Grunion

California grunionDid you know that in America, on the beaches of California all the way down to Mexico, there are small fish which reproduce in the sand, out of water, and by the light of the full moon?

Their name is grunion, they look like anchovies and their moonlight display is fascinating to see !

With the high spring tides between January and August, grunion ride the breaking waves as high up the beach as possible so that they can spawn in the sand.

The female wriggles her body into the sand with her head sticking up and deposits her ova. The males curl around the females to fertilize the ova, which then become eggs. Our small parents-to-be then face a race against time, as they can only survive for around 20 minutes out of water !

After fertilizing the eggs the grunion must return to the sea as quickly as possible by riding the ebb tide back out to sea. Their offspring is born at the next spring tide. Of course, this incredible mating ritual is only possible on clean beaches ! What better reward for all those who take care of the environment !

Introducing grunion

Grunion

Grunions are small slender fish which, depending on the species, range between 18 and 25 cm in length and look like anchovies. They belong to the genus Atherina (like the sand smelt in Europe) although there are only two North American species which lay their eggs in the sand.

The California Grunion (Leuresthes tenuis) inhabits the Pacific coast from south of San Francisco to Baja-California.

The Gulf Grunion (Leuresthes sardina) is endemic to the Gulf of California (Sea of Cortez) ; it can be found in Mexico, along the Baja California peninsula, and spawns mostly in the northern part of the Gulf.

In France, Grunion are also called athérine mexicaine (a former FAO name).

 

Their slender body is greenish-brown along the back and upper body and they have a silvery underbelly. There is a dark stripe along the length of their sides which clearly demarcates the upper and lower pigmented zones. Their jaws are weak and their teeth are tiny and sometimes completely absent. They have a spiny first dorsal fin. They feed on planktonic prey (larvae, crustaceans…) and various benthic organisms.

The grunion’s unique courtship ritual 

Grunion's reproduction

From January to August (with peaks in April and May), at nighttime and under the full moon, grunion swim in from the open sea on the high spring tide to spawn in the sand.

They ride the breaking waves as high up the beach as possible.

The female grunion digs a nest (4 to 5 cm deep and 5 or 6 cm long) by wriggling her body in the sand using her tail before depositing her eggs. 



Males then compete frenetically to mate with the female. They wrap themselves around her and release their milt onto her ova. The mating couples don’t have much time as, out of water, they tire quickly. The mating union lasts only for the duration of the sea’s slack water period, and the grunion ride back out to the open sea with the ebb (falling) tide.

The life of a grunion

Their eggs incubate for two weeks in the sand under the beach during the neap tide period. Here they are continually warmed by the sun and kept moist by water percolating down through the sand.

The young fish hatch at the following spring tide and are flushed out of the sand by the sheer force of the waves, and drift out to the open sea with the ebb current.

Grunions have numerous predators. From the air (pelicans, double-crested cormorants, gulls...), from the sea (jacks, barracuda, tuna, sea-lions, whales...) and from the land (crabs...humans).

This member of the Atherinidae family (Leuresthes tenuis or grunion) is an example of a species adapting in a highly complex way to the rhythm of the ocean tides. It spawns « out of water » at night on the sandy beaches of the Pacific coast, above the water line and during spring tides (…). Males and females spawn in the open air, for up to about twenty minutes, without going into oxygen deficit.  

Incubation in the sand

Grunion egg

Respecting the rhythm of the tides is crucial for fish whose eggs are emerged along coastal shores, as is the case for grunion (Leuresthes tenuis) in California.

Grunion embryos are capable of postponing hatching for several weeks, by surviving on their fat reserves, although they are able to hatch after 9 days if tidal water reaches them. […]

Hatching is induced when they are agitated in water.

 

 

 

Threats to marine life, fishing and the law

Humans eat these fish fried. But tourist activities such as motorised vehicles on the beach also do serious damage to the beach, which is the biotope grunion depend on to reproduce.

As a precaution, grunion fishing is regulated.

In California, it is illegal to fish in April and May, the two main spawning months. In addition, anyone over 16 requires a licence to fish grunion, and they must only be caught by hand.

Grunion and pollution

Grunion only reproduce on clean beaches. They therefore act as an indicator, as an ecological marker.

 

Vital Statistics

  •  
  • There are 2 species of the genus atherina which spawn in the sand.
  • On average grunion can survice for 10 mn out of water without falling into oxygen debt.
  • An adult grunion is some 18 to 25 cm in length, although length varies with the species.

Other sand spawing fish :

The Grass Puffer in Japon (Takifugu niphobles)

The mummichog and its close relatives in the eastern USA (Fundulus heteroclitus).

 

Sources

Articles La Danse des grunions - V. PLATT - Paru dans Dauphin Hebdo n°74 du 20 mars 1991, pp 4-5  

Ouvrages

  • La Mer de Cortez. Ed. Flammarion, 1988 COUSTEAU et PACCALET - – Coll. L’Odyssée, pp 149-159 Ref. NAUSICAA : B 545 COU
  • Le Livre de la mer. Ed. Larousse-Bordas, 1998, p 201
  • Sexualité et reproduction des poissons – J. MELLINGER - Ed. CNRS, 2002, p 119 et p 269

Sites Internet

Spécialistes des grunions

Photos NAUSICAA, Yvette TAVERNIER
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