1000s of Sea Hares in Hawaii

June 20, 2005
From: Mike Roberts

Hi
Leslie Harris suggested I send you the information and pictures I posted on Digitaldiver.net of a mass of sea hares and one of their predators. I hope this is of interest to you.

Locality: Ulua Beach, Wailea, Hawaii (Maui). Depth: 40-50 ft. Length: varied. 05 and 06 May 2005. Sand. Photographer: Mike Roberts

A couple of weeks ago, I came a across a phenomenon I have seen only twice before in many years of diving Maui. If I wasn’t a photographer or if I had been in that “WA  state of mind,” I would have likely missed it altogether. 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 striatus] to be exact, 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 – for a sea hare anyway – to those that seem freshly hatched

Cute little guys but they were the 'hors deuvres' of the day. Stripebelly puffer fish just rested next to the line of food, looking like so many Jabas while gorging themselves well beyond capacity.

Then I saw a nudi I had never seen before. [see separate message #14082 ] He was a mean machine with body lines that would make Fisher jealous. He was certainly built for speed!  This efficient little machine was just zooming along in haphazard patterns until he caught the “scent” of a hare in his path. Then, without missing a beat, the hare would be sucked into his powerful manifold and he would move on to the next one. Even though there were dozens of these sleek nudis driving around in circles and slurping up the little morsels, even though the puffers stationed along the rivers were too fat to swim, the hares didn’t seem to understand they were lunch. But it didn’t matter in the big scheme. They had numbers on their side. And they just kept marching south.

[See Digitaldiver.net -http://www.digitaldiver.net/index.php ]

Mike Roberts

mike@tortuga-web.com

Roberts, Mike, 2005 (Jun 20) 1000s of Sea Hares in Hawaii. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/14074

Dear Mike,
Thanks for another record of this fascinating behaviour. We have two earlier records on the Forum of this mass swarming of Stylocheilus in Hawaii [Cory Pittman - #424; John Hoover - #6681]. Have a look at my comments on this phenomenon attached to those messages, and the mass mortality Fact Sheet, where I discuss the mass swarming behaviour found in many Sea Hares. We can't really say why there are so many together like this, so using terms like migration and even aggregation or swarming suggest things which we are not sure about. I  tend to think these animals settle out of the plankton and grow close to where they settle only becoming noticeable when the oldest cohort in the population reach adulthood. Perhaps a clue is their food. They feed on a brownish layer of blue-green algae or more properly cyanobacteria. If you look at your upper photo you will see a brownish stain across the sand which is what they are feeding on. I suspect environmental conditions may cause the cyanobacteria to be arranged unevenly on the sand and the 'rows' or 'streams' of Stylocheilus may reflect the distribution of the brown 'stain' they feed on. Whether they are actually 'going anywhere' - like in a spiny lobster migration - is most doubtful.

What makes your record particularly interesting is the observation of the two predators, a species of puffer fish, and the cephalaspidean sea slug Philinopsis cyanea . I have put your photos of Philinopsis in a separate message #14082. Your message on Digitaldiver.net also generated another interesting record of mass swarming in Stylocheilus striatus from the Caribbean [see message #14080]. Any time you witness nudibranchs and related sea slugs 'doing things' like this - think of us obsessive students of sea slugs - such messages are always welcome.
Best wishes,
Bill Rudman

Rudman, W.B., 2005 (Jun 20). Comment on 1000s of Sea Hares in Hawaii by Mike Roberts. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/find/14074

