''Crangon franciscorum'' is a species of shrimp in the family

Crangonidae Crangonidae is a family of shrimp, of the superfamily Crangonoidea, including the commercially important species ''Crangon crangon''. Its type genus is '' Crangon''. Twenty-four genera are included in the family: *'' Aegaeon'' Agassiz, 1846 ...

which is endemic to the

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estu ...

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environmen ...

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

, and found from

Puget Sound Puget Sound ( ) is a sound of the Pacific Northwest, an inlet of the Pacific Ocean, and part of the Salish Sea. It is located along the northwestern coast of the U.S. state of Washington. It is a complex estuarine system of interconnected ma ...

in the north to

San Diego, California San Diego ( , ; ) is a city on the Pacific Ocean coast of Southern California located immediately adjacent to the Mexico–United States border. With a 2020 population of 1,386,932, it is the eighth most populous city in the United Stat ...

in the south. The species is especially abundant in San Francisco Bay, despite population fluctuations due to environmental stresses. Its common names include bay shrimp, sand shrimp, common shrimp, grass shrimp, black shrimp, California shrimp and black tailed shrimp. The species has been commercially fished from 1869 to the present.

Role in the Bay food web

This shrimp species is "an important part of the estuarine food web" in the greater San Francisco Bay. It feeds on bivalves,

amphipods Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far describ ...

and foraminiferins, and is prey for various fish, including striped bass,

white sturgeon White sturgeon (''Acipenser transmontanus'') is a species of sturgeon in the family Acipenseridae of the order Acipenseriformes. They are an anadromous fish species ranging in the Eastern Pacific; from the Gulf of Alaska to Monterey, Californ ...

, Pacific tomcod and shellfish such as

Dungeness crab The Dungeness crab (''Metacarcinus magister'') is a species of crab inhabiting eelgrass beds and water bottoms along the west coast of North America. It typically grows to across the carapace and is a popular seafood. Its common name comes f ...

. Its diet is "heavily influenced by predator size, temperature-salinity preferences, and prey availability."

Lifecycle

The shrimp are short-lived, with a lifespan ranging up to 18 months for males and 30 months for females. The males spawn once, while longer-lived females spawn twice. Some evidence indicates the species may be protandrous hermaphrodites, which means that surviving males are transformed into females after one year of life. This may account for the longer lifespan of females. The two most important natural environmental factors affecting the health of the shrimp population are water temperature and salinity. The shrimp thrive in

brackish water Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...

, with a preferred salinity of 14 parts per thousand (ppt) when young to 24 ppt when ready to spawn. In contrast, open ocean waters have a salinity of about 35 ppt. The species prefers a water temperature of about 18 °C, or 65 °F. Because these factors vary within the bay, based on the seasons each year, and the variations in the inflow of fresh water in heavy rainfall years as opposed to drought years, the shrimp migrate around the bay, seeking optimal conditions, and their population levels fluctuate dramatically. When heavy flows of fresh water enter the bay, mature females migrate to the saltier parts of the central bay, or out the

Golden Gate The Golden Gate is a strait on the west coast of North America that connects San Francisco Bay to the Pacific Ocean. It is defined by the headlands of the San Francisco Peninsula and the Marin Peninsula, and, since 1937, has been spanned by t ...

to the Gulf of the Farallones. There, they mate with males which prefer a slightly saltier environment. The females then incubate from 2,000 to 8,000 eggs and when hatched, the young shrimp migrate back to the shallower and less salty estuaries around the bay. As the newly hatched shrimp develop and mature, they gradually migrate to "deeper, cooler and more saline water".

Environmental stresses

When exposed to sewage in a laboratory, the species shows behavior described as "avoidance of the toxicant". It is "sensitive to pollution in estuaries". In the late 19th and early 20th centuries, San Francisco Bay became increasingly polluted. The Gold Rush, which proceeded commercial shrimp fishing, caused the first significant environmental damage to San Francisco Bay. Hydraulic mining techniques introduced tens of millions of cubic meters annually of rock and soil debris into the Sacramento and San Joaquin Rivers, the main sources of fresh water put into the bay. The accumulation of sediment on the bottom of the bay reduced total water volume and changed tidal patterns. Hydraulic mining was banned by an 1884 court decision. The size of the bay shrank as dikes were built to create farmland and salt ponds, and areas of the bay were filled to create real estate, such as San Francisco's Marina District, Treasure Island, and Foster City. The vast majority of the tidal marshes around the bay, where the shrimp thrive, were lost to development by the 1980s. The

Central Valley Project The Central Valley Project (CVP) is a federal power and water management project in the U.S. state of California under the supervision of the United States Bureau of Reclamation (USBR). It was devised in 1933 in order to provide irrigation an ...

, which began in 1933, and the

California State Water Project The California State Water Project, commonly known as the SWP, is a state water management project in the U.S. state of California under the supervision of the California Department of Water Resources. The SWP is one of the largest public wate ...

, which began in 1960, were efforts using dams, reservoirs, and canals, to divert water from the Sacramento and San Joaquin River systems for use in agricultural irrigation and urban development. By the 1980s, the annual inflow of fresh water into the bay was reduced to about 40% of historic levels. Following prolonged droughts, the shrimp population declines, and following wet years, it rebounds. In 1996, after two wet years, the shrimp population was 20 times larger than it was in 1980, following the severe drought of the late 1970s.

Commercial fishing

This species was by far the most common species of shrimp in San Francisco Bay in the 19th century, and was also the most important species for its commercial shrimp fishery from the 1870s to the 1930s, accounting for about 90% of the shrimp catch.

References

{{Taxonbar, from=Q4485789 Caridea Crustaceans of the eastern Pacific Ocean Endemic fauna of California Crustaceans described in 1856 Taxa named by William Stimpson Fauna without expected TNC conservation status