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Speculation on the Taxonomy of Hexabranchus
 
by Cory Pittman
I've updated this article to better reflect my current thinking as of fall, 2011.

This whole thing started in 2002 as a result of ongoing conversations with Scott Johnson. For some time, we'd been discussing the status of the Hawaiian Hexabranchus spp. He'd been arguing for multiple species and leaning toward a three species interpretation as reflected in Bertch & Johnson, 1981 and Kay, 1979 while I'd been arguing for lumping everything under H. sanguineus. Then, he sent me copies of all his photos and Pauline sent me copies of all the photos taken by her and Mike Severns. It turned out that Scott had a good growth series for H. aureomarginatus and Pauline had a good growth series for H. pulchellus. So, with both in hand (plus some others from John Hoover), I had enough data to look at the ontogeny. As a result, I also switched to favoring multiple species but re-sorted everything in accordance with a two species interpretation. This is illustrated in the following composite. The left hand column, showing H. aureomarginatus, is based entirely on photos by Scott (some cropped and/or flipped). In the right hand column, showing H. pulchellus, the 2nd, 4th, and 5th photos are by Scott while the others are either by Pauline or Mike Severns. As you can see, both species have complex ontogenies with changes in pattern continuing throughout the animal's life. And, based not only on these photos but on several dozen others (plus observations of live animals), the differences between them appear to be consistent and without obvious intergrades. Since it seems unlikely that such well-defined and complex differences could result from environmental factors in sympatric populations, I would argue that this supports the conclusion that they are valid species. Differences in the radius of coiling and height of the egg masses would also seem to support that conclusion.

Hexabranchus Growth Series
 
Growth series: left = Hexabranchus aureomarginatus, right = Hexabranchus pulchellus

After looking at the Hawaiian material, I got curious about what was happening in the rest of the Indo-Pacific. So, I decided to try an exercise in "data mining" in order to increase my sample of available photos. Since "everyone" photographs Hexabranchus, I got busy with Google, Yahoo, Alta Vista, etc and pulled up as many Hexabranchus photos as I could find on line. That gave me a reasonably large sample to work with (around 1300 links at the time--many more are available now) and I've been trying to make sense of them ever since.

As of my last reassessment, Hexabranchus spp. appear to fall into four quite well-defined groups:  three broadly sympatric in the Indo-Pacific plus one confined to the Caribbean. Supporting evidence for distinction of the sympatric Pacific groups is also provided by the fact that no photos I've seen of paired animals show mixing between them.

The following summary is only intended as a brief description of this result. Therefore, I'm just highlighting a few of the distinguishing characteristics. I'll expand on them and go into greater detail on request. Also, I should state up front that all of this is tentative, pending confirmation by DNA. In addition, it's not intended as commentary on other work such as Valdez (2002) that reached different conclusions.

To illustrate the groups,  I've added links "sorting" the photos posted on four major sites (The Sea Slug Forum, Nudipixel, Medslugs and Umiushi-zukan). I've also added links to a few other photos to illustrate forms not included on those sites and listed references to some text sources.
__________________________________________

GROUP  #1

In juveniles, the inner face and tip of the rachis are red but the outer face lacks red pigmentation. As the animals approach maturity, opaque white pigment develops on the outer face of the rachis starting at the tips of the terminal branches and expands downward to cover the entire outer face and base in the terminal phase. Pinnae may vary from yellow to red depending on population/species but do not develop white-frosted tips. The group is also characterized by the presence of contrasting lateral patches on the notum that are truncated dorsally (at least at some stage in their development). It differs from groups #3 and #4 in that it lacks violet/blue spots on the notum in early juveniles and from group #2 in that the notum lacks both a broadly reticulate pattern and a "quilted" surface texture in adults.

Three behavioral traits may also support its separation from groups #3 and #4 (while possibly uniting it with group #2):  On average, the egg masses are relatively loosely coiled compared to groups #3 & #4 suggesting a larger turning radius while laying. Photos apparently taken in the field and anecdotal reports by Pauline, John and Scott for H. aureomarginatus suggest that adult animals may remain in the open more commonly during the day than in groups #3 & #4. Beginning at maturity and coinciding with the appearance of the opaque white pigment on the rachis, the branchia are generally held in a more erect posture than in groups #3 and #4. Combined with the frequency of missing or regenerating branchia in photos (particularly in the Red Sea), this suggests that the contrasting branchia may be acting as lures in a manner similar to the dorsal "horns" in Ceratosoma (perhaps, it would be interesting to check them for concentrations of antifeedant compounds?).

