The blue king crab, Paralithodes platypus, is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of king crab

King crab

King crabs, also called stone crabs, are a superfamily of crab-like decapod crustaceans chiefly found in cold seas. Because of their large size and the taste of their meat, many species are widely caught and sold as food, the most common being the red king crab, Paralithodes camtschaticus.King...

 which lives near St. Matthew Island

St. Matthew Island

St. Matthew Island is a remote island in the Bering Sea in Alaska, WNW of Nunivak Island. The island has a land area of , making it the 43rd largest island in the United States. Its most southerly point is Cape Upright which features cliff faces which exceed...

, the Pribilof Islands

Pribilof Islands

The Pribilof Islands are a group of four volcanic islands off the coast of mainland Alaska, in the Bering Sea, about north of Unalaska and 200 miles southwest of Cape Newenham. The Siberia coast is roughly northwest...

, and the Diomede Islands

Diomede Islands

The Diomede Islands , also known in Russia as Gvozdev Islands , consist of two rocky, tuya-like islands:* The U.S. island of Little Diomede or, in its native language, Ignaluk , and* The Russian island of Big Diomede , also known as Imaqliq,...

, Alaska, with further populations along the coasts of Japan

Japan

Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...

 and Russia

Russia

Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

. Blue king crabs from the Pribilof Islands are the largest of all the king crabs, sometimes exceeding 18 pounds (8.2 kg) in weight.

Fisheries

Commercial blue king crab harvest around the eastern Bering Sea began in the mid-1960s and peaked in 1981 with a catch of 13228000 pounds (6,000.1 t). The Pribilof Island harvest by the United States peaked in 1980 at 10935000 lb (4,960 t) and was closed in 1988 due to population decline, then again in 1999 after being opened for three years. The St. Matthew fishery peaked in 1983 with 9453500 lb (4,288 t) but experienced a similar decline and was closed in 1999. It was opened in 2009, and was featured on The Deadliest Catch. The St. Matthew stock is rebuilding but the fishery remains closed, while the Pribilof stock has not drastically improved. Diomede blue king crabs have never been harvested commercially, but support a subsistence fishery for the Native Village of Diomede, Alaska

Diomede, Alaska

Diomede is a city in the Nome Census Area of the Unorganized Borough of the U.S. state of Alaska, located on Little Diomede Island...

, population 170.

Colder water slows the rate of crab growth and crabs at northern latitudes are often smaller than more southern crabs. Commercial harvest of blue king crabs at the Pribilof Islands is limited to males with a carapace

Carapace

A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...

width (CW) over 6.5 inches (16.5 cm) and St. Matthew Island is limited to crabs with CW greater than 5.5 in (14 cm), corresponding to crabs over 4.7 in (11.9 cm) carapace length (CL). Diomede blue king crabs are similar in size to St. Matthew Island crabs.

Migration

Female blue king crabs migrate seasonally from depths of 400–600 ft (121.9–182.9 ) in winter to shallow depths of 20 foot for females with eggs and 150 foot for females without eggs. The average depth for male crabs of commercial size is 230 ft (70.1 m), although crabs can commonly be caught at shallower depths in winter.

Reproduction

Pribilof Island blue king crab mate and produce eggs in late March to early May. Female blue king crabs generally brood their eggs externally for 12–14 months. Since blue king crabs need more than 1 year to brood their eggs, they miss a breeding cycle just before the larvae hatch and only produce eggs every other year, although first-time breeders can often produce eggs in subsequent years. Females release larvae around the middle of April in the Pribilof Islands, while Pribilof crabs held at warmer temperatures in the laboratory may release larvae as early as February.

Female blue king crabs in the Pribilof Islands grow to the largest size before they are reproductively mature. Fifty percent of crabs are mature at 5 in (12.7 cm) CL. St. Matthew Island females can become sexually mature at 3 inches CL and Diomede crabs are similar. Larger female crabs from the Pribilof Islands have the highest fecundity, producing 162,360 eggs or 110,033 larvae per crab. There is a reduction in fecundity of approximately 33% between the egg and larvae stages. In Japan, an average of 120,000 larvae were released from each blue king crab. Diomede blue king crabs release an average of 60,000 larvae per female.

Environmental variables such as tides, temperature, salinity, light, phytoplankton blooms, and predation are seasonally pulsed and likely serve as cues for larval release. Release of larvae over a longer period may serve to give the female a larger window for larvae to correspond with any favorable environmental conditions that may exist, also known as “bet-hedging”. In the laboratory, Pribilof larvae hatch over the course of about one month, and Diomede larvae hatch over the course of 2–3 weeks. These differences may be due to water temperature in the lab which has a clear effect on embryonic and larval development, and is probably slightly different from hatch timing in a natural environment.