Hypselodoris imperialis

Aquarium portrait of 60 mm individual collected at Magic Island, 10 feet deep

(Photo by Hans Bertsch, 7 August 1977)

Hypselodoris imperialis (Pease, 1860)

            White notum sprinkled with numerous small yellow dots. Marginal blue line, with varying numbers of inwardly (mid-dorsally) directed flanges. Inside the individual flanges are different numbers of yellow dots (from 0-6). Adult length to about 50 mm.

            It can be distinguished from three other Indo-Pacific species with similar color patterns. Two exhibit trailing behavior. Hypselodoris ghardaqana (Gohar & Aboul-Ela, 1957) has a thin purple marginal line without the flanges, and its yellow dorsal maculations are larger in size and fewer in number. Hypselodoris sp. 11 (in Gosliner, Valdés & Behrens, 2018: 197) has pink mottling on the dorsal surface of the notum and the posterior portion of the foot; the yellow spotting on the dorsum appears denser, and it has red pigmentation rather than purple-blue (indigo) on the gills’ branches. Hypselodoris bollandi Gosliner & R. Johnson, 1999, does not exhibit trailing behavior. Its white notum has numerous yellow spots; it has a thin purple marginal line, and the purple can extend onto the center of the dorsum; the bright orange-tipped rhinophores and gills are quite obvious.

            Because no detailed study of the taxonomy, distribution, and natural history of H. imperialis has been published, below is a more extensive discussion of this species. It is a sort of “everything you didn’t know you wanted to know”!

I. Taxonomy and Original Descriptions

            The original descriptions of Doriprismatica imperialis Pease, 1860, and Chromodoris godeffroyana Garrett in Bergh, 1877, described the same unique color pattern. Pease wrote: “Colour pale cream white, and spotted above and on the sides with rich yellow; the spots are small, irregular, and very slightly raised. The mantle is margined with purple, and there are a few broken rings of the same colour on the side and upper posterior end of the foot, each ring having a yellow centre.” Bergh’s Latin description of Garrett’s species stated: “Color paginae superioris sicut faciei inferioris pallii pallide flavus punctis ochraceis ubique sparsis; margo pallii et apex podarii nigri punctis flavis ornati.” Both describe yellow spots in the purple margins. Bergh (1879) published Garrett’s drawing of the Tahitian specimen, clearly showing the yellow specks inside the marginal purple “rings.” He included (Bergh, 1879: 12, footnote 2) Garrett’s English description of godeffroyana, which states the “margins of the mantle broadly margined and lobed with deeper purple black,” without mentioning the illustrated yellow specks in the purple marginal lobes. Bergh’s first discussion of imperialis (changing the genus from Pease’s Doriprismatica to Chromodoris) describes the dorsal coloration as “Color lacteus, marginae palliali purpureus; supra et lateribus maculis numerosis luteis, lateribus et dorso podarii ocellis luteis” (Bergh, 1880: 25) is unclear whether the yellow specks also occur within the purple margin. In this work, he published a drawing by Pease of the Hawaiian animal; this was found by Garrett among the collections of the Godeffroy Museum in 1874, after Pease’s death. Contrary to Pease’s original description, it does not show any yellow specks in the purple margins. Bertsch & Johnson (1981) published color photographs they identified as living Chromodoris godeffroyana from Oahu, clearly showing the yellow specks within the purple marginal region. Note that the specimens reported as Chromodoris imperialis by Kay & Young (1969), Kay (1979), and Bertsch & Johnson (1981) are actually Goniobranchus albopunctatus (Garrett in Bergh, 1879). The synonymy of H. godeffroyana with the earlier H. imperialis has been accepted by multiple authors, including Moretzsohn & Kay, 1995.

            After the generic placement of this species in Doriprismatica and Chromodoris, it was placed in Risbecia (by Rudman, 1984; see also Gosliner & R. Johnson, 1999, and R. Johnson, 2011), but it is currently accepted as a member of the genus Hypselodoris (see R. Johnson & Gosliner, 2012, and Epstein et al., 2018).

