Because it makes them better hunters, scalloped hammerhead sharks ( Sphyrna lewini ) have evolved a unique method to avoid losing body heat when they dive for prey in deep, cold waters: they close their gills.

Numerous fish and marine-mammal species are known to dive from the warm surface to deeper waters to hunt. However, ectothermic, or ‘cold-blooded’, animals face the challenge of how to conserve their body temperature to keep their metabolism active enough for hunting when the surrounding water can be just a few degrees above freezing.

“The most rapid point of heat loss for any fish, even a high-performance fish, is always at the gills,” says Mark Royer, a postdoctoral researcher in shark physiology and behaviour at the Hawaiʻi Institute of Marine Biology in Kaneohe, part of the University of Hawaiʻi. Because of the high volume of warm blood flowing through the gills, they are “essentially just giant radiators strapped to your head”, he says.

Some fish, such as the whale shark ( Rhincodon typus ), are able to conserve their body heat when diving through sheer size. Others, such as tuna, marlin and the family that includes great white ( Carcharodon carcharias ) and mako sharks ( Isurus oxyrinchus ), have evolved specialized heat-exchange systems at the gills that avoid too much body heat being lost.

The scalloped hammerhead has neither of these advantages or adaptations, yet has been tracked doing rapid, repeated dives to depths of around 800 metres.

To understand how sharks were coping with the temperature changes, Royer and his colleagues developed a device consisting of instruments that measured depth, water temperature, location and movement, as well as a probe embedded into muscles near the dorsal fin that recorded the shark’s core temperature. The device was designed to break off after several weeks, float to the surface and send out a signal to enable its recovery.

Three scalloped hammerhead sharks captured off the Hawaiian coast were tagged with the device.

In a paper published in Science ¹ , the team reported that the sharks would dive several times — six in an evening, for one shark — into deep water at temperatures of 5–11 °C, around 20 °C colder than at the surface, and remain there for 5–7 minutes at a time before surfacing.

Body temperature remained constant for most of the dive until the final stage of their ascent back to warmer waters, when it would decline rapidly.

Keeping warm

Royer suggests that the sharks are keeping their core temperature stable by simply not opening their gills or mouth during the dive; effectively ‘holding their breath’. “If you don’t have water going over your gills, then you won’t be dumping your body heat into the environment,” he says.

The drop in body temperature near the surface coincides with the sharks’ steep ascent flattening out slightly, which Royer suggests is the moment they start to allow water to flow over the gills. “They can slow themselves down, open their gills and start breathing again [because] the water that they’re in, it’s not as cold as it is at the bottom,” he says.

To shut down oxygen intake in this way suggests that the scalloped hammerhead must be able to deal with plummeting blood-oxygen levels during dives, says Mark Meekan, a fish ecologist at the University of Western Australia Oceans Institute in Perth, although the mechanism is yet to be discovered. “What they could be doing is slowing the heart muscle, slowing the pumping of blood around the body,” Meekan says. The shark’s tissues and blood could have evolved to hold more oxygen per unit of volume — akin to the adaptation seen in people who live at altitude — or might be able to deal with the molecular by-products of anaerobic respiration, which can be toxic at high levels.

Marine biologist Colin Simpfendorfer, at James Cook University in Townsville, Australia, says the study shows how sharks are well adapted to the limits of their environment. “Diving to over 1,000 metres from tropical temperatures at the surface down to just a couple of degrees centigrade to feed is a fairly extreme movement to do on a regular basis,” Simpfendorfer says.

