Red Crab

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The invasion of the red crabs!

Links to other Galatheidae's homepages

Look the life cycle of Pleuroncodes planipes along
the west coast of Baja California

Red crab swimming with a typical position

Dense pelagic swarm of Pleuroncodes planipes

Other galatheids

Pleuroncodes monodon

Cervimunida johni

Galathea australiensis

Carlos J. Robinson and Jaime Gómez-Gutiérrez (1998) The red-crabbloom of the west coast of Baja California, México. J. Plankt. Res. 20(10): 2009-2016.

The red crab Pleuroncodes planipes, (Stimpson, 1860) is an important component of the pelagic and benthic ecosystem along the west coast of Baja California, México. This crustacean of the Galatheidae family, langostilla as known commonly in México, is a very abundant grazing organism in the coastal upwelling at the southern part of the California Current System and a common prey for some marine vertebrates and invertebrates. In its benthic phase, this crab is detritophagous in low oxygen concentration environments. Juvenile and young adults may be found at the bottom as well as in the water column. However, when they reach their second year, larger than 32 mm in standard caparace length, they become strictly benthic. The seasonal inshore-offshore movements of benthic P. planipes are well studied, as well as the inshore-offshore larval drift.

However, the diel movements of this species are not well understood. Although, migration from the bottom to mid-water during the night is a common behaviour, P. planipes may be found also on the surface during the summer like observed with Munida gregaria(New Zealand) aggregated by internal waves and surface Langimur circulation. The red crab do migrate towards the surface. Pelagic and benthic phases can be found simoultaneously only inshore and during the seasonal reproductive months (Spring, February-March), the benthic phase that may migrate into the pelagic zone for reproduction.

Our personal observations using hydroacoustics records indicate horizontal movements and dispersion during the night in the water column. During the day, however, the pattern is less regular, the red crab may be found compacted and very closed to the bottom, compacted and in continuous layers in depth water, in dense groups in mid-water or near to the surface. One of the problems when assessing pelagic stocks and behaviour of anchovies and sardines in the west coast of Baja California is the sudden, huge appearance of red crab in middle water during dusk in areas where during the day they were absent. Although hydroacoustically it is possible to discriminate between the anchovy's target strength and the red crab's, the problem remains when it is necessary to obtain mid-water net samples to validate the observations. A few minutes of trawl are more than enough for a net to be lost or damaged due to the presence of these crabs. The sudden appearance may be an expansion of the red crab observed in the middle water layer, they are very active swimmers during the night, or perhaps they arise from the bottom.

Our results show that benthic P. planipes rises from the bottom towards the surface at dusk from very compacted patches settled on the continental shelf. The behaviour is fast and abruptly. Using the time when the swarm was first recorded (about 1845 h in transect two and 1815 h in transect three) and the time when the ship passes over the same area again (1945 h and 1915 h) it is estimated that the behaviour lasted less than 60 minutes. The way that red crab rises from the bottom indicates that the spatial distribution in the benthic habit of this crustacean is in form of dense patches, rather than homogeneous.

Echograms on transect two. The white continuos layer is the bottom. Colors represent echo-intensity. Yellow and red represent 1480 millivolts, green and yellow 920, blue and green 560. The 920 millivolts interval represents approximately the target strength used to describe the behaviour of the red crab. White color means a high degree of compactness of echoes. (a) At 1100 h starting in the oceanic zone. (b) At 1700 h starting in the oceanic zone. Notice on the continental shelf a tower-like patch rising from the bottom.