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without ocelli
 

ocelli detail
 

interrupted lines
 

side
 

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young, 4 mm
 

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drifting
 

swarming


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GALLERY



 
Stylocheilus striatus
(Quoy & Gaimard, 1832)
 
Stylocheilus striatus
Maximum size:  57 mm (Ostergaard, 1955).

Identification:  This is a small, papillate sea hare with low parapodia. Its body is cream marked with fine, longitudinal brown lines that may be interrupted in some animals. It is decorated with blue and gold ocelli.when mature (young animals lack ocelli and the ocelli gradually become more prominent and numerous with age). Mature animals lack a shell although it's present in very small juveniles.

Natural history:  Stylocheilus striatus is probably the most common opisthobranch in Hawaiian waters, often representing over 50% of the animals in quantitative samples. It is found in rocky habitats and Halimeda kanaloana beds in protected to highly exposed locations. It occurs in tide pools and subtidally from depths of < 1 to 34 m (< 3 to 110 ft). It is primarily nocturnal although it often remains in the open during the day. It feeds on cyanobacteria (blue-green algae), particularly of the genus Lyngbya, and the blue and gold ocelli may represent aposematic coloration "advertising" the presence of toxins concentrated from its food. During any month of the year (though more commonly in spring/summer), when populations are high, it can form vast "locust-like" swarms that migrate across the bottom stripping the substrate of cyanophytes as they advance. (Note 1) When food is exhausted, entire populations may release from the bottom and drift off on the current with the animals using mucous strings as "sails." (Note 2) Most drifting seems to occur at night although it's possible that populations may release during the day in response to stress. (Note 3) Because of this behavior, large numbers may suddenly appear in new areas when currents concentrate and deposit drifting swarms. Stylocheilus striatus releases a purple dye when disturbed (though the "sensitivity" varies) and lays a tangled, golden-brown egg string that hatches in six to seven days in the laboratory.

Distribution:  Big Island, Maui, Lanai, Molokai, Oahu, Kauai, Niihau, French Frigate Shoals, Laysan, Midway and Kure: circumtropical.

Taxonomic notes:  This is the species listed in Kay, 1979 and Hoover, 1998 as Stylocheilus longicaudus (Quoy and Gaimard, 1824). It is referred to as the "lined sea hare" in Hoover, 1998 & 2006. It was first reported from Hawaii in Gould, 1852 (as Stylocheilus lineolatus) and is listed in Edmondson, 1946 and Ostergaard, 1955 as Notarchus lineolatus. It's listed in Ostergaard, 1950 as Notarchus striatus. Aclesia areola Pease, 1860 is a synonym.

Photo:  CP: ocellate; 50 mm: Hekili Point, Maui; July 12, 2005.

Observations and comments:

Note 1:  We've observed vast swarms composed of many thousands of animals on the bottom on several occasions, both by day and night. By day, they seem to form more concentrated "windrows." At night, they seem to spread out more, presumably while feeding. Repeated observations suggest that they migrate slowly across the bottom.

Note 2:  On several occasions at Hekili Point, Maui we've noted large populations appearing or disappearing over a few days in an area of the trough that we visit regularly. It is well protected by an offshore crest but has a moderate long-shore current that tends to concentrate drifting material. On at least three occasions during low tides at night, we've observed very large numbers of S. striatus on the bottom with moderate to very large numbers drifting in the water column suggesting recent settlement or release. One such sequence may be particularly informative. In fall, 2007 we'd noted an area with unusually rich cyanophyte growth (primarily Lyngbya majuscula) in the trough west of the point. During several visits between Oct. 30 and Nov. 25 we'd seen only a few scattered S. striatus in the area. However, during low tide on the night of  Nov. 26 we found immense swarms covering the bottom. During low tide on the night of Nov. 27, we encountered many thousands of animals drifting in the water column starting in the cyanophyte area and continuing down-current for 100-200 m. When the bottom was examined in detail during the day on Nov. 28, it was found that the cyanophytes had been stripped from the substrate and there were only moderate numbers of scattered S. striatus remaining. Subsequently, moderate numbers were seen through Dec. 11 during several visits. From Nov. 25 till Nov. 28 the surf was low and there was no sign of mass mortality. Such observations would seem to support the suggestion that the species uses a "release and drift" strategy to move between patchy concentrations of their food algae.

Note 3:  On May 30, 1992 in the trough just east of Hekili Point, an area with somewhat restricted circulation and depths of 1-2 m (3-6 ft), we saw many hundreds drifting in the water column when returning to shore from a snorkel in the late afternoon. It was at high tide and during a period of  unusually low surf and warm weather. Although the resident population in the area hadn't been previously noted, the circumstances suggest that they had recently released from the bottom. The drifting animals were of all sizes and the water seemed unusually warm. This might support the suggestion that populations may also release and drift in response to high temperature or other stress. The animals appeared healthy and there was no sign of mass mortality.
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