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Whale tenants: barnacles offer clues about water flow patterns in humpback whales
Pioneering study compiles 24 years of records to understand attachment patterns of crustaceans on the bodies of filter-feeding whales
The barnacles Coronula diadema are epibionts of humpback whales – Photo: Wikimedia Commons
Humpback whales do not travel alone. Along their migratory routes, they carry passengers. Their tenants, crustaceans known as barnacles, attach to these animals much like oysters attach to rocks.
A study conducted at USP’s Institute of Biosciences (IB) links the size, shape, and position of barnacles on whale bodies to the flow of water passing over the cetaceans. The researchers found that crustaceans of the species Coronula diadema vary across different areas of the same animal’s body.
“Barnacles change shape depending on where they are on the whale”, said Teresa de Filippo, the researcher who is the study’s lead author. The authors point out that the pressures of water circulation can shape the morphology of epibionts — animals that attach to other animals. The greater the turbulence, the smaller the barnacle.
The results were published in the scientific journal Evolutionary Ecology as an outcome of Filippo’s undergraduate research project.
Teresa de Filippo – Photo: LinkedIn
The crustaceans live on the whales’ bodies – Photo: Wikimedia Commons
Where they live
“One of our first hypotheses was that whale behavior would change the barnacle’s shape”, explained Teresa de Filippo. This would suggest that male aggressiveness during the breeding period would determine the shape and robustness of associated crustaceans, at least those that remained attached.
“But that did not happen”, she added. The analyses conducted by the team show no significant differences between barnacles from male and female cetaceans. “Perhaps the behavior is not that different”, said Filippo.
The barnacles studied were collected between 2000 and 2024 from stranded humpback whales along the coast between southern Bahia and Rio Grande do Sul. The researchers conducted morphometric measurements on 136 samples. Each crustacean was photographed and had its geometry analyzed, enabling comparisons of differences in shape and size among individuals.
Barnacles are sessile animals, i.e., fixed in place and hermaphroditic – Photo: Wikimedia Commons
The results show a clear pattern: barnacles attached in the genital region, near the caudal peduncle, are smaller, thicker, and have a wider base. The study suggests that turbulence generated by the tail may be associated with the reduced size and increased robustness of the crustaceans, the more intense swimming motion in the whale’s posterior region promotes the development of more resistant crustaceans.
Those attached to the jaws, pectoral fins, and belly, by contrast, tend to be larger and more elongated. In these regions, barnacles benefit from a gentler and more efficient water flow for nutrient transport, allowing for greater development.
“It is a very interesting result”, said Tammy Arai, Filippo’s supervisor. She highlighted that the discovery of variability among barnacles within the same individual “is an unprecedented finding”. Until now, it was thought that disparities would be greater between different individuals.
Arai noted that the study is pioneering in suggesting an additional component for the study of epibionts in marine animals. “There is no other study that has measured how much hydrodynamics affects barnacles on whales and dolphins”, she said.
The researchers pointed out that barnacles are hermaphroditic, possessing both male and female sexual organs. Therefore, there is no sexual dimorphism: all barnacles are, in principle, similar. Environmental factors, such as feeding and hydrodynamics, shape each individual.
Tammy Arai – Photo: LinkedIn
Humpback whales feed by filtering water – Photo: Wikimedia Commons
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Outside the box
“We need studies to better understand how barnacles manage to adapt to whales”, said Filippo. To that end, she sees the integration of different areas within biology as a solution.
“Animals grow at the intersection. They don’t grow in a little box”, said the biologist. It is not yet possible to definitively determine the causes of why barnacles develop different shapes. For that, studies are needed that encompass different fields. “Why is this happening? Is it because of genetics? Because they are not receiving food properly? Does it have to do with hydrodynamics?” she asked.
The article Phenotypic plasticity in shell morphometrics of the barnacle Coronula diadema (Linnaeus, 1767) reflects cetacean hydrodynamics is available online.
More information: e-mail tedefilippo@usp.br, with Teresa de Filippo, or iwasa-arai@usp.br, with Tammy Arai.
*Intern under the supervision of Fabiana Mariz
**Intern under the supervision of Moisés Dorado
English version: Nexus Traduções, edited by Denis Pacheco
Barnacles change their characteristics depending on their position on the humpback whale’s body – Photo: Wikimedia Commons
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A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.
A reprodução de matérias e fotografias é livre mediante a citação do Jornal da USP e do autor. No caso dos arquivos de áudio, deverão constar dos créditos a Rádio USP e, em sendo explicitados, os autores. Para uso de arquivos de vídeo, esses créditos deverão mencionar a TV USP e, caso estejam explicitados, os autores. Fotos devem ser creditadas como USP Imagens e o nome do fotógrafo.