja California peninsula, these bent morphs made up almost a third of the barnacle population, whereas, in cooler waters further north, such as California, these adaptations were conspicuously absent.

This discrepancy suggests that the adaptive responses of barnacles are complex and influenced by multiple factors, including the nature of the predatory threat and possibly genetic constraints.

Implications of morphing barnacles

Karolina Zarzyczny, a co-author of the study, indicates that the reasons behind the absence of bent morphs in cooler regions could include the ineffectiveness of such a defense against smaller, less aggressive cold-water predators or the lack of genetic capability in these populations to produce such morphs.

The discovery underscores the plasticity of marine organisms in the face of environmental stressors and hints at a broader ecological upheaval as species either adapt, migrate, or face extinction.

The researchers emphasize that this is but a glimpse into the ongoing and future impacts of tropicalisation on marine ecosystems. There is a pressing need for further research to unravel the complex drivers behind these adaptations and the long-term consequences for marine biodiversity and ecosystem function.

As barnacles morph to survive, they serve as a stark reminder of the rapid changes occurring in our oceans and the resilience of life in adapting to the new conditions of our Anthropocene era.

More about barnacles

Barnacles, those crusty, stalwart fixtures of the sea, lead a life that is as fascinating as it is peculiar. These marine organisms, often spotted clinging to rocks, ship hulls, and even whales, belong to a group of crustaceans known as Cirripedia.

Despite their unassuming appearance, barnacles play an essential role in the marine ecosystem and have intriguing biological attributes that are worth exploring.

Barnacle biology

Barnacles start their life as free-swimming larvae, much like their crustacean cousins, the crabs and lobsters. They pass through several larval stages in the plankton before settling down and undergoing a dramatic transformation. Once they find a suitable surface, they affix themselves headfirst using a powerful adhesive and build a hard, calcareous shell around their bodies, where they will spend the rest of their lives.

This shell consists of multiple plates that can open to allow the barnacle to feed and close to protect it from predators and desiccation during low tides. Barnacles feed by extending featherlike appendages known as cirri to sweep tiny particles of food—primarily plankton—from the water into their mouths.

Reproduction and growth

Reproduction in barnacles is a unique process. Most species are hermaphroditic, possessing both male and female reproductive organs. Despite this, they typically cross-fertilize with neighbors. Their long, extendable reproductive organs allow barnacles to reach out to nearby individuals for this purpose.

Once fertilized, the eggs develop into larvae within the shell, eventually being released into the sea to join the ranks of the plankton. Barnacles grow by adding new material to the edges of their shell plates and by molting their inner body parts.

Ecological importance

Barnacles significantly contribute to the marine environment. They serve as a food source for a variety of animals, including birds, fish, and sea stars. Their presence on the backs of whales and shells of turtles adds to the complex habitats these larger animals support.

Moreover, barnacles help scientists as bioindicators to assess the health of marine environments. Their sensitivity to changes in water temperature and chemistry makes them reliable indicators of the ecological effects of climate change.

Barnacle challenges, adaptations, and morphing

Barnacles face numerous challenges, from the threat of predation to the constant battering by waves. They have developed remarkable adaptations to survive these conditions.

For instance, the strength of their adhesive is unparalleled, and their ability to close their plates tightly helps them endure the harsh ebb and flow of the tide. In addition, as discussed above, certain species of barnacles have learned how to morph into different shapes to confuse predators.

Human interactions

Humans have a long history with barnacles, mostly due to their habit of attaching to boats. They can cause significant problems for marine vessels, leading to increased drag and fuel consumption. This phenomenon, known as biofouling, has prompted extensive research into antifouling technologies.

In some cultures, barnacles are also a culinary delicacy, with the gooseneck barnacle being a prized dish in parts of Europe.

Ongoing research and morphing barnacles

Scientists continue to study barnacles, not only to understand their biology and ecology but also to mimic their adhesive properties for biomedical applications. Barnacle glue holds promise for surgical adhesives, a testament to the potential benefits these creatures offer beyond their ecological roles.

In summary, barnacles are not just a fixture of the marine landscape but dynamic organisms with complex lives. They showcase the marvels of adaptation and serve as an integral part of the oceanic world, impacting both the environment and human industries in profound ways.

The full study was published in the journal Biogeography.

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