II. Distribution

            A. Localities across the Pacific. With only a few exceptions, H. imperialis occurs exclusively in the Hawaiian Islands (Oahu and Maui). A single specimen of its synonym, H. godeffroyana, was reported by Garrett from Tahiti, about 4000 km south of Hawaii. Scott Johnson found a single specimen of H. imperialis in 1983 at Enewetak Atoll (see his website: www.underwater.com/nudi/chromodorids/e367 ), some 3400 km to the southeast. It measured 37 mm in length and was found under a rock in 5 meters of water on the lagoon-side reef of Enewetak Island, a member of the Marshall Islands, east of the Federated States of Micronesia.

            Karin Fletcher recently informed me of a DNA sequence of an H. imperialis based on specimens collected from “India: Southeast Coast.” It had been submitted to GenBank (30 March 2014) by M. Mohanraj and S. Bragadeeswarm. We view this record as highly problematic: it was not cited by Epstein et al., 2018; we have found no images of their living animal; there is no indication by them as to how the identification was made nor if they had compared it with CO1 information from Hawaiian specimens. Moreover, its citation in the UniProtKnowledgebase states the posting is “unreviewed,” i.e., a record that awaits full manual annotation. To say the least, this record needs confirmation.

            B. In the Hawaiian Islands. On two islands, its frequency of occurrences has varied during the past 50 years. During intensive studies by Scott Johnson and myself on Oahu from 1976-1980 and 1982, it was commonly observed at the Ala Wai Yacht Harbor Canal (~21º 17' N; 157º 50" W). On some days we found 11-15 animals on a single dive, but on other days the count reached highs of 28-35. Sometime in 1983, the species apparently disappeared from Oahu. Cory Pittman has been studying the nudibranchs on Maui since 1978. The first H. imperialis he saw was in the fall of 2000. After that, the population increased rapidly, and from 2009 to the spring of 2019 they were one of the most commonly seen species on Maui (Cory Pittman, pers. comm., January 2021).

            C. Bathymetry. This species has not been reported from the intertidal, neither on Oahu nor Maui. Neither was it found in earlier intertidal studies (Kay & Young, 1969; Kay, 1979). The reports by Pease, Garrett and Bergh do not specify the collection depth. On Maui most H. imperialis were found subtidally between depths of 3-10 m (Pittman, pers. comm., January 2021) On Oahu we found them from 1.5-15 m deep (see bathymetric chart in Bertsch & Johnson, 1980), although most were in the 1-4 m depth range.

III. Pairing and Trailing

            Hypselodoris imperialis is one of several Indo-Pacific species, including H. tryoni (Garrett, 1873), that exhibit “trailing behavior” (Gosliner, Valdés & Behrens, 2018: 197-198), where the animals are usually found in a touching queue. Based on 166 scuba divers from various sites on Maui, 8.4% of H. imperialis sighted occurred as “singletons,” that is, one per sighting; 91.6% were in groups of two or more (Pittman, pers. comm., Jan. 2021). At the Magic Island site facing the Ala Wai Canal on Oahu, Scott and I made 17 dives between 11 March to 3 August 1978, counting 306 observed animals, in 148 total sightings. Of these, 23 (7.5%) of the sightings were singletons, 107 (69.9%) were pairs, 5 (4.9%) were threesomes, 12 (15.7%) were foursomes, and once 6 were together. The average was 2.07 individuals per sighting. This species usually occurs and will be observed in pairs.

