Swarming behaviour of Stylocheilus striatus, October 2005

Some exemples of swarming behaviour around the world.

Stylocheilus striatus can sometimes be found in large numbers : swarning behaviour. This behavior was observed in different place in the world.

January 2017, in Mayotte island, Remy Eudeline, million of specimens on N'Gouja

December 2015, in l'ïle de la Réunion, Philippe Bourjon, million of specimens on Saint Gilles

January 2014, in Mayotte Island, Norbert Verneau on Petite Terre.

October 2010, in Mayotte, Alain Barrère and November 2010 in Reunion Island, Philippe Bourjon on Saint Gilles

December 2009 in Reunion Island, Philibert Bidgrain on Saint Gilles

October 2008, in Madagascar, Sully Bachel on Sainte Marie Island

October 2008, in Mauritius island, Yann Von arnim on "ferme marine de Mahebourg"

October 2005, in Reunion and Mayotte Island, Philibert Bidgrain, Daniel Nardin and Stéphanne Ciccione, see below for more details

    June 2005, in Grace Bay, Provo, Turks & Caicos Islands, Tropical West Atlantic by Martin Banford

" Usually when someone says there were millions, they are exaggerating, but if they had not managed to make up a million, they were not far off. There were large areas of dispersed ones, and then two long lines almost like battle fronts of these things, moving steadily eastward."

May 2005, in north side of Little Cayman Island (Caribbean) by Jim Chambers

"Each group was about the size of a large dinner plate, and there were a lot of "trailers" behind each main group. The largest sea hares were about 4-5 cm (stretched out), and there were many smaller ones about 2-2.5 cm. My best guess is that each group had at least 200-300 sea hares, and they were moving perhaps one foot per minute (rough guess) away from shore."

May 2005, in Ulua Beach, Wailea, Hawaii (Maui) by Mike Roberts

"At first glance, it is like swimming by windrows of decay or algae rolling in the surge. These “rivers” are meandering across the sand flowing together, then apart. Some are separate small “streams,” while others are wide flowing “channels.” With a little closer observation, it becomes apparent that these are slugs. Literally thousands upon thousands of Lined Sea Hares Stylocheilus striatuys forming rivers of slugs climbing on top of each other with a definite plan of going south. On an even closer inspection the individual slugs could be spotted in all sizes from large to those that seem freshly hatched "

August and September 1994 and May 1992 in Hekili Point, Maui by Cory Pittman

"The most spectacular examples I've observed occurred just west of Hekili Point, Maui in 1994. The night of Aug. 21, however, I found many hundreds of animals on the bottom and hundreds more drifting in the water column. On Sept. 26, I again found large numbers both on the bottom and drifting. I also found many on the bottom and moderate numbers drifting on Sept. 30. Most of the drifting animals seen in 1994 were relatively large and I saw many copulating pairs on the bottom. Another observation that might be of interest occurred on May 30, 1992 (this time in the trough just east of Hekili Point)"

In hawai (Kay, 1964) , immense aggregations of thousands of animals have beeen witnessed.

Reunion Island swarming observation (photos : Philibert Bidgrain):

Swarming behaviour was observed in Reunion island ( "trou d'eau lagoon" at La Saline)

Daniel Nardin observed this behaviour for the first time on October 26 2005.

Three days after I observed these strange numerous populations. I found different kind of group.

On the lagoon, there are many large group who are actively crawling on the substrate. Each group is very important, may be more than one thousand in some cases.

2 November 2005, after two days of important surge on the lagoon, all these seaslugs have disappeared....

A synchronous swarm with Lizard Island (obseved by Nerida Wilson)

S. striatus are currently also swarming at Lizard Isand, Great Barrier Reef. The first sighting was of a developed swarm about the 26th October.

It is interesting that it appears synchronous in two different oceans....



In this photo you can see a
portion of a large group

swarming of stylocheilus striatus

Photo below : Detail of a large group. You can see the very dense
population of S.striatus ...

Detail of a large group
swarnmng of stylocheilus striatus



In this photo, you can remark the heterogenous size of Sea Hares, taken in the same group.



swarming of stylocheilus striatus


Early on the morning, just after the sunrise, I observed along the shore in 30 to 80 cm of water, some small large as dinner plate and inactive group, partially hiden in the sandy substrate.

swarning of stylocheilus striatus


I observed on October 29 2006, along the shore in 30 to 80 cm, groups of about hundred specimens who are spawning small filament on dead coral or little rock scattered on the sandy substrate.

swarning of stylocheilus striatus

swarming of stylocheilus striatus


On October 29, I put in my aquarium a little fragment of dead coral covered with Stylocheilus spawning.

Four days later, the eggs have disappeared, and I observed floating on the surface, small veliger larvae.

You can note that in larvae stade, a classical gasteropod shell could be observed...

Veliger larvae

Ciliary movement of Stylocheilus striatus veliger larvae.

veliger larvae
veliger larvae
veliger larvae

Mayotte Island swarming observation (photos: Stéphane Ciccione):


Stéphane Ciccione

N'Gouja's seagrass meadow.

Mayotte island ( one of the Comoro island, Mozambic channel)

13 October 2005. Two weeks before the swarming behaviour observed in Reunion island

stylocheilus striatus