Scientists have shown that ancient whale migrations have been recorded in the most unusual of places – in the shell plates of the little barnacle hitchhikers which commonly sprinkle the skin of these majestic creatures. These coronulid barnacles act as little time capsules which sequentially record the whales’ geographic location through layered shell deposits which capture isotopic ratios of the surrounding environment.
This is amazing news for whale researchers as migration has long been hypothesized to perform a pivotal role in mysticete (large whales which feed using a filtering mechanism made up of baleen plates) evolution but there are very sparse records of migration in the fossil record – until now.
Coronulid barnacles are a group of commensal barnacles that are found on several mysticete lineages. While many barnacle species attach themselves to rocks or other hard surfaces, whale barnacles enjoy certain advantages such as a safe surface to live on, a free ride to some of the richest waters in the world and a chance to breed with other barnacles when the whales congregate during the mating season.
Coronulid barnacles live enclosed in a shell of six calcite plates and attach themselves by burrowing into the skin of their host whale. They are so effective at securing themselves that they remain attached to the whale throughout the rest of their lives and sometimes the shells remain attached even after the barnacle has died.
They grow a few millimeters each month by depositing new layers of calcium carbonate at the base of their shell. As the oxygen is extracted from the surrounding water and built into the calcite shell, it keeps an accurate record of the oxygen isotope ratios present in the surrounding ocean at the time at which the shell layer is formed.
These isotope ratios can be used to determine the migration routes of baleen whales because each area of the ocean has a unique isotope signature, which is determined by environmental factors such as ocean temperature and isotopic composition. This means that a year’s worth of barnacle shell growth reflects the annual migration of the host whale. By matching the isotope ratios, specifically the δ18O, to the corresponding area of the ocean, researchers can determine where the whales have been and therefore map their migration routes.
While this approach has been used to reconstruct the movement patterns of other species, stable isotope systems usually rely on slow growing tissue, such as tooth enamel, which also has a high preservation potential and is resistant to chemical change. As baleen whales filter-feed, they do not have hard enamel-based teeth or a comparable tissue which is why it has been so hard to reconstruct their ancient migration routes to date.
Barnacle Time Capsules
However, a new technique pioneered by PhD student Larry Taylor, from the Department of Integrative Biology at the University of California, may provide an alternative solution to the problem. Taylor, together with his supervisor and other researchers published an article in 2019 demonstrating that not only do modern barnacle shells accurately reflect the current migration routes of mysticetes, but also that this information is preserved in fossilized coronulid barnacles.
This study specifically looked at current migration routes of humpback and gray whales from the northeast Pacific and their associated barnacle species, Coronula diadema and Cryptolepas rhachianecti respectively. Currently, most modern baleen whales migrate annually from cool, seasonally productive, high-latitude oceans during the summer months before returning to the warm tropical oceans during the winter to breed. The migration routes have already been mapped for these two species: with breeding grounds in the tropical water around Central America, Mexico and Hawaii and feeding areas in the cooler waters in the Gulf of Alaska, Bering Sea and Californian current.
To test the hypothesis, both present and fossilized barnacle shells were collected, and calcite samples were taken sequentially along the growth axis of the barnacle shells. The oxygen isotope ratio was measured using mass spectrometry and matched to corresponding areas in the ocean. The known migration routes were compared to the routes constructed from just the isotope signals present in the barnacle shells and were found to match, demonstrating the accuracy of this method for determining whale migration routes.
Ancient Whale Migrations
Having established the reliability of this method in modern whales, Taylor decided to focus on the humpback whale barnacle (C. diadema) for determining ancient whale migration routes, as their barnacle is more common and has been found from more than a dozen sites worldwide, three of which were used for this study. As coronulid barnacles are species-specific, it is possible to tell which species of whale the fossilized barnacle was attached to, even if it was no longer attached to the whale. These shells were tested for oxygen isotope ratios and compared to those found in modern shells and to shells from a stationary mollusc found in the same fossil bed. The fossilized shells exhibit similar trends and ranges to those found in modern shells, although they are slightly enriched which is to be expected based on the difference in climatic conditions. The fossilized whale barnacles were also found to have larger δ18O ranges than the stationary molluscs, which is also to be expected given that the whale barnacles traveled vast distances and experienced a wider range of oceanic conditions than the stationary molluscs.
One of the main insights of the paper is that whale migrations have been occurring for thousands of years and are fairly similar to those currently undertaken. This is of great interest to many mysticete researchers as selection for migration is thought to have become prevalent during the era. The Plio-Pleistocene is hypothesized to have been a critical turning point in mysticete evolution as species diversity and morphological disparity decreased and gigantism became established in the clade. Scientists believed this is linked to selective pressures for migration as migrating would allow baleen whales to adapt to changing climatic conditions and the increasing patchiness in ocean productivity that was brought about by seasonal upwelling.
Although researchers are particularly interested in past whale migrations and when they became established, this research also has potential to be applicable to modern scenarios too. It is predicted that changes in whale migratory patterns will occur, likely to be linked to climate conditions. If scientists can understand the link between how changing ocean conditions impact migration routes, this may help us predict how these creatures will be affected by climate change.