Related messages

  1. Sea hare from Isla Marada, Florida
    From: W. Robert Hudgins, April 8, 2010
  2. Aggregation of Stylocheilus in the Philippines
    From: Marcel Tanke, July 21, 2008
  3. Re: Line of slugs and aggregations
    From: Jean-François Hervé, November 24, 2007
  4. Line of slugs and aggregations
    From: Kim Friedman, November 5, 2007
  5. Stylocheilus swarming in Cuba
    From: Alfredo Barroso, September 6, 2007
  6. Re: Meeting of the Sea Hares
    From: Ann Clear, March 26, 2007
  7. Meeting of the Sea Hares
    From: Ann Clear, March 23, 2007
  8. Re: Swarming of Stylocheilus striatus
    From: Everett M.Turner Jr., February 22, 2007
  9. Swarming of Stylocheilus striatus
    From: Everett M.Turner Jr., February 21, 2007
  10. Stylocheilus striatus from Brazil
    From: Fabio Amorim, March 22, 2006
  11. Stylocheilus striatus from Western Australia
    From: Karen Stackpole, March 13, 2006
  12. Re: 1000s of Sea Hares in Hawaii
    From: Chris Porter, November 30, 2005
  13. Aggregation of Stylocheilus striatus in Indonesia
    From: Paul Whitehead, November 29, 2005
  14. Swarming of Stylocheilus striatus at Bonaire
    From: Les Wilk, November 7, 2005
  15. Reunion Is - Swarming and spawning of Stylocheilus
    From: Philibert Bidgrain, November 5, 2005
  16. Re: Stylocheilus striatus swarms at Reunion Island
    From: Nerida Wilson, November 3, 2005
  17. Stylocheilus striatus swarms at Reunion Island
    From: Philibert Bidgrain, October 31, 2005
  18. Stylocheilus striatus from Wollongong, New South Wales
    From: Sascha Schulz, September 14, 2005
  19. Stylocheilus striatus swarming - Turks & Caicos Ids
    From: Martin Banford, July 29, 2005
  20. Swarming Sea Hares in Hawaii
    From: Jim Spears, July 8, 2005
  21. Re: 1000s of Sea Hares in Hawaii
    From: Mike Roberts, June 23, 2005
  22. Stylocheilus swarmimg in the Caribbean
    From: Jim Chambers, June 20, 2005
  23. White Hawaiian Pleurobranch?
    From: Keoki Stender, April 15, 2005
  24. Stylocheilus striatus from Brazil
    From: Vinicius Padula, February 16, 2005
  25. Stylocheilus striatus or S. striata ?
    From: Marina Poddubetskaia, January 5, 2005
  26. Stylocheilus striatus from Jamaica
    From: Ross W. Gundersen, October 21, 2003
  27. Re: Stylocheilus striatus (?) from Grenada
    From: Marli Wakeling, August 23, 2003
  28. Stylocheilus striatus from Grenada
    From: Marli Wakeling, August 22, 2003
  29. Sea hares & Lyngbya majuscula
    From: Steve Arquitt, May 24, 2003
  30. Stylocheilus striatus from Lord Howe Island
    From: W.B. Rudman, January 27, 2003
  31. Stylocheilus striatus from Sulawesi
    From: Mary Jane Adams , January 8, 2003
  32. Stylocheilus striatus from Florida
    From: Linda Ianniello, October 22, 2002
  33. Ink Gland in Stylocheilus striatus
    From: Angela Capper, September 3, 2002
  34. Early larvae of Stylocheilus striatus
    From: Duncan Revell, May 25, 2002
  35. Feeding in Stylocheilus longicauda
    From: Jan Drexel, May 14, 2002
  36. Stylocheilus striatus from Balai
    From: Stuart Hutchison, March 25, 2002
  37. Stylocheilus striatus - veliger larvae
    From: Duncan Revell, February 18, 2002
  38. Stylocheilus striatus - life history studies
    From: Duncan Revell, February 13, 2002
  39. Re: Green Nudibranch
    From: Timothy Loyd, February 24, 2001
  40. Anatomy of a spotted sea hare
    From: Brandon, February 22, 2001
  41. Green Nudibranch
    From: Timothy Loyd, February 21, 2001
  42. Sea Bunnies- Florida?
    From: John E. Billings, December 15, 2000
  43. Microcoleus & Lyngbya
    From: Clay Carlson, October 24, 2000
  44. Stylocheilus striatus and secondary metabolites
    From: Lloyd Godson, October 22, 2000
  45. Which sea hare is this one?
    From: Phanor Montoya, September 29, 2000
  46. Stylocheilus from the Caribbean
    From: Barry Lipman, September 27, 2000
  47. Re: Another aquaria slug-type creature
    From: Michelle Catalano, June 8, 2000
  48. Another aquaria slug-type creature
    From: Michelle Catalano, June 7, 2000
  49. Aplysiid from Coffs Harbour, NSW
    From: Carol Buchanan, April 28, 2000
  50. Aplysia sp.? from Papua New Guinea
    From: Mary Jane Adams, March 20, 2000
  51. Stylocheilus as food
    From: Bill Rudman, July 23, 1999
  52. Breeding & rearing Stylocheilus longicauda
    From: Gary Henderson, March 11, 1999
  53. Re: Where do I buy Sea Hares?
    From: Mark Lanett, February 3, 1999

Show factsheet and all related messages