Environment

Why Are Abalones Threatened With Extinction?

Abalones are a group of sea snails and marine mollusks that are threatened with extinction.

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Abalone is a name given to any group of small to large sea snails, which are marine gastropod molluscs in the family Haliotidae. Abalones are also known by other names such as mutton shells or mutton fish, particularly in Australia, and in England they are known as ormer. Other common names include sea ears, ear shells, or paua. There are currently between 30 and 130 species of abalone recognized worldwide. Abalone are characterized by shells that have an open spiral structure with several respiratory pores on the outer edge of the shell. The inner layer of the shell is typically thick and is made up of nacre material, which is extremely iridescent in many species, giving it an array of strong colors that can change. This attribute makes the shells attractive to humans, and they are often used as jewelry or decorations. The flesh of the abalone is also consumed, either raw or cooked, by several cultures around the world.

Description

Abalones are generally grouped as black, green, white, or pink abalones, and vary considerably in size, ranging from about 0.79 in to 7.9 in, while the largest in the genus is approximately 12 in. The shells of abalones are convex in shape and vary from oval to round, but could also be flattened or arched. Among most species, the shell has two or three whorls and a flat spire. The last whorl is auriform, and is known as the body whorl, which resembles an ear, and this is where the name "ear shell" is derived. The species Haliotis asinine (black abalone) has a different shape, which is somewhat distended and elongated. Additionally, other species like Haliotis cracherodii (black abalone) have unusual shells, such as an ovate shape with picky ribs and exerted spire. The snails can cling to surfaces with their muscular and broad feet at sublittoral depths. Some species, like Haliotis cracherodii, were widespread in intertidal zones. Abalones lay a large number of eggs once, ranging from 10,000 to 11 million. The larvae of abalones are lecithotrophic, which means adults do not provide any assistance.

Threat of Extinction

Abalones currently face an imminent danger of extinction due to several factors such as acidification of the oceans, overfishing, and diseases. In fact, it is believed that they could be extinct within a few years if the current rate of ocean acidification is not halted or reversed. In the United States (US), green, pink, and white abalones have been included on the federal list of endangered species and measures for restoration have been proposed in the areas near Santa Barbara Island and San Clemente Island. It has also been proposed to introduce farming of abalone, which could then be eventually released into the wild. In the US, abalones were previously a common sight for divers in parts of California, but in recent years the populations have dropped significantly and are now very rare.

Overfishing

As a result of the unique reproduction behaviors of the abalones, they can be easily depleted by intense fishing that target groups of animals. Commercial harvesting of abalone began in California in the early 1970s, reaching peak levels in the mid-1970s, and was later banned in the 1990s. There were previously size limits and seasons for fishing, which were measures to reduce the number of abalones caught. However, these precautions were not effective, and the abalone population continued to dwindle, which makes current recovery efforts challenging. Similarly, illegal harvesting (poaching), particularly of black abalone, has been a major problem in remote areas along the coast of Central California, where black abalone existed in large numbers. Illegal harvesting of black abalone has significantly its global population, and negatively affected its ability to reproduce and sustain its population. In 2018, the California Fish and Game Commission announced that the state recreational abalone fishing season would remain closed until April 2021 in an attempt to give the abalone time to regenerate. This decision was based on studies by marine biologists that indicated the abalone populations of abalone were not healthy. The recreational abalone fishing season was previously open from April until November.

Diseases

One of the diseases that has significantly affected the abalone population, particularly black abalone, is withering abalone syndrome. The disease, which is fatal and typically affects the digestive system of the abalone, was first observed in 1985 in parts of Southern California, and has since spread both north and south. The disease resulted in catastrophic abalone deaths, particularly black abalone, which is now endangered. Cultured white abalone populations have also been affected, as well as pink abalone. The pathogens that cause the disease are presently found along the coastal region of California. It is believed that all black abalone in the wild have been infected, but it becomes fully manifested and more prevalent in Southern California, where the water is relatively warm. Deaths associated with the disease are linked to the warming effects of the surface water in the oceans, which could be caused by thermal discharges or the effects of El Nino. Other diseases which have emerged in the recent past include sabellidosis, vibriosis, and the herpes virus.

Significance of Abalones

Decreasing populations of abalone have a significant impact on the health of coastal food webs, which is also expected to deteriorate with falling population numbers. Abalones play a significant role in keeping the population of algae in check in order to avoid exponential algal blooms in the oceans. Significant increases in algae will block the sunlight that penetrates oceans, leading to a decrease in the amount of oxygen generated through photosynthesis, which is critical for other animals in the oceans. With the falling population of abalones, other secondary predators like sea lions and sea stars, which rely on them as a source of food, will starve to death or find other prey. Such a situation could result in a decrease of both the population of predators as well as other animals that become targeted as prey in the absence of abalone. 

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