IV. Unique patterns and trying to track their movements

            A. Color pattern variations. The variations in color pattern reported in the first descriptions of this species are quite significant. Surrounding the notum is a band of navy blue, which periodically extends inwards over the dorsum as small fingers or flanges (called “rings” by Pease). In each flange several yellow dots may occur or be completely absent. The number and sequencing of these dots on each side of the body is unique for each individual. Thus, there is no need to use artificial dyes, notches in the mantle, or other marking devices that would disturb or damage the animal or change its appearance. Individuals can be identified by counting the dots in each flange: this is recorded simply as a series of numbers, e.g., left side 0121411, right side 120011001. Because of this unique pattern for each individual, we theorized that one could re-find and re-identify an individual animal (Johnson & Bertsch, 1979), and calculate the distance it had traveled between sightings.

            This identification method was recently used by Lindsay et al., 2018, who reported that individual specimens of Diaulula sandiegensis (Cooper, 1863) and D. odonoghuei Steinberg, 1963, were identified in the field by their unique dorsal spotting patterns. They also reported finding the same individuals over a 3-4 day period during a series of low tides, and then relocated them in the same general area about two weeks later. However, because their study was concerned with using DNA and morphology to identify pseudocryptic species, no movement/distance traveled information was provided.

            B. Habitat site. Magic Island is a man-made structure consisting of rock fill that has formed a 10 m vertical cliff face on the western side of the Ala Wai Canal, a dredged channel serving the boat harbor, and draining refuse from the Manoa and Palolo Valleys. We found the H. imperialis in abundance on the cliff face. This vertical wall contains many holes and crevices, has a high sponge cover, and large amounts of other filter feeders such as tunicates and sessile coelenterates–the alcyonarian Telesto and hydroids. Underwater, the visibility is poor, and there is a brown silty muck at the bottom of the channel. Silt coats the rock face, often covering the turquoise sponge Dysidea fragilis, the prey item of H. imperialis.

             C. Establishing grid on the cliff face. Using scuba, we marked off a 50 m horizontal line. Data collection consisted of recording each individual’s marking (dot pattern and size), its location on the horizontal line and its depth (resulting in grid coordinates of the location), and whether it was associated with another (in which case each animal’s distinct dot pattern was recorded). We recorded these data with pencil on an underwater slate that had a ruler attached. Photographic records were not used; our Nikonos II cameras only housed film rolls of 36 exposures. These raw data are presented in Spreadsheet 1.

            D. Identifying individuals as re-found. We then ordered the codes in Column D in numerical sequence. We re-found a total of 7 individuals (marked as “MATCH” on Spreadsheet 2 . Five individuals were re-found once, one was re-seen twice, and one was re-seen three times.

            E. Distances between sightings. To calculate the distances between findings, we first converted the original feet depth measurements into meters for consistency in graphing. We then used the Distance Formula (an application of the Pythagorean Theorem): d = √ (X2 - X1)2 + (Y2 - Y1)2 to give a straight-line measurement between the two sighting sites. We chose this vector analysis as the distance traveled, because we could not observe continuously all their movements, which most likely included some meandering from the straight line. These distances and days between sightings are given in Table 1.

            Total distances traveled between sightings ranged from 0.5 m to 26.028 m. The average distance traveled per day for each individual ranged from 0.06 m to 5.5 m. The average distance between sighting for all individuals was 1.1 m/day. Millimeter precision is an artifact of the calculating formula, so we rounded off the averages at the cm level.

            Basically, it can be said that the animals are moving, but not too far. Any distances the animals moved back inside and among the rocks of the cliff face could not, of course, be measured.

            F. Questions and problems. Since they seem not to move far, why didn’t we encounter more animals a second, third or more times? The unscientific answer is, “Hmmm, good question!”

            Several issues need addressing:

            1) “In and Out.” On the vertical, bouldered rock face, there are numerous interstices or spaces between and behind each rock where the animal can move. It seems probable and parsimonious that the slugs do not remain on the canal-facing side of the rocks, but will move into those hidden areas not visible to a research diver in the water.

            2) How can we determine the relation between “match,” “very close,” and “close”? Should some of the latter be considered “matches”? We have chosen to use only the “matched” sightings in our calculations.

