A familiar shoreline barnacle has been recorded boring into deep-sea sharks and taking nutrients straight from their flesh, representing a total shift from filter feeding to parasitism.
This change represents an evolutionary turning point in a living organism, showing how a conventional marine creature can move into a profoundly different lifestyle.
A transformation in a fjord
Far below the surface of Norway’s Sognefjord, small lantern sharks are now found with yellow, stalk-like growths securely fixed in their skin.
Henrik Glenner of the University of Bergen (UiB) dissected these growths and established that they were barnacles known as Anelasma squalicola. Their altered stalk enters the shark’s tissue and acts as a feeding structure.
Instead of attaching to rocks and filtering plankton from the surrounding water, the species sends root-like filaments into living flesh, obtaining nourishment directly from its host.
The animal still has the overall form of a normal stalked barnacle, making its alteration an unusual evolutionary link that warrants closer study into how such a transition happens.
From rock to shark
Most barnacles start life as free-floating larvae, then fasten themselves to solid surfaces and develop protective shell plates.
In ordinary species, feathery structures known as cirri-jointed feeding limbs that comb plankton from the water-move rhythmically to catch food.
Anelasma squalicola retains these cirri, but they no longer serve a feeding purpose, meaning its implanted stalk provides its only nutrition.
This structural adaptation represents a clear departure from suspension feeding and indicates complete dependence on a living host.
A documented evolutionary connection
A comprehensive phylogenetic study, using DNA to map evolutionary relationships and published in 2014, verified that Anelasma squalicola developed from standard stalked barnacles rather than from established parasites.
The genetic evidence identified it as the nearest living relative of Capitulum mitella, a rock-dwelling species collected as food in East Asia.
This position showed that parasitism emerged from free-living filter feeders, not from lineages that were already parasitic.
The discovery offered a rare instance of a substantial lifestyle change retained in a living animal, instead of being reconstructed solely from fossils.
A deep-water hotspot
Lantern sharks in Sognefjord, including species such as Etmopterus spinax, inhabit depths of roughly 100 to 1,000 metres.
Within these dark waters, scientists have identified unusually large numbers of sharks hosting the barnacle, pointing to a concentrated population.
As these sharks are seldom encountered by people, the scale of the infestation was not recognised until focused sampling commenced.
This concentration suggests that conditions within the fjord may support the parasite’s survival and transmission.
Why here and now
Barnacles have been present for hundreds of millions of years and have left a substantial fossil record charting gradual alterations in shell plates and body forms.
However, sharp changes between feeding methods are rarely witnessed directly in species alive today.
Glenner notes that fossil evidence indicates these transitions can happen quickly on an evolutionary timescale, leaving few signs of intermediate forms.
Seeing this development within a restricted fjord gives scientists an uncommon opportunity to examine how new ecological roles arise.
How Anelasma squalicola feeds
After attachment, the parasite’s stalk grows into branching tissue, increasing its internal surface area within the shark.
These rootlets take up nutrients from nearby tissue and fluids directly, rather than collecting food from the water column.
Microscopic examinations have found that the digestive tract is still present but is frequently empty, supporting the conclusion that blood and tissue provide most of its nourishment.
This internal attachment also holds the barnacle securely in place despite currents and movements by the host.
Risks to sharks
Black lantern sharks in the fjord seem able to live with several parasites simultaneously, at times carrying pairs close to their dorsal fins.
Nevertheless, the penetrating stalk injures tissue and probably redirects energy away from growth and reproduction.
Sharks have escaped parasites of this kind through approximately 400 million years of evolution, which makes this relationship notably recent.
Should the barnacle spread beyond the fjord, deep-water shark populations in other areas could experience comparable pressure.
An uncertain future
Scientists consider two potential futures for this unusual lineage.
The parasite may either fail to extend its range and vanish as an evolutionary experiment, or succeed in establishing itself across other oceans.
Few records beyond Norway indicate that it is still globally uncommon, although isolated instances have been documented near Greenland.
Ongoing monitoring will establish whether this fjord-limited occurrence develops into a broader marine concern.
A living evolutionary moment
Findings from Sognefjord demonstrate that a well-known crustacean can stop filtering plankton and take up a blood-feeding lifestyle along a single evolutionary branch.
Whether this parasite persists or disappears, it provides an unusual view of dramatic biological change occurring in plain sight.
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