Size matters for fast fish, but that’s not all – The Japan News

This compares with 3.1 kph of a 2.2-ton whale shark, a speed not unexpected considering its huge body. Sunfish and salmon were slowpokes: An 87-kilogram sunfish swam at 2.2 kph and a 3.3-kilogram salmon at 2.7 kph.

It is natural that larger fish can swim faster. When taking weight differences into account, six species, including tuna and some belonging to the shark family, swam 2.7 times faster than fish with lower body temperatures. The annual migrating distance of fast fish was 2.5 times longer.

Among all animals, the swimming speeds of fish with high body temperatures are close to those of marine mammals such as whales that are homeothermic animals. On the other hand, the speeds of fish with low body temperatures were as low as those of reptiles, such as sea turtles.

Dark muscles

It was found that muscle contractions increased with higher body temperature. Additionally, the larger the body, the more effectively the animals retain heat.

“As a rule, the speeds of marine animals are decided by body temperature first, followed by body size,” Watanabe said.

However, there are exceptions. The speed of a 90-ton blue whale is 8 kph, while that of a 24.6-kilogram emperor penguin is 7.7 kph.

Among vertebrate animals, the shark belongs to a group of cartilaginous fish (see below) that emerged about 400 million years ago, and its skeleton differs from those of bony fish, such as tuna. But their muscles and blood vessels are similar because they have to maintain a high body temperature.

One common point is dark muscles where many blood vessels are clustered together. Fish with low body temperatures have a small number of these muscles next to the surface of their bodies, while bluefin tuna and great white sharks have most of this deep muscle mass.

Another similar point is the special structure of the rete mirabile, a complex of arteries with warm blood and veins with cold blood close to each other in the body. The rete mirabile works to prevent heat escaping from major parts of the body.

The same but different

Regardless of species, fish that swim fast for a long period generally have an oval body or a crescent-shaped tail to reduce resistance. Evolutionary development in which body characteristics and biological patterns of different species become similar is referred to as “convergence.”

“Although bluefin tuna and great white sharks are distant genealogically, they resemble each other not only in appearance but also in their physiological functions to keep their body temperatures high,” said Masanori Nakae, a researcher of ichthyology at the National Museum of Nature and Science. “This resemblance was caused by convergence.”

Bluefin tuna and great white sharks are representative of hunters in outer seas (see below). Their swimming ability is indispensable for surviving in outer seas where there is virtually no place for them to conceal their bodies.

“The ability to keep a high body temperature and move energetically is believed to help them survive in such formidable environments,” Nakae said.

Migrating patterns vary

Marine animals have different migrating patterns. Humpback whales migrate north to south and according to the season.

They ingest a great deal of nutrition in the summer in the Arctic and Antarctic seas, which are rich in krill, and raise their calves in warm seas at low latitudes in winter.

On the other hand, bluefin tuna and great white sharks travel east to west. As there is no great change in climate, it is unclear why they do this.

■ Cartilaginous fish

There are about 1,200 species of fish, including sharks and rays, that have cartilaginous skeletons. As the jawbones can be separated from the skull, they open their mouths wide to catch prey. Their backbones are made of cartilage, but they have no ribs to protect their internal organs. Their bodies are highly flexible. However, their teeth and scales are hard due to calcification, so they become fossilized and their bodies are easily identifiable, when they die. Because the structure of their bodies is so simple, they retain the characteristics of primitive fish.

■ Outer seas

Areas located outside relatively shallow continental shelves. They occupy about 90 percent of the total surface of the oceans. Except for areas where deep-sea water bubbles up, the outer seas are poorer in nutrition than coastal areas, where minerals drain into the seas from continents and rivers. The outer seas are less dense with animals and plants. Plankton lives near the surface of the sea, where sunlight reaches it. From the sky, the surface of the outer seas appears to be dark blue, while it looks bright when viewed from below. For this reason, many fish in the outer seas have an indistinct coloration — a bluish black back and a silvery white belly.Speech


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