Mesoplodon

The genus *Mesoplodon* contains 16 different species of toothed whales. It is the largest genus within the cetacean order. These animals are considered the most poorly known group of large mammals on Earth. Scientists are still discovering them today. For example, a new species was described as recently as 2021. The name *Mesoplodon* comes from Greek words. It translates to "armed with a tooth in the centre of the jaw." This name reflects the unique way their teeth are positioned.

Physically, these whales are relatively small. They range in size from the tiny pygmy beaked whale to the larger strap-toothed whale. Their bodies have a spindle shape, which means they are thick in the middle and taper at the ends. They possess small dorsal fins located between two-thirds and three-quarters down their backs. Their flippers are both short and narrow. The head is small and tapered with a semicircular blow hole. This blow hole is sometimes asymmetric. The beak blends into a small melon, which is the fatty organ in the forehead, without a visible crease.

Each species has large teeth that vary in size, shape, and position. Some of these teeth can even look like tusks. In certain species, the lower jaw forms a massive arch. This arch can extend above the rostrum, or snout. This shape is sometimes compared to a playground slide. There is also sexual dimorphism, which refers to physical differences between males and females. While much is unknown, females tend to be the same size or larger than males in some species. Males often have bolder colors and unique dentition, or tooth patterns. Males are also frequently covered in scars left by the teeth of other males.

Both males and females often carry round scars on their skin. These specific marks are caused by bites from cookiecutter sharks. Many species are very difficult to study because they are rarely observed. They are deep divers that spend much of their time far below the surface. Many of them feed entirely on squid. When they do come to the surface, they are typically very slow swimmers. They do not make obvious blows when breathing. They also never raise their flukes, or tail fins, above the water. Some species are so uncommon that they have never been seen alive by humans.

History shows that our understanding of these whales is constantly growing. Many species were identified in the 1800s. Sowerby described *M. bidens* in 1804. Blainville described *M. densirostris* in 1817. Other species were named much later, such as the pygmy beaked whale in 1991. Perrin's beaked whale was described in 2002. We also know about four extinct species that lived in the past. These include *M. longirostris*, *M. posti*, *M. slangkopi*, and *M. tumidirostris*. Fossils of these whales have been found in places like South Africa and the North Pacific.

Conservation is a major concern because we lack population estimates. We do not know how many of these whales are left in the ocean. They have been accidentally captured in drift nets. They have also been hunted occasionally by the Japanese, though never directly. It is currently unknown what effect these activities have on their total population. Because we have almost no information on their gestation, or pregnancy, periods, it is hard to track their numbers. We also lack data regarding their longevity or how they nurse their young.

*Mesoplodon* is part of a complex biological system of marine mammals. It is closely related to bottlenose whales and giant beaked whales. One relative, the Longman's beaked whale, was once part of this genus. However, Joseph Curtis Moore moved it to its own genus called *Indopacetus*. This classification is still used by researchers today. The study of these whales helps scientists understand the diversity of life in the deep ocean. As new species are found, our map of the marine world continues to change.