The story goes that Richard Ellis once referred to right-whale dolphin species (genus Lissodelphis) as being elongated to the point of nearly being eel-like. These sentiments have been shared by (repeated by?) other publications (e.g. Stewart et al., 2002) and strangely omitted by others. The species are occasionally depicted as being fairly normal shaped delphinids, but photographs confirm that yes, these are very elongated cetaceans.
Lissodelphis borealis, based off of Reeves et al. 2002 and Perrin, 1991. Until I get permission to use the image, this quick illustration will have to do. For those of you with access, I highly recommend looking at the Perrin picture. FR for this picture is about 8, it is around 10 for Perrin, 1991 but apparently reaches 11 (keep reading).
As the pictures indicates, the local nickname "snake porpoise" doesn't seem that far fetched. These are of course not porpoises but dolphins (delphinidae) in the subfamily Lissodelphininae. Two species are in the genus, a more elongated northern species (L. borealis) and a more massive southern one (L. peronii). Other genera in the subfamily, recently demonstrated to be monophyletic, include Cephalorhynchus and "Lagenorhynchus" (Harlin-Cognato & Honeycutt, 2006). None of the other species are elongated and all of them posses dorsal fins, although a few do have a similarly stark black and white coloration (C. commersonii, L. cruciger). The family Delphinidae appears to have originated around 16 million years ago (Arnason et al., 2004) and coupled with the lack of similarity with relatives and the lack of a fossil record (Newcomer et al., 1996) suggests that this body plan derived quite recently.
The southern right-whale dolphin (L. peronii) was first described in 1804 and is distinguished by its northern relative by a coloration pattern with more white, a greater girth, and proportionally larger flippers (Newcomer et al., 1996). The coloration for both species is variable: all black and all white individuals are known as well as L. borealis specimens with a similar coloration pattern (and "swirled" L. borealis). See this picture for typical coloration. L. peronii is a circumpolar animal that normally inhabits 35 to 62 degrees South (up to 12 degrees) while L. borealis (described 1848) inhabits the north Pacific between 35 and 51 degrees N. Both have been noted as being very common species, estimated to be in the tens of thousands (Jefferson & Newcomer, 1993 and Newcomer, et al., 1996). Both are fairly small cetaceans with L. borealis reaching 3.1 m and 113 kg and L. peronii reaching at least 2.97 m and 116 kg (it has fewer samples). Right-whale dolphins are gregarious creatures with groups occasionally numbering over 1000, although L. borealis has been seen singly and L. peronii is sometimes seen in small groups. Other species of cetaceans are often affiliated with these species, and one species of pinniped has been associated with L. borealis. Habitat-wise these are cold water species affiliated with the open ocean or deep water. There does not seem to be much indication for why such a unique body could evolve from looking at the current information on the ecology of the species.
Quite unexpectedly, L. peronii has been recorded swimming at 22 km/h and L. borealis at over 40 km/h. Presumably L. peronii can travel faster but has not been adequately measured. 40 km/h would make L. borealis one of the faster species of dolphins, and Fish 1996 noted that another species of very fast dolphin, Dall's porpoise (Phocoenoides dalli) had a very divergent body plan. Using a fineness ration (body length/maximum thickness) the body shape with the least amount of drag was calculated to be 4.5; Dall's porpoise had a value of 4 and L. borealis had a value of 11. A value that high seems quite extreme, and (since figures are not available) I wonder if it is approaching the FR for Basilosaurus. Since longer shapes produce more drag, how are these species some of the fastest dolphins? Fish notes the FR is of course not the only indicator of performance, and streamlining, such as a fusiform body, also plays a large role. The species are noted as being dorso-ventrally instead of laterally (Stewart et al., 2002), although I'm not sure how that would streamline the animal. It is unfortunately noted by most workers that many aspects of the biology of this genus is unknown (Rankin et al., 2007).
The story doesn't have to stop there. The only thing noted about the skeleton of L. borealis (and presumably L. peronii) is that all aspects of it are weakly built (Jefferson & Newcomer, 1993), that is prior to Buchholtz & Schur 2004. It is first noted that Lissodelphis has an abnormally high lumbar vertebrae count; typically in delphinids with high counts lumbar vertebrae are "capped" at 24 with additional caudal vertebrae added, but Lissodelphis may have more lumbars than caudals. Surprisingly their total count (86-89) isn't the highest, but is exceeded by Lagenorhynchus albirostris (91) and approached by other members of the genus (77+), which, er, may be polyphyletic (Harlin-Cognato & Honeycutt, 2006). Buchholtz & Schur suggest multiple origins for this trait, as Lissodelphis combines primitive traits (no reversal of vertebrae orientation or syncliny, low torso metapophyses, short neural spines) with derived ones (vertebral count, disk shaped ("discoidal") lumbar vertebrae, and a secondary rise in the torso). The authors treat the strange spinal characteristics as further anomalies, although the mention of a "leaping style of locomotion" should be significant. Jefferson & Newcomer 1993 and Newcomer et al. 1996 mention that both species make low angle leaps at high speed with a great amount of surface disturbance but don't point out that this is anomalous. Presumably the vertebral derivations are directly related to this behavior, but there are many aspects of this species that are still baffling.
The only other species of cetaceans completely lacking any sort of dorsal structure (fin or ridge) are the right and bowhead whales (Balaenidae), although they are not noted as being fast swimmers and there seems to be a tendency for polar cetaceans to reduce their dorsal structures. The caudal fins in at least L. borealis seem quite small as well, so these species must have some mechanism to stabilize. Clearly there is much to be learned about these species, and I'm sure those interested in underwater robots mimicking animals will find this species of interest.
So that's the story so far for this anomalous genus. One of these days we'll have a more complete picture of whatever odd mechanics are occurring, but probably never fully complete. I have the feeling that this elongation is not comparable to that of some archaeocetes, but we'll have to wait and see. It seems like there's always something left to be discovered and described about species, so unfortunately I can never write a truly complete post. Oh well.
I'm currently waiting for something to ship to me and when it does, expect a post of monstrous proportions...
Arnason, Ulfer et al. 2004. Mitogenomic analyses provide new insights into cetacean origin and evolution. Gene vol. 333.
Buchholtz, Emily A. & Schur, Stephanie A. 2004. Vertebral osteology in Delphinidae (Cetacea). Zoological Journal of the Linnean Society, 140, 383–401.
Harlin-Cognato, April D. and Honeycutt, Rodney L. 2006. Multi-locus phylogeny of dolphins in the subfamily Lissodelphininae: character synergy improves phylogenetic resolution. BMC Evolutionary Biology 6:87.
Fish, Frank E. 1996. Transitions from Drag-based to Lift-based Propulsion in Mammalian Swimming. Amer. Zool. 36:628-641.
Jefferson, Thomas A. & Newcomer, Michael W. 1993. Lissodelphis borealis. Mammalian Species. No. 425, p. 1-6.
Newcomer, Michael W. et al. 1996. Lissodelphis peronii. Mammalian Species. No. 531 p. 1-5.
Perrin, William F. 1991. Why are there so many kinds of whales and dolphins? BioScience vol. 41 n. 7, 460-461.
Rankin, Shannon et al. 2007. Patterned burst-pulse vocalizations of the northern right whale dolphin, Lissodelphis borealis. J. Acoust. Soc. Am. 121 (2), p. 1213-1218.
Stewart, Brent S. et al. National Audubon Society Guide to Marine Mammals of the World. Knopf, 2002.