            3) Do the spot numbers in a flange change? There are growth-related changes in the size and number of spots on the center of the dorsum of H. imperialis (compare Scott Johnson’s photos of small individuals under 20 mm in length with the images of large adults). Scott's Images: h148 o06 and h148 o12 A small number of large spots changes to a larger number of smaller spots. This is well known among other chromodorids, such as Felimida norrisi (Farmer, 1963) and Chromolaichma dalli (Bergh, 1879) (illustrated in Bertsch & Aguilar Rosas, 2016: 265, 267). Lindsay et al., 2016, reported color pattern growth changes for the “ringed” Diaulula sandiegensis and the “spotted” D. odonoghuei. Dorsal spots enlarged in both species with growth. As the spotted morphs grew, additional spots appeared on the edge of the mantle (this did not occur in the “ringed” species). The question remains, are there spot changes within the purple flanges along the mantle margin?

            4) Were there human mistakes in the counting? Possibly, but certainly not to the extent it would significantly affect the number of observed matches.

V. For future studies

            The daily movements of many animals–especially ones considered charismatic, e.g., sharks, whales, wolves, tigers, and elephants–have been and are being tracked for very good reasons. They are all key endangered members of key endangered ecosystems, with obvious relationships to humans and the health of our fragile, interconnected global ecosystem. Tracking a 50 mm long nudibranch occurring on the rocky cliff face alongside a yacht harbor exit? Why not? No matter how important or unimportant (by some standards) an animal may seem, it is a living being and contributes to the overall functioning of our planet’s single ecosystem. Besides, regardless of the priority one may place on the species, it’s an interesting (albeit small) part of much larger concerns, such as the effects of global climate change.

            For a study of this nature to succeed, two things are needed: a relatively large population of the species, and a “grid-able” area. We found these at our Magic Island site. Our biggest concern was safety. Many times we would look at a part of the rocky cliff face and see paint scars left by a boat (yacht?) that had scraped the rocks. If we heard a boat while diving, we would become quite alert, often dropping to a lower depth, not wishing to be between a rock and a hard boat.

            The grid should be clearly (but unobtrusively) marked out for replicable repeated sampling. Photographic series of images covering the same grid area over time would be very significant, a method used in recording successional changes on settling plates (e.g., Birkeland, 1977, Lindeyer & Gittenberger, 2010). Each survey should cover the same grid area and in the same sequence. The surveys should probably be conducted daily. If diver accessibility is a problem, one might program a small ROV to make rows of horizontal passes across the grid, photographing the site in the process.

            In summary, be safe, and communicate your enthusiasm for, and discoveries of, these fouydroyant creatures. DNA techniques are determining evolutionary relationships and new cryptic or pseudocryptic species. Our observations can find out what they are doing, and how they are living their lives.


            I am deeply grateful to Scott Johnson for his collaboration in the design of this project, in the field work, and in the analysis of the data. Our teamwork was essential to this study of “The Imperial Slug.” I also thank Cory Pittman of Maui and Karin Fletcher of Washington State for valuable comments and information they shared. And of course to all the divers who helped us on our Earthwatch Expeditions on Oahu during the summers of 1978, 1979 and 1980, mahalo nui loa.

Literature Cited

Bergh, L. S. R. 1877. Malacologische Untersuchungen 11. In: C. Semper (ed.), Reisen im Archipel Philippinen. Wissenschaftliche Resultata, 2 (2) 12: 429-494.

Bergh, L. S. R. 1879. Neue Nacktschnecken der Südsee. Journal de Museum Godeffroy 5 (14): 1-50.

Bergh, L. S. R. 1880. Malacologische Untersuchungen. In: C. Semper (ed.), Reisen im Archipel der Philippinen. Wissenschaftliche Resultata, 2 (4) 1: 1-78.

Bertsch, Hans & Luis E. Aguilar Rosas. 2016. Invertebrados Marinos del Noroeste de México / Marine Invertebrates of Northwest Mexico. Instituto de Investigaciones Oceanológicas, UABC, Ensenada, xxxii + 432 pp.

