In the vast expanse of our ocean realm, where dominance is often determined by speed and skill, a special group of fast-swimming predators has risen to the top. These apex predators, known as ‘regional endotherms,’ possess unique characteristics that support their fast and predatory lifestyle. These features include elevated body temperatures, centralized red muscles, and thick-walled hearts.
However, within this elite group, there exists a surprising anomaly – the endangered basking shark (Cetorhinus maximus). These gentle giants reach lengths of up to 40 feet (12 meters), making them the second-largest fish in the world. Unlike their fierce counterparts, basking sharks are colossal filter-feeders, consuming tiny planktonic organisms such as copepods and krill. They use specialized gill rakers to filter these organisms from the water. Basking sharks are highly migratory, traveling long distances between feeding grounds. During the summer months, they can often be found near coastal areas where plankton blooms occur. Their slow cruising speeds allow them to efficiently filter-feed while conserving energy. These majestic creatures have become popular in the world of ecotourism, as enthusiasts seek awe-inspiring encounters with these gentle giants.
Despite their ecological importance and growing popularity among tourists, basking sharks face various threats. Historically, they were extensively hunted for their liver oil, fins, and meat, leading to significant population declines. Although hunting has largely ceased, they are still at risk of accidental capture in fishing gear, especially in gillnets and trawls. This unintended capture poses a significant threat to their survival, particularly in areas crucial to their feeding and breeding activities. Additionally, their frequent presence near the water’s surface makes them highly vulnerable to boat strikes, marine pollution, and habitat degradation caused by human activities along the coast. These cumulative threats, combined with their slow reproductive rate, have led to basking sharks being classified as an endangered species. Urgent conservation efforts are necessary to protect these gentle giants and maintain the ecological balance they contribute to in our oceans.
Unlike their fierce counterparts, the basking shark is a filter-feeder. Until recently, it was believed to have the typical anatomy and physiology of fully ectothermic fishes. However, a study by lead author scientist Haley R. Dolton of Trinity College Dublin has shown otherwise. In 2020, Dolton’s team conducted beach dissections on two stranded male basking sharks in England. They discovered unique anatomical features, such as red muscles distributed medially in the trunk and a thick-walled ventricle in the heart. In the following year, hearts were collected from two more stranded basking sharks in Ireland. The examination of their ventricles revealed an unexpected average of about 47% outer compact myocardium, which is commonly associated with highly active regional endotherms.
Further investigation using electronic biologging tags provided even more intriguing insights. The tags allowed for the measurement of subcutaneous white muscle temperatures of free-swimming basking sharks without the need for capture or handling. These measurements consistently showed an elevation of 1.0 to 1.5°C above the ambient water temperature. This discovery is reminiscent of temperature elevations observed in other regional endothermic species but is distinct from fully ectothermic sharks like the whale shark.
Based on these anatomical and physiological findings, the research team believes that basking sharks, despite being planktivorous, challenge the traditional classification of fully ectothermic fishes. The presence of unique characteristics associated with regional endotherms, such as red muscles and elevated subcutaneous body temperatures, may explain their close relationship to other regionally endothermic shark species. These findings may also provide insight into their cruising speeds and trans-oceanic movements. Additionally, these traits likely contribute to their ability to navigate cooler waters while efficiently filter-feeding at relatively fast speeds, overcoming substantial drag.
The newfound understanding of basking shark physiology is highly significant for their management and conservation. The authors state that it “changes our understanding of the physiology of this species and might help improve distribution and population forecasting in the future.” As the enigma of the endangered basking shark is gradually unraveled, the research team is in awe of its remarkable adaptation and surprising endothermic traits. These findings defy traditional categorization and invite us to delve deeper into the wonders that lie beneath the ocean’s surface.
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