It seems likely that it represents a complex rather than a single species. The available photos suggest that there are at least five populations that are sufficiently consistent internally but sufficiently distinct from each other to raise the possibility that they are valid endemics. Listed, roughly from west to east, they are: H. sanguineus, H. sp. #3, H. sp. #4, H. sp. #7 and H. aureomarginatus. All of them are allopatric, however, and I don't know what's happening in the gaps. So, of necessity, the splits remain tentative pending DNA work. And, there are enough regions that aren't covered by my sample to keep open the possibility that there may be more than five species.

Hexabranchus sanguineus (Ruppell & Leuckart, 1831)

Confined to the Red Sea. Sympatric with H. sp. #2.

Distinguished from all other group #1 spp. except H. sp. #3 by the lack of white pigment of some type on the notum. Distinguished from H. sp. #3 by its bright red background when mature and the presence of a white marginal line in mature animals.

Photos of juveniles are unavailable. But, based on patterns of variation within the group and the youngest available photos, they are probably translucent pink and may lack a marginal white line. With growth, the background darkens to orange/red, dark red lateral patches appear on the notum and a dark red marginal band appears on the dorsal surface. With maturity, the background continues to darken and white markings become more prominent on the rachis and rhinophores. A narrow white marginal line may appear converting the marginal red band to submarginal and the background may become dark enough to obscure the lateral patches and submarginal band. In terminal animals, the marginal white line often becomes wider and more prominent.

It is probably more closely related to H. sp. #3 than other members of the group.

See the following linked photos for illustration:

Sea Slug Forum: 17493, 15168, 15167, 13977, 6724, 5160, 5153, 3789, 3754, 2964, 747 (bottom), hexasang (middle)
Nudipixel: 2127, 2516, 1833, 2125, 2126, 5141, 5142, 5383, 5384, 8502, 17001, 10285, 10729, 10730, 11140, 11328, 12353, 16645, 11136, 11544, 17214, 12478, 12479, 13380, 13381, 19127, 19139, 23389, 24028, 27519, 27522, 28555, 28964, 29545, 33914, 33915, 33916, 38327, 40124
Umiushi-zukan:  23690
Medslugs: 001, 002, 003, 004, 008, 009, 010, 011, 016, 017, 018, 019, 020, 025, 031, 032, 033, 034, 045, 054, 055, 056, 057

Also, the following text sources:

Coleman, 2001: p 20
Coleman, 2008: p 41; p 315--4th photo
Debelius, 1996: p 123--photo 6; p 198--photo 1
Debelius & Kuiter, 2007: p 256--3rd photo
Humann & Deloach, 2010: p 296--3rd photo
Yonow, 2008: pp 145 & 146

Hexabranchus sp. #3

Found along the coast of Eastern Africa from South Africa to Kenya, then east through Mauritius and the Seychelles to Sri Lanka. Sympatric with H. sp. #1 and H. sp. #2.

Distinguished from all other group #1 spp. except H. sanguineus by the absence of white pigment on the notum. Distinguished from H. sanguineus by lack of a bright red background in mature animals and by the presence of a white marginal line in juveniles that usually disappears in adults.

Photos of juveniles are unavailable. But, based on patterns of variation within the group and the youngest available photos, they are probably translucent yellow with a marginal white line. With growth, the background darkens to yellow, dark red lateral patches appear on the notum and a dark red submarginal band (sometimes divided) appears on the dorsal surface. With maturity, the background continues to darken to bright yellow and white markings become more prominent on the rachis and rhinophores. The white marginal line may be lost converting the submarginal red band to marginal. In terminal animals, the background may darken to a somewhat dusky yellow-orange but does not achieve the deep red found in H. sanguineus.

It is probably more closely related to H. sanguineus than to other members of the group.