Bertsch, Hans & Scott Johnson. 1980. Preliminary analysis of the geographic and bathymetric distribution of Hawaiian chromodorids (Gastropoda: Opisthobranchia). The Festivus 12 (6): 73-77.

Bertsch, Hans & Scott Johnson. 1981. Hawaiian Nudibranchs. Honolulu, Oriental Publishing Co., 112 pp.

Birkeland, Charles. 1977. The importance of rate of biomass accumulation in early successional stages of benthic communities to the survival of coral recruits. Proceedings, Third International Coral Reef Symposium (Rosenstiel School of Marine and Atmospheric Science, University of Miami): pp. 15-21.

Epstein, Hannah E., Joshua M. Hallas, Rebecca Fay Johnson, Alessandra Lopez & Terrence M. Gosliner. 2018. Reading between the lines: revealing cryptic species diversity and colour patterns in Hypselodoris nudibranchs (Mollusca: Heterobranchia: Chromodorididae). Zoological Journal of the Linnean Society (2018) XX: 1-74.

Gosliner, Terrence M. & Rebecca F. Johnson. 1999. Phylogeny of Hypselodoris (Nudibranchia: Chromodorididae) with a review of the monophyletic clade of Indo-Pacific species, including descriptions of twelve new species. Zoological Journal of the Linnean Society (1999) 125: 1-114.

Gosliner, Terrence M., Ángel Valdés & David W. Behrens. 2018. Nudibranch and Sea Slug Identification. INDO-PACIFIC, second edition. Jacksonville, Florida, New World Publications, Inc. 451 pp.

Johnson, Rebecca F. 2011. Breaking family ties: taxon sampling and molecular phylogeny of chromodorid nudibranchs (Mollusca, Gastropoda). Zoologica Scripta 40 (2): 137-157.

Johnson, Rebecca Fay & Terrence M. Gosliner. 2012. Traditional taxonomic groupings mask evolutionary history: a molecular phylogeny and new classification of the chromodorid nudibranchs. PloS ONE 7 (4): e33479 [15 pp.]

Johnson, Scott & Hans Bertsch. 1979. A population study of the nudibranch Chromodoris godeffroyana. Western Society of Malacologists, Annual Report 11: 8.

Kay, E. Alison. 1979. Hawaiian marine shells. Reef and Shore Fauna of Hawaii. Section 4: Mollusca. Bernice P. Bishop Museum Special Publication 64 (4): xviii + 653 pp.

Kay, E. Alison & David K. Young. 1969. The Doridacea (Opisthobranchia: Mollusca) of the Hawaiian Islands. Pacific Science 23 (2): 172-231.

Lindeyer, Frederike & Adriaan Gittenberger. 2010. Ascidians in the succession of marine fouling communities. Aquatic Invastions 6 (4): 421-434.

Lindsay, Tabitha, Julie Kelly, Anton Chichvarkhin, Sea Craig, Hiroshi Kajihara, Joshua Mackie & Ángel Valdés. 2016. Changing spots: pseudocryptic speciation in the North Pacific dorid nudibranch Diaulula sandiegensis (Cooper, 1863) (Gastropoda: Heterobranchia. Journal of Molluscan Studies (2016): 1-11.

Pease, William H. 1860. Descriptions of new species of Mollusca from the Sandwich islands. Proceedings of the Zoological Society of London 28: 18-37.

Rudman, W. B. 1984. The Chromodorididae (Opisthobranchia: Mollusca) of the Indo-West Pacific: a review of the general. Zoological Journal of the Linnean Society 81 (2/3): 115-273.

Hans with granddaughter Adriana Ivette Cadena (of Tenellia ivetteae fame) and her cat Akane, safely quarantined in Imperial Beach.

Dr. Hans Bertsch
Imperial Beach, Calif
May, 2021
Send Hans email at hansmarvida@sbcglobal.net