Gastropteron pacificum

Swimming animals

Image at left, Langley Harbor, Whidbey Island, Washington, 29 January 2022

Image at right, Holmes Harbor, Whidbey Island, Washington, 22 November 2007

Both images taken by Jan Kocian

Gastropteron pacificum Bergh, 1893

            This is an intriguing cephalaspidean. It crawls and it swims. It is known from deep water and from the shallow subtidal.

            The animal is a beautiful, photogenic “butterfly of the sea.” The body color is translucent creamish, with pinkish red specks. Like all Gastropteron, it has enlarged parapodial flaps (lateral extensions of the foot). It swims quite vigorously by flapping them (Farmer, 1970). Haefelfinger & Kress (1967) illustrated the swimming behavior of the Mediterranean Gastropteron rubrum (Rafinesque, 1814). They made frame-by-frame drawings from a motion picture they had taken of a swimming animal. Note that although aplysiids and gastropterids swim by flapping their parapodia, these structures grow from different sections of their bodies.

            In recent years, scuba divers in the Salish Sea and Puget Sound region of Washington and nearby Vancouver Island have reported numerous sightings of this species from shallow subtidal depths (Rudman, 2001-2007). Karin Fletcher (of Port Orchard, Washington) wrote me, “It is not a rare species up here, meaning that without effort I can always find a few swimming in the water column in the autumn/winter.” These pelagic sightings seem to be usually of solitary individuals. There are over 70 observations of Gastropteron pacificum from this region that are posted on the iNaturalist website ( ), both of individuals swimming or crawling on the bottom. Their data seem to indicate a seasonality in the peak numbers of sightings, from October to December.

            When breeding and egg laying they occur in dense benthic concentrations. On 14 February 2019, Jan Kocian (from Whidbey Island, Washington) was diving at Langley Harbor, on a flat, sandy plain in 30 feet of water. He observed clusters of Gastropteron, none of which was swimming. All the animals were just laying on the sea floor with their white egg balloons.

He also found these egg-laying concentrations at the same locality on 5 March 2008.

            Anne Hurst (1967) described the egg masses and veligers of Gastropteron pacificum. She considered the egg mass to be of “Type C,” that is, “in the form of an ovoid or globular jelly bag attached by a jelly string. Ths is common amongst cephalaspideans” (Hurst, 1967: 256). The egg mass of G. pacificum “is almost globular and of clear jelly. It contains widely separated rounded capsules containing spherical pink eggs. The smooth-walled capsules each have a short string-like protrusion from one point on their surfaces and this does not appear to be attached elsewhere. As the eggs develop to form a ball of cells, the pink colour becomes concentrated and at one side of it is a group of yellowish cells, the whole being surrounded by a narrow layer of greenish cells” (Hurst, 1967: 268). She found egg masses in January, February, March, May, and June. Egg capsule dimensions range from 181-220 µ, and the animals take 14-15 days to hatch.

            The veliger shells are uninflated, appearing 9-10 days after the egg mass has been laid. They average 158.2 µ (+ 3.6) in length, 105.8 µ (+ 3.8) in width, and 102.6 µ (+ 4.4) in depth (Hurst, 1967: Table 9).

            Mis-information has abounded about this species, regarding both its nomenclatural history and its distribution, despite the marvelous paper by Eveline Marcus (1971). There have been two confusions.

            TAXONOMIC: Numerous publications and online references (including WoRMS, accessed 2 February 2022) erroneously cite Bergh, 1894, as the date of this species’ description. Bergh actually named this species in 1893 in his article “Die Gattung Gastropteron,” published in the Zoologische Jahrbücher Abtheilung für Anatomie und Ontogenie der Thiere, 30 December 1893. In his 1894 publication reporting on opisthobranchs collected by the Albatross expedition of 1891, Bergh actually duplicates what he had previously published, with just a couple of very minor emendations. Although he cites his 1893 paper, his repeated 1894 description (pages 202-205) begins with Gastropteron pacificum Bergh, n. sp., rather than the correct Bergh, 1893. This may have contributed to the confusion over the publication date. Note also that the type specimens of G. pacificum were not collected during the Albatross expedition, but by William H. Dall during his August 1874 expedition to the Aleutian Islands.

