Wednesday, October 22, 2008

The Sperm Whale's Jaw; or Sorry, Gerald Wood

In length, the Sperm Whale’s skeleton at Tranque measured seventy-two Feet; so that when fully invested and extended in life, he must have been ninety feet long; for in the whale, the skeleton loses about one fifth in length compared with the living body. Of this seventy two feet, his skull and jaw comprised some twenty feet, leaving some fifty feet of plain back-bone. Attached to this back-bone, for something less than a third of its length, was the mighty circular basket of ribs which once enclosed his vitals.

- Excerpt from "Measurement of The Whale's Skeleton" - Chapter 103 of Moby-Dick; or The Whale by Herman Melville

Earlier in the chapter, Melville/Ishmael tells us that a Sperm Whale of the largest magnitude measures between eighty-five and ninety feet long and by his reckoning weighs at least 90 tons, or as much as a village of 1,100. By my own reckoning in this previous post, I pegged a 19.5 meter (67'11") bull at around 90 tonnes (~100 tons) and I estimated a 27.5 m (90 foot) bull to weigh a stupefying 210 tonnes/230 tons. If the whale of Tranque was as big as Melville/Ishmael claimed*, it would have rivaled the largest blue whales (Balaenoptera musculus) for size.

* How can a 72 foot skeleton possibly correspond to a 90 foot whale? A ~20 foot mandible would be at a 1:3.6 ratio to the skeleton length, which agrees well with the 1:3.8 m estimate below. The modified Gore et al. (2007) formula (see further below) estimates the whale's length at 21.4 meters (70 feet) using a skull length of 6 meters - could this have been based off a real specimen?

Skeletons of gigantic sperm whales (Physeter macrocephalus sometimes P. catodon) do not currently exist in museums as far as I know (if they ever did) and the only possible evidence of colossal bulls rests in preserved mandibles. In my previous post I discussed a 5 meter (16'4.75") mandible in the British Museum which Wood (1982) estimated to have come from a 25.5 meter bull; I extended Wood's reasoning to estimate a 5.5 meter mandible from the Nantucket Whaling Museum to correspond with a 27.5 meter bull. Wood (1982) noted that a 14.7 m whale had a mandible:body length ratio of 1:6.2 and a 16.28 m whale had a ratio of 1:5.4; he extended the graph to estimate a 1:5.1 ratio for his monstrous mandible and I extended it further to 1:5 for the even bigger mandible. I'm now quite certain that all of these estimated lengths and ratios are, in fact, hogwash.

The book, The Marine Mammals in the Anatomical Museum of the University of Edinburgh (courteously digitized by Google) notes that a 15'10" (4.83 m) mandible probably corresponded with a sperm whale 60 feet (18.3 m) in length - let's assume that this estimate is accurate. This whale would have had a mandible:body length ratio of around 1:3.8; if we assume there is little allometric change, the 5 m mandible would thus correspond to a 19 meter bull and the 5.5 meter mandible would correspond to a 21 meter bull. Given that the record length sperm whale was 20.7 meters in length, I'd say that these seem like plausible figures. But is this accurate?

Unfortunately, there is a lack of data on sperm whale mandible length:body length. However, if we assume that the length of the mandible is nearly the same as that of the skull, this gives us more data to work with. I'll give a range of estimates just to be safe, the first will assume a 1:1 mandible:skull ratio and the second will assume that the mandible is roughly 90% of the skull's length. Data on head length should be avoided since soft tissue can make it considerably longer. Gordon (1991) used data on sperm whale spermaceti length in comparison to body length to come up with a formula which Gore et al. (2007) in turn used to estimate body length from skull length. It is as follows:

Total body length = 9.75 − 0.521 (SL) + 0.068 (SL^2) + 0.057 (SL^3)

Where SL = skull length

This is quite a bit more complicated than what I was using as it appears to take allometry into account (where I previously sorta ignored it). Here is what happens when we plug in numbers:

5 meter mandible:
5 meter skull = 15.97 meters (52 feet)
5.5 meter skull = 18.42 meters (60 feet)

Wood's 25.5 meter estimate was probably off by 138% to 160%.
Using a 1:3.8 ratio gives estimates that are off by 114% to 119%

5.5 meter mandible:
5.5 meter skull = 18.42 meters (60 feet)
6.0 meter skull = 21.384 meters (70 feet)

My 27.5 meter estimate (based off of Wood's allometry) is probably off by 128% to 149%
Using a 1:3.8 ratio gives estimates that are off by 113% and 106%

A 6 meter skull was also present in the (fictional?) Tranque whale, and Melville/Ishmael's estimate for skeletal length (72 feet) is off by 103%. The estimation for body length in the flesh is off by 129%.

Hard data is needed on the mandible:body length ratio in sperm whales so these equations can be refined. I'd say that as is, the data suggests that the 5 and 5.5 meter mandibles do not correspond with bulls greatly exceeding the known 20.7 meter record. It could be possible that these mandibles are the product of abnormal growth analagous to acromegaly in humans (as suggest by Alan Hazen). There is currently no non-anecdotal evidence of bulls greatly exceeding 20.7 meters and while specimens somewhat larger than this probably existed in the past due to a larger average length, sperm whales do not appear to rival blue whales for the title of the