So far, scalloped hammerheads are the first fish found to do this, but Simpfendorfer says other sharks and fish might have the same adaptation. “There is a big advantage in being able to hunt when you’re warm and everything else is cold.”

article_text: Because it makes them better hunters, scalloped hammerhead sharks (Sphyrna lewini) have evolved a unique method to avoid losing body heat when they dive for prey in deep, cold waters: they close their gills. Numerous fish and marine-mammal species are known to dive from the warm surface to deeper waters to hunt. However, ectothermic, or ‘cold-blooded’, animals face the challenge of how to conserve their body temperature to keep their metabolism active enough for hunting when the surrounding water can be just a few degrees above freezing. “The most rapid point of heat loss for any fish, even a high-performance fish, is always at the gills,” says Mark Royer, a postdoctoral researcher in shark physiology and behaviour at the Hawaiʻi Institute of Marine Biology in Kaneohe, part of the University of Hawaiʻi. Because of the high volume of warm blood flowing through the gills, they are “essentially just giant radiators strapped to your head”, he says. Some fish, such as the whale shark (Rhincodon typus), are able to conserve their body heat when diving through sheer size. Others, such as tuna, marlin and the family that includes great white (Carcharodon carcharias) and mako sharks (Isurus oxyrinchus), have evolved specialized heat-exchange systems at the gills that avoid too much body heat being lost. The scalloped hammerhead has neither of these advantages or adaptations, yet has been tracked doing rapid, repeated dives to depths of around 800 metres. To understand how sharks were coping with the temperature changes, Royer and his colleagues developed a device consisting of instruments that measured depth, water temperature, location and movement, as well as a probe embedded into muscles near the dorsal fin that recorded the shark’s core temperature. The device was designed to break off after several weeks, float to the surface and send out a signal to enable its recovery. Three scalloped hammerhead sharks captured off the Hawaiian coast were tagged with the device. In a paper published in Science1, the team reported that the sharks would dive several times — six in an evening, for one shark — into deep water at temperatures of 5–11 °C, around 20 °C colder than at the surface, and remain there for 5–7 minutes at a time before surfacing. Body temperature remained constant for most of the dive until the final stage of their ascent back to warmer waters, when it would decline rapidly. Royer suggests that the sharks are keeping their core temperature stable by simply not opening their gills or mouth during the dive; effectively ‘holding their breath’. “If you don’t have water going over your gills, then you won’t be dumping your body heat into the environment,” he says. The drop in body temperature near the surface coincides with the sharks’ steep ascent flattening out slightly, which Royer suggests is the moment they start to allow water to flow over the gills. “They can slow themselves down, open their gills and start breathing again [because] the water that they’re in, it’s not as cold as it is at the bottom,” he says. To shut down oxygen intake in this way suggests that the scalloped hammerhead must be able to deal with plummeting blood-oxygen levels during dives, says Mark Meekan, a fish ecologist at the University of Western Australia Oceans Institute in Perth, although the mechanism is yet to be discovered. “What they could be doing is slowing the heart muscle, slowing the pumping of blood around the body,” Meekan says. The shark’s tissues and blood could have evolved to hold more oxygen per unit of volume — akin to the adaptation seen in people who live at altitude — or might be able to deal with the molecular by-products of anaerobic respiration, which can be toxic at high levels. Marine biologist Colin Simpfendorfer, at James Cook University in Townsville, Australia, says the study shows how sharks are well adapted to the limits of their environment. “Diving to over 1,000 metres from tropical temperatures at the surface down to just a couple of degrees centigrade to feed is a fairly extreme movement to do on a regular basis,” Simpfendorfer says. So far, scalloped hammerheads are the first fish found to do this, but Simpfendorfer says other sharks and fish might have the same adaptation. “There is a big advantage in being able to hunt when you’re warm and everything else is cold.” vocabulary:

