Monday, June 9, 2008

Tasmacetus

For the latest on Tasmacetus



The introductory post mentioned how the family Ziphiidae was the most poorly known amongst cetaceans, and this monotypic genus is certainly no exception. While 42 strandings have been documented, depictions of the coloration were erroneous (and quite variable) prior to Reeves et al. 2002 - although the textual description still seemed outdated. Pitman et al. 2006 were the first to describe the coloration and established four definitive sightings*. The lightly colored melon and long dark beak are distinctive features, but particularly noteworthy are a dorsal "cape" on a light background, a "flipper stripe" and no apparent difference in the coloration of males and females. These coloration characteristics are typical of delphinids and Pitman et al. note that the presence of teeth in both jaws may be of some significance. The authors also point out that there is a superficial similarity to the more northernly Indopacetus which is something of a tropical Hyperoodon planifrons doppelgänger....

*Five prior sightings did not mention the coloration and are therefore suspect. Videotape of one indicated an "unidentified but distinctively patterned species of Mesoplodon". The authors later state that the coloration of several mesoplodonts is unknown.


This all begs the question of where exactly Tasmacetus fits into ziphiid phylogeny. Lambert 2005 places it as a basal taxa, possibly in a clade with Berardius while Bianucci et al. 2007 notes that a mix of archaic characters and derived characters puts it in an unresolved position. Since the most prominent archaic trait is the teeth in both jaws it is worth pointing out that Ninoziphius (a relative of Berardius and/or Tasmacetus? - Bianucci et al. 2007), Messapicetus and Ziphirostrum (members of Ziphiinae e.g. relatives of extant Ziphius - Bianucci et al. 2007) all appear to have teeth in the upper and lower jaws (Lambert 2005) and the extant Mesoplodon grayi has teeth in the upper jaw only (Reeves et al. 2002) suggesting multiple tooth loss events or reversals (or both?) in ziphiids. It should be noted that male Tasmacetus still have a pair of enlarged anterior teeth in addition to 72-96 others (Mead and Payne 1975). Geisler and Sanders 2003 place Tasmacetus as the sister group to other ziphiids which share nine synapomorphies (2 deal with teeth) and it is noteworthy that the molecular phylogeny of May-Collado and Agnarsson 2005 is in agreement with this. Mead 2007 notes that Tasmacetus has a non-derived stomach anatomy - but so do Hyperoodon, Ziphius and some mesoplodonts.

The range of Tasmacetus. Numbers indicate strandings - two were from Juan Fernandez Islands and six were from Tristan da Cunha. Dots indicate sightings, the dot from Tristan da Cunha indicates two sightings. So far it is known from about 33 to 50 degrees South. After fig. 1 of Pitman et al. 2006


Tasmacetus lives in an area with notoriously bad weather and small amounts of landmass so it is not clear if it (along with the pygmy right whale, hourglass dolphin and mesoplodonts) are rare or rarely encountered. MacLeod et al. 2003 theorize (on the basis of one example and the teeth) that Tasmacetus is a specialist on bottom-dwelling fish but Pitman et al. 2006 mention another example which had a stomach entirely filled with cephalopods. The specimens were from Argentina and Tristan de Cunha, respectively. We are going to need a lot more data to make any conclusions on the physiology of this genus, but at least we know how to identify it at sea!

The following is modified from Pitman et al. 2006:

Length: The largest accurately recorded specimen was a 6.6 m (21'8") female. Their Table 1 has 7 m (23') and 7.35 m (24') "bulls" - but these are unconfirmed. It is comparable in size to Ziphius and larger than mesoplodonts (judging from Reeve et al. 2002).

Head: The dark beak lengthens with age and is proportionally similar to Indopacetus and some mesoplodonts; it is longer than those of Hyperoodon or Ziphius. The pale melon is prominent (unlike in mesoplodonts) to a degree that resembles Indopacetus or Ziphius. It does not seem to become more prominent with age in males. Aerial observations reported a "blow" whereas those on ships did not (it may just be inconspicuous).

Body: According to Reeves et al. 2002 it is cigar-shaped and thus Mesoplodon-like. The pale shoulder "patch" is considered a diagnostic feature (in addition to the long dark beak and pale melon). Aerial observations should find the black "cape" from the blowhole to mid-dorsal fin and pale gray afterwards (in contrast to the pale melon) diagnostic. The dorsal fin is smaller than Hyperoodon and Indopacetus and falcate - but similar to Ziphius and some mesoplodonts.

School Size: The four observations gave schools of 3-6, higher than the usual 1-3 for mesoplodonts and Ziphius. According to Reeve et al. 2002, school size for Indopacetus is 15-20 average (and up to 100).


An adult male Tasmacetus - the coloration is described as blue-gray/brown in adults and olive brown in younger specimens. Aerial observations show a distinctly light melon, dark ("black") cape and light gray behind the dorsal fin.


Pitman et al. 2006 is available online for free, by the way and I couldn't help but notice that every website out there on the genus was in need of an update.


Coming up next, more ziphiids of course.



References:


Bianucci, Giovanni et al. 2007. A high diversity in fossil beaked whales (Mammalia, Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. A high diversity in fossil beaked whales (Mammalia, Odontoceti, Ziphiidae) recovered by trawling from the sea floor off South Africa. Geodiversitas 29 (4) : 561-618.

Geisler, Jonathan H. and Sanders, Albert E. 2003. Morphological Evidence for the Phylogeny of Cetacea. Journal of Mammalian Evolution, Vol. 10, Nos. 1/2

Lambert, Olivier. 2005. Systematics and phylogeny of the fossil beaked whales Ziphirostrum du Bus, 1868 and Choneziphius Duvernoy, 1851 (Mammalia, Cetacea, Odontoceti), from the Neogene of Antwerp (North of Belgium). Geodiversitas 27 (3) : 443-497.

MacLeod, C. D. et al. 2003. Review of data on diets of beaked whales: evidence of niche separation and geographic segregation. J. Mar. Biol. Ass. U.K. , 83, 651-665

May-Collado, Laura and Agnarsson, Ingi. 2006. Cytochrome b and Bayesian inference of whale phylogeny. Molecular Phylogenetics and Evolution 38, 344–354

Mead, James G. 2007. Stomach Anatomy and Use in Defining Systemic Relationships of the Cetacean Family Ziphiidae (Beaked Whales). The Anatomical Record 290:581–595

Mead, James G. and Payne, Roger S. 1975. A specimen of the Tasman Beaked Whale, Tasmacetus shepherdi, from Argentina. Journal of Mammalogy, Vol. 56, No. 1, pp. 213-218
Pitman, Robert L. et al. 2006. Shepherd's Beaked Whale (Tasmacetus shepherdi): Information on appearance and biology based on strandings and at-sea observations. Marine Mammal Science 22 (3) 744-755

Reeves, Randall R. et al. 2002. National Audubon Society Guide to Marine Mammals, Alfred A. Knopf, New York.

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