First record of pelagic swarms of Munida gregaria (Anomura: Galatheidae) in the Beagle Channel, Argentina

GUSTAVO A. LOVRICH; PATRICIA PÉREZ BARROS; M. CAROLINA ROMERO; FEDERICO TAPELLA; MARIANO J. DIEZ; M. PAULA SOTELANO; GABRIELA SCIOSCIA; M. CECILIA AVALOS

Coquimbo, Chile

Congreso; Mid-Year Meeting The Crustacean Society; 2007

Universidad de Coquimbo / The Crustacean Society

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:12.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman";} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Pelagic swarms of Munida gregaria have been well documented off New Zealand but only recorded in the continental shelf of Southern South America back in the 1930’s. Since then, such aggregations have never been formally reported, although local mariners have sporadically experienced this phenomenon, even inside the Beagle Channel. In previous studies, both morphospecies M. gregaria and M. subrugosa were the main constituents of benthic samples. On the 28 and 29 May 2007, swarms of several hundreds of animals per m3 occurred at 1 m depth in Bahía Ushuaia (54º 50´S; 68º 17´W) and extending in depth to at least 4 m. Samples from these swarms, as well as from the sea-bottom were taken with an epibenthic trawl of 1.8 m mouth opening and 10 mm mesh size. The pelagic swarms were constituted of 100% individuals of the morphospecies M. gregaria, of an average size of 8.6±0.6 and 8.5±0.5 mm CL for males and females, respectively. Males outnumbered females in a 1.4:1 ratio (Ho 1:1; Gw= 10.6; p=0.001). All individuals were in postmoult or intermoult stage, with no epibionts.  The benthic sample was constituted of 95% M. gregaria and 5% M. subrugosa. Average sizes of M. gregaria were 20.6±1.9 and 19.0±1.2 mm CL for males and females, respectively, and 75% of the females were ovigerous. All individuals were in postmoult or intermoult stage and epibionts were in >43% of the sampled animals. The stomach content of the pelagic crabs is described. Stranded M. gregaria similar in size to those from the described swarms were found the previous month along the Beagle Channel shore, and are interpreted as to be part of the same phenomenon. We discuss the schooling behaviour as a response to high benthic densities.