See the following linked photos for illustration:

Sea Slug Forum: 20510, 20413, 20409, 14788, hexasang (bottom two)
Nudipixel: 1063, 4921, 16748, 17022, 12921, 13423, 14363, 24014, 25145, 32250, 41212, 41213, 42468
Medslugs: 012, 013

Also, the following text sources:

Debelius, 1996: pp 198 & 199--photos 2 & 4 (terminal form)
Debelius & Kuiter, 2007: p 256--1st photo

Hexabranchus sp. #4

Found throughout the coastal waters of both western and eastern Australia, east to Lord Howe and New Caledonia, north through Bali and Borneo to Taiwan and Southern Japan, then east to at least Saipan. Sympatric with H. sp. #1 and H. sp. #2.

Distinguished from all other group #1 spp. by the presence of minute white flecks on the notum, often in clusters. Distinguished from H. sp. #7 and H. aureomarginatus by the absence of subcutaneous white rosettes at all growth stages.

Juveniles are translucent gray with a marginal white line and a frosting of white flecks. With growth, the background becomes translucent yellow with dusky areas, then darkens to light orange. Dark orange lateral patches appear on the notum and a dark orange submarginal band may develop. With maturity, the white flecks often form more pronounced clusters (or fuse into cream patches) while the background continues to darken. Secondary dark patches often develop between the lateral patches and the submarginal band. The marginal white line is often lost rendering the submarginal orange band marginal and some animals may develop a narrower and darker marginal band creating a "two-toned" effect. In terminal animals, the background may darken to red obscuring the lateral patches and marginal band.

The populations included under this species are almost certainly more closely related to each other than to the other nominal spp. listed in group one. However, H. sp. #4 shows much more intraspecific variation than the others. So, it's possible that future DNA work could result in further splits.

See the following linked photos for illustration:

Sea Slug Forum: 17274 (top & center-right), 4193, 23266
Nudipixel: 1491, 17220, 18964, 26011, 29517, 32551
Umiushi-zukan: 16507, 16486, 11044, 6733, 19807, 5677, 7666
Mikadoumiusi (front page): 0306, 0307, 0314, 0315, 0316
Gary Cobb (front page): 9276 (top), 9279, 9342, 9337, 9309, 9310, 9331, 93411, 7363 (top)
potatocod.com (front page): 001, 002, 003

Also, the following text sources:

Coleman, 2001: p 41--4th, 8th & 11th photos
Coleman, 2008: p 314--5th photo; p 315--5th & 6th photos
Debelius & Kuiter, 2007: p 255--3rd photo
Hazime, et. al.,1986: p 224--2nd photo
Herve, 2010: p 182--6th photo
Marshall & Willan, 1999: p 207--Fig. 73
Nakano, 2004: p 119--3rd photo
Ono,  2004: p 127--4th photo
Wells & Bryce, 1993: p  94--photo 108

Hexabranchus sp. #7

Found in Raratonga and French Polynesia. Sympatric with H. sp. #1.

Distinguished from all other group #1 spp. except H. aureomarginatus by the presence of white subcutaneous rosettes. Distinguished from H. aureomarginatus by the absence of a broad yellow-white marginal band and retention of the dorsal rosettes in the terminal phase. Animals from French Polynesia have a yellower background with fewer white rosettes and some faint, pale mottling on the notum (at least in sub-terminal animals).

Photos of juveniles are unavailable. But, based on patterns of variation within the group and the youngest available photos, they are probably translucent pink without a marginal white line. With growth, subcutaneous white rosettes develop and expand to cover most of the animal. Dark yellow-brown lateral patches appear on the notum and a dark pink submarginal band develops. Meanwhile, the background may darken through yellow to orange. With maturity, the background continues to darken to red obscuring the lateral patches and submarginal band while the white rosettes (as well as white markings on the rachis and rhinophores) become more prominent.

It is probably more closely related to H. aureomarginatus than to other members of the group.

See the following linked photos for illustration:

Bishop Museum Cook Island Biodiversity Database (front page): rrjs1, rrgm5, rrjs6
Nuditahiti (front page): Hexabranchus sp. 2, Fabian #1, Fabian #2, Fabian #3

Also, the following text sources:

Salvat and Bacchet, 2011: p 221--sp. 1

Hexabranchus aureomarginatus
Ostergaard, 1955


Found throughout the main Hawaiian Islands and north to Midway Atoll. Sympatric with H. pulchellus.