            DISTRIBUTIONAL: In MacFarland’s posthumously-edited and published opus magnum (1966: 4) appears the following sentence: “The species [Gastropteron pacificum] has also been taken off San Francisco and along the west coast of Central America from the Gulf of California to the Galapagos Islands by the U.S.S. Albatross in 1881.” However, the Albatross was not launched until 19 August 1882, so something is amiss. Eveline Marcus (1971) writes, “I did not find any other reference to these localities [the Gulf and the Galápagos Islands] in Bergh’s publications (1893, 1894), nor did Tokioka & Baba (1964) in their careful review of the genus Gastropteron mention the Galápagos. I suppose that the title of Bergh’s second paper: ‘Reports on the Dredging Operations off the West Coast of Central America to the Galapagos, to the West Coast of Mexico, and in the Gulf of California, in Charge of Alexander Agassiz, carried on by the U. S. Fish Commission steamer “Albatross”, during 1891, Lieut. Commander Z. L. Tanner, U. S. N., Commanding,’ has brought about this error, which should not creep into further lists.” At that time, there had been no primary references to the collecting of Gastropteron pacificum farther south than the San Diego region (Bertsch, 1969, and Marcus, 1971). Hence it is wrong to cite MacFarland (1966) as including the Gulf of California and the Galápagos in the distribution of G. pacificum.

            But that’s not the end of the story. Karin Fletcher (pers. comm., 2 February 2022) sent me links to the collection records of specimens held in various university museums.

            In “Computarización de la colección de moluscos holoplanctónicos del Golfo de California y Golfo de Tehuantepec” (Aceves Medina, 2021, ) are three records of Gastropteron pacificum from Mexican waters. It reports one specimen from the central Gulf of California, south of Guaymas, Sonora, 26.8º N; 110.1º W, January 2007, and two specimens from the southern Golfo de Tehuantepec, 13.9º N; 95.7º W, July 2007 and June 2010.

            In 2003 Ángel Valdés identified a specimen of Gastropteron pacificum in the Benthic Invertebrate Collection of the Scripps Institution of Oceanography. The specimen (Accession Number 1973.031) was dredged from 1157-1454 m depth, south of Punta Guiones, Costa Rica (9.5667̊ N; 85.6833̊ W), on 20 April 1973 by Carl Hubbs and Spencer Luke, from the R/V Agassiz.

            There are two specimens of Gastropteron pacificum in the National Museum of Natural History “collected from Colombia and Ecuador.” However, there are no data on the place, date, nor collector, hence until further study I consider them dubious (or inquirendum) references.

            So now we do have actual specimen collecting data for localities in the Gulf of California, southwestern Mexico, and Costa Rica.

            The known range of G. pacificum is from its type locality at Unalaska, Aleutian Islands (approximately 50.7º N; 165.6º W) to San Diego, California, and from the central Gulf of California (south of Guaymas) and the Golfo de Tehuantepec, Mexico, to Punta Guiones, Costa Rica. It occurs in shallow subtidal regions, and to over 1100 m deep.


            I am very grateful to Jan Kocian for his spectacular photographs and for sharing collecting data, and to Karin Fletcher for her pain-staking assistance with references and comments on draft versions of this BOW.


Bergh, R. 1893. Die Gattung Gastropteron. Zoologische Jahrbücher. Abtheilung für Anatomie und Ontogenie der Thiere 7: 281-308. (30 December 1893)

Bergh, R. 1894. Reports on the dredging operations off the west coast of Central America to the Galapagos, to the west coast of Mexico, and in the Gulf of California, in charge of Alexander Agassiz, carried on by the U. S. Fish Commission Steamer “Albatross,” during 1891, Lieut. Commander Z. L. Tanner, U. S. N., Commanding. XIII. Die Opisthobranchien. Bulletin of the Museum of Comparative Zoology at Harvard College 25(10): 125-233.

Bertsch, Hans. 1969. A note on the range of Gastropteron pacificum. The Veliger 11(4): 431-433.

Farmer, Wesley M. 1970. Swimming gastropods (Opisthobranchia and Prosobranchia). The Veliger 13(1): 73-89.

Haefelfinger, H.-R. & A. Kress. 1967. Der Schwimmvorgang bei Gastropteron rubrum (Rafinesque 1814) (Gastropoda, Opisthobranchiata). Revue Suisse de Zoologie 74(3): 547-554.

Hurst, Anne. 1967. The egg masses and veligers of thirty northeast Pacific opisthobranchs. The Veliger 9(3): 255-288.

MacFarland, Frank Mace. 1966. Studies of opisthobranchiate mollusks of the Pacific Coast of North America. Memoirs of the California Academy of Sciences 6: xvi + 546 pp.

Marcus, Eveline. 1971. Range of Gastropteron pacificum Bergh, 1893. The Veliger 13(3): 297.

Rudman, W.B., 2001-2007. Gastropteron pacificum Bergh, 1894. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Tokioka, Takasa & Kikutaro Baba. 1964. Four new species and a new genus of the family Gastropteridae from Japan (Gastropoda: Opistobranchia). Publications of the Seto Marine Biological Laboratory 12(3): 201-229.

Cat Akane with Ivette and Hans

Dr. Hans Bertsch
Imperial Beach, Calif
Feb., 2022
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