{'Ectothermic': '体温受外界环境影响的动物，即冷血动物', 'Sphyrna lewini': '斑锤头鲨的学名，是一种大型鲨鱼', 'Rhincodon typus': '鲸鲨的学名，是一种大型鲨鱼', 'Carcharodon carcharias': '大白鲨的学名，是一种大型鲨鱼', 'Isurus oxyrinchus': '镰刀鲨的学名，是一种大型鲨鱼', 'Metabolism': '新陈代谢，指的是有机体内物质的合成和分解过程', 'Radiators': '散热器，是一种用来散热的装置', 'Hawaiʻi Institute of Marine Biology': '夏威夷海洋生物研究所，位于夏威夷州Kaneohe，是夏威夷大学的一部分', 'Anaerobic respiration': '厌氧呼吸，是指在缺氧条件下的呼吸过程', 'Oceans Institute': '海洋研究所，位于澳大利亚西部城市珀斯，是西澳大利亚大学的一部分', 'James Cook University': '詹姆斯库克大学，位于澳大利亚昆士兰州汤斯维尔，是澳大利亚最大的研究型大学之一'} readguide:

{'reading_guide': '本文讲述了斑锤头鲨（Sphyrna lewini）如何通过关闭鳃来避免在深冷水域捕食时失去体温的独特方法。文章指出，斑锤头鲨可以在温度仅比冰点高几度的深水中进行多次快速潜水，而且能够保持体温稳定。研究人员推测，斑锤头鲨可以通过在潜水期间不张口不张鳃来保持体温稳定，这表明它们必须能够应对潜水期间血氧水平的急剧下降。文章最后指出，斑锤头鲨是第一种被发现具有这种适应性的鱼类，但其他鲨鱼和鱼类可能也具有相同的适应性。'} long_sentences:

{'sentence 1': 'To shut down oxygen intake in this way suggests that the scalloped hammerhead must be able to deal with plummeting blood-oxygen levels during dives, says Mark Meekan, a fish ecologist at the University of Western Australia Oceans Institute in Perth, although the mechanism is yet to be discovered.', 'sentence 2': 'Marine biologist Colin Simpfendorfer, at James Cook University in Townsville, Australia, says the study shows how sharks are well adapted to the limits of their environment.'}

sentence 1: 这种方式关闭氧气摄入表明，斑锤头鲨必须能够应对潜水期间暴跌的血氧水平，澳大利亚西部大学海洋研究所的鱼类生态学家马克·米肯（Mark Meekan）说，尽管机制尚未发现。

句子结构分析：这是一个复合句，主句是“这种方式关闭氧气摄入表明，斑锤头鲨必须能够应对潜水期间暴跌的血氧水平”，其中“这种方式关闭氧气摄入”是定语从句，修饰“表明”；“斑锤头鲨必须能够应对潜水期间暴跌的血氧水平”是状语从句，修饰“表明”；“澳大利亚西部大学海洋研究所的鱼类生态学家马克·米肯（Mark Meekan）说”是宾语从句，修饰“说”；“尽管机制尚未发现”是状语从句，修饰“说”。

语义分析：句子表达的意思是：这种关闭氧气摄入的方式表明，斑锤头鲨必须能够应对潜水期间暴跌的血氧水平，澳大利亚西部大学海洋研究所的鱼类生态学家马克·米肯说，尽管机制尚未发现。

sentence 2: 澳大利亚詹姆斯·库克大学的海洋生物学家科林·辛普芬多弗（Colin Simpfendorfer）说，这项研究表明，鲨鱼已经很好地适应了其环境的限制。

句子结构分析：这是一个复合句，主句是“澳大利亚詹姆斯·库克大学的海洋生物学家科林·辛普芬多弗（Colin Simpfendorfer）说，这项研究表明，鲨鱼已经很好地适应了其环境的限制”，其中“澳大利亚詹姆斯·库克大学的海洋生物学家科林·辛普芬多弗（Colin Simpfendorfer）说”是定语从句，修饰“说”；“这项研究表明，鲨鱼已经很好地适应了其环境的限制”是状语从句，修饰“说”。

语义分析：句子表达的意思是：澳大利亚詹姆斯·库克大学的海洋生物学家科林·辛普芬多弗说，这项研究表明，鲨鱼已经很好地适应了其环境的限制。