Distinguished from all other group #1 spp. by a yellow-white marginal band that's retained throughout the life of the animal and by the paint-like patches of white pigment that develop in the terminal phase. Distinguished from H. sp. #7  by restriction of subcutaneous white rosettes to the spaces between the lateral patches and their appearance only in sub-terminal animals.

Juveniles are translucent gray with a yellow marginal band. With growth, the background becomes cloudy and a few subcutaneous white rosettes develop on the notum. Dark red lateral patches appear on the notum and a red submarginal line may develop. With maturity, the background darkens to red obscuring the lateral patches and submarginal line. In terminal animals, the white rosettes "emerge" and fuse forming irregular patches of dense, superficial white pigment. Patches of such pigment may also appear elsewhere on the body and the marginal band may become white.

It is probably more closely related to H. sp. #7 than to other members of the group.

See the left side of the composite photo for illustration as well as the species page on this site.

Also, see the following linked photos:

Sea Slug Forum: 4953, 4952, 4949
Medslugs: 059, 061
Nudipixel: 1529, 3223, 6537, 6540, 6541, 7541, 8643
Umiushi-zukan: 1338

Also, the following text sources:

Bertsch and Johnson, 1981: cover; pp 31, 32 & 33
Hoover, 1998: p 172
Kay, 1979: p 472--Fig. 151-G

GROUP #2


In juveniles, the rachis is uniformly translucent yellow-gray with no red lines on either face. This lack of red lines is retained throughout the life span and neither opaque white lines nor cloudy white pigment develop with maturity. Rather, in mature and terminal animals, bright yellow pigment appears apically on the outer face of the rachis and brick-red blotches appear basally. The pinnae vary from red to yellow and do not develop white frosting. It's also distinguished from groups #3 and #4 by the lack of spotting on the notum in juveniles.

As in group #1, the egg masses appear to be more loosely coiled than in groups #3 & #4 suggesting a larger turning radius when laying. The available photos suggest that it often remains in the open during the day and the relatively high frequency of paired animals in photos suggests that mating animals may stay together longer than in other Hexabranchus spp. It appears to be restricted to a relatively deep habitat (seldom less than 70 ft) and the lack of variation over its broad geographic range suggests that the larvae may remain in the plankton for a longer period than in other Hexabranchus spp. The branchia may be held in a more erect posture in adults but the sample size of juvenile and transitional animals is too small to be sure. The uniformly "quilted" appearance of the notum in resting mature animals suggests a possible difference in dorsal musculature. There may also be a tendency toward basal fusion in the paired branchia giving large animals the appearance of having fewer than six gills.

Although the group is found throughout the Indian Ocean and Western Pacific, it seems likely that it is monotypic based on the minimal variation in color.

Hexabranchus sp. #2

Found from South Africa north to the Red Sea, across the Indian Ocean to Australia, north to Japan and east to Fiji. Sympatric with H. sanguineus, H. sp. #1, H. sp. #3 and H. sp. #4.

Distinguished from all other Hexabranchus spp. by the broadly reticulate pattern on the notum of mature animals, bright yellow rhinophores, broad reddish spots on the underside of the mantle, etc. It may also reach the largest size of any Hexabranchus sp.

Juveniles are solid violet or pink with white marginal lines and red rhinophores. With growth, large dark patches appear on the notum and become better defined and more extensive with age, ultimately "connecting" to create the broadly reticulate pattern of dark pigment seen in adults. The background becomes translucent gray ventrally and yellow to pink dorsally with the marginal line and rhinophores becoming yellow. Dark blotches also appear between the foot and the margin. With maturity, the background may darken further but  remains at least slightly lighter than the reticulate pattern. Mature animals also develop a broad dark submarginal band on the notum, usually containing lighter blotches or lines. The overall tone of the dark pigment varies from dusky yellow to dark red but the underlying pattern remains the same (suggesting, perhaps, that the red pigment is acquired from its food?).

See the following linked photos for illustration:

Sea Slug Forum: 20682, 19636, 14178, 6046, 5818, 2965, 2634, 20975 (bottom)
Nudipixel: 446, 6869, 6878, 13505, 21933, 13285, 31198
Medslugs: 015, 026
Umiushi-zukan: 22710, 20585, 16851, 15416, 283, 2874, 26203
Slug Site (front page): 125, 243 (yellow form)

Also, the following text sources:

Colin & Arneson, 1995: p 185--photo 866
Coleman, 2008: p 24--2nd photo; p 313--2nd photo; p 315--1st & 2nd photos; back cover
Debelius, 1996: p 197; pp 198-199--photos 7, 8, 9 & 10
Debelius & Kuiter, 2007: p 255--2nd photo
Hazime, et. al.,1986: p 224--1st photo
Herve, 2010: p 181
Nakano, 2004: p 119--4th photo
Takamasa 2003: p 73--1st & 3rd photos
Yonow, 2008: pp 147 & 148

GROUP #3

In group #3 juveniles, the branchia have a red line on both the inner and outer face of the rachis, a characteristic that is retained throughout the life of the animal. With increasing size, a frosting of cloudy white pigment appears on the tips of the pinnae. In mature and terminal animals, cloudy white pigment appears on the base of the rachis and expands apically with growth (but doesn't completely obscure the red lines even in very large animals). Unlike in groups #1 & #2, juveniles have violet/blue spots on all or part of the notum and the egg masses, on average, are tightly coiled suggesting a small turning radius while laying. Also, there is no difference between juveniles and adults in branchial posture. Unlike in groups #1 & #2, mature animals seldom remain in the open during the day.

It's found throughout the Indo-Pacific and appears to contain at least two species.

Hexabranchus sp. #1:

Found from South Africa north to Kenya, across the Indian ocean to Australia, north to Japan, South to Lord Howe Island and east to the Marshalls and French Polynesia. Sympatric with H. sp. #2, H. sp. #3, H. sp. #4, and H. sp. #7.

Juveniles have violet/blue spots on the notum, narrow white submarginal lines on both the dorsal and ventral surfaces of the notum, cream rhinophores with orange tips, a cream background and a variably developed bluish marginal/submarginal band. Early transitional animals develop a darker background and loose the spots in the center of the notum (although the size at which the central spots are lost is variable and some very young animals may not show them at all). The submarginal white lines are also lost. Transitional and mature animals have white pigment on the rhinophore lamellae, white specks on the notum, orange rhinophores and indistinct reddish spots on the ventral submargin. Mature animals have a broad white marginal band that extends onto the ventral surface of the mantle, a broad red submarginal band (often interrupted with a narrow white line) that is irregularly scalloped and sharply margined medially and a broad cream mid-lateral area variably flecked with red. The lateral patches are poorly defined and arcuate medially. In terminal animals, dark pigment fills in the mid-lateral area and (generally lighter) brown/red pigment may obscure both the dorsal white spots and submarginal ventral red spots. The white pigment on the lamellae fades to cream and the outer edges of the lateral patches darken to form diffuse arcs. However, details vary considerably. Also, there are at least three fairly well defined color forms that differ in major aspects of the banding:  "red banded" in which the marginal white band is absent and the submarginal red band continues across the margin to the ventral surface, "split banded" in which the medial scallops on the submarginal red band connect to a second dark band occupying part of the mid-lateral area and "red submargined" in which a red submarginal band occurs below the white marginal band on the ventral surface. All of them appear to be otherwise identical to sympatric specimens of the typical form suggesting that they are forms of #1 but it might be good to check their DNA "just to be sure." In very large animals, the margins tend to become frillier than in other Hexabranchus spp. suggesting a possible morphological difference. Some transitional and early-mature animals may show more "granular" white spotting on the notum than average, particularly in Australia (perhaps another instance for a DNA check?). Also, a faint blue marginal/submarginal band may be retained in some terminal animals and a narrow red line may be present on the rhinophore collar.

The presence of considerable intraspecific variation in this species suggests the possibility that future DNA work could result in splitting off additional species.

The following composite illustrates the ontogeny of this species beginning just after the loss of  the dorsal spots. The component photos (some cropped and/or flipped) were taken by Scott Johnson in the Marshalls.
 
Marchalls Hexabranchus Growth Series
 
Growth series: Hexabranchus sp. #1

Also see the following linked photos for illustration:

Sea Slug Forum: 19813, 17274 (center-left and bottom), 11688, 9954, 9646, 8966, 8847, 6289, 6288, 6852, 3756, 2095, 1013, 803, 747 (top), 22431, 22662, 23392, hexasang (top), 18683, 14701, 14676, 8417, 6586, 2890, 1041, 21256, 20185, 17652, 16000, 11576, 3721
Nudipixel: 2642, 362, 402, 812, 962, 1654, 1683, 1932, 1961, 2893, 2922, 4307, 4308, 4454, 4475, 4610, 5101, 5362, 5612, 5815, 5839, 6518, 6792, 7124, 7335, 7895, 7952, 8015, 8196, 8197, 8198, 8199, 8200, 8223, 636, 1147, 3909, 3910, 7605, 9257, 9415, 10859, 10869, 11058, 11321, 11368, 11589, 11595, 11794, 11896, 14650, 14852, 17444, 17504, 9970, 11011, 17283, 17284, 12689, 13244, 13436, 13651, 13786, 15655, 16003, 17906, 18606, 18752, 18774, 18775, 18965, 19059, 19282, 20290, 20387, 20410, 20419, 20870, 20871, 20944, 20947, 21095, 21575, 21576, 22145, 22218, 22219, 22360, 23498, 24026, 24156, 24521, 24734, 24947, 25198, 25678, 25927, 26291, 26393, 26472, 26550, 26566, 26567, 26697, 26781, 26782, 26926, 27116, 27129, 27806, 13304, 13324, 13551, 13661, 14370, 16025, 21443, 23127, 27536, 21616, 24351, 24588, 25651, 5778, 1490, 3024, 4422, 4423, 6102, 7314, 6103, 7867, 10552, 16331, 12721, 18819, 18820, 18821, 18823, 18822, 25109, 22813, 27968, 28054, 29196, 29669, 29677, 29813, 30337, 30495, 30496, 30847, 31063, 31849, 31942, 32194, 32594, 33083, 33107, 33329, 33851, 33920, 34200, 34493, 34609, 34610, 35359, 35541, 35549, 35671, 36291, 38238, 39421, 39628, 39859, 39998, 40062, 40145, 40298, 40421, 40670, 40837, 41145, 41283, 41819, 42023, 42378, 42395, 42462, 42469, 42536
Medslugs: 024, 029, 030, 035, 043, 005, 006, 021, 007, 027, 028, 036, 037, 038, 039, 040, 041, 042, 014, 022, 023, 044, 046, 052, 063, 050, 051, 053, 060, 047, 048, 049, 058, 062, 064, 065, 066
Umiushi-zukan: 21819, 19894, 21143, 20250, 19907, 19724, 19576, 17976, 18604, 17796, 17797, 17240, 16850, 16162, 15545, 14872, 14734, 17240, 13965, 13896, 13530, 13015, 12740, 12347, 12260, 7203, 11565, 10973, 10822, 10224, 10205, 9391, 9265, 9301, 85918403, 8299, 769370097136, 70156991594156975686, 5540, 5536 (top), 3965, 4664, 4665, 4451, 37953561, 3200, 3201, 3189, 2569, 1408, 1197, 945, 730, 130(left), 22201, 19806, 18930, 17619, 15466, 14467, 12138, 3661, 9489, 9182, 6943, 1687, 492, 493, 23288, 23474, 23539, 23656, 23983, 23988, 24016, 24231, 24327, 24753, 24797, 24839, 25321, 25510, 25922, 26406, 26446, 26946, 27040, 26968, 28273, 28491, 29148, 29205, 29517, 30041, 30222, 30679, 29159

Also, the following text sources:

Coleman, 1989: cover; pp 7 &  8
Coleman, 2001: p 36; p 41--1st, 2nd, 3rd, 5th, 6th, 7th, 10th & 12th photos
Coleman, 2008: p 24--1st photo; p 28; p 38; p 313--1st, 3rd, 4th & 5th photos; p 314--1st, 4th, 6th & 7th photos; p 315--3rd & 7th photos
Colin & Arneson, 1995: p 185--photo 865
Debelius, 1996: p199--photos 3 & 5
Debelius & Kuiter, 2007: p 10; p 255--1st, 4th & 5th photos; p 256--2nd, 5th, 6th & 7th photos
Gosliner, 1987: p 87
Gosliner, et. al., 1996: p 161
Gosliner, et. al., 2008: p 101
Hazime, et. al.,1986: p 224--3rd photo
Herve, 2010: p 180; p 182--1st, 2nd, 4th, & 5th photo
Humann & Deloach, 2010: p 296--1st, 2nd, 4th, & 5th photos
Nakano, 2004: p 119--1st & 2nd photos
Ono, 1999: p 76
Ono, 2004: p 127--1st, 2nd & 3rd photos
Salvat and Bacchet, 2011: p 221--sp. 2
Suzuki, 2000: p 48
Takamasa 2003: p 72; p 73--2nd photo

Hexabranchus pulchellus Pease, 1860

Found throughout the main Hawaiian Islands and north to Midway Atoll. Scott was uncertain whether an unphotographed animal he saw on Kure was this species or H. aureomarginatus. Sympatric with H. aureomarginatus.

Distinguished from other group #3 spp. by the combination of mid-dorsal spots with an absence of submarginal white lines in early juveniles, the presence of a distinctly yellow background in transitional animals, the lack of white pigment on the lamellae in transitional and mature animals, the lack of a well defined inner margin on the submarginal red band in mature animals, different derivation and development of the white mottling, the consistent presence of a crisply margined red submarginal band on the ventral surface in mature animals, the presence of a very narrow marginal white line in mature animals and a sharply margined white band on the rhinophore collar in mature animals.

Juveniles are translucent gray with violet-blue spots covering the notum. There is no submarginal white line. With growth, the background darkens to yellow, cloudy white patches appear and the dorsal spots decrease in size. With maturity, the background darkens through orange to red and cream mottling appears on all surfaces. In terminal animals, a few flecks of dense white superficial pigment may appear on the notum, white bands develop on the rhinophore collars and a narrow white marginal line usually develops.

Due to the large number of correlated distinguishing characteristics, it seems likely that the Hawaiian population will ultimately be confirmed as endemic. However, H. pulchellus was described from immature specimens and used by Kay, 1979 (and others) within the context of a three species interpretation. So, if the two species interpretation for Hawaii is confirmed, there may well be an earlier available name. It's also conceivable that it might be found to extend further south or east when material becomes available from Johnston Atoll and/or the Line Islands.

See the right side of the composite photo for illustration as well as the species page on this site.

Also see the following linked photos:

Sea Slug Forum: 21297 (not healthy), 4954
Nudipixel: 3361, 3362, 40622, 40927

Also, the following text sources:

Bertsch and Johnson, 1981: p 30
Debelius, 1996: p199--photo 6
Debelius & Kuiter, 2007: p 256--4th photo
Hoover, 1998: p 173
Kay, 1979: p 472--Fig. 151-H

GROUP #4

Based on limited photos, juveniles have a translucent cream rachis without red lines on either face. Cloudy white pigment is present on the base of the rachis and the pinnae are lined in red. As the animals grow, the red pigment on the pinnae may become faint while the cloudy pigment on the base may become more prominent.

Although the branchial pigmentation places it outside group #3, the presence of spots in juveniles and the tightly coiled egg mass suggest that it may be more closely related to that group than to groups #1 & #2.

It appears to be monotypic and to be limited to the Caribbean.

Hexabranchus morsomus Ev. Marcus & Er. Marcus, 1962

Found only in the Caribbean.

Distinguished from Pacific Hexabranchus spp. by the red rhinophores with pale tips and by the limitation of longitudinal markings to a very narrow marginal red line. Also, by the presence on the notum of scattered conical pustules, usually with red tips.

Juveniles are translucent cream, becoming densely frosted with opaque white with growth. A fine red marginal line is present and the red tips on the pustules become more prominent with age. With maturity, red mottling appears between the pustules and usually increases with age (sometimes obscuring the red pustule tips and marginal line).

Some authors place it in a separate genus as Caribranchus morsomus (Marcus & Marcus, 1962).

See the following linked photos for illustration:

Sea Slug Forum: 7817
Nudipixel: 16819, 36274

Also the following text sources:

Debelius & Kuiter, 2007: p 256 (as C. morsomus)
Kay & Schoenberg-Dole, 1991: p 70
Valdes, et. al., 2006: pp 116-118

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Tentatively, that makes at least three species of Hexabranchus in the Indo-Pacific if only the three basic sympatric lineages are ultimately supported and, perhaps, eight (or more?)  if the suggested allopatric species and/or color variants prove to be distinct. Plus, of course, one in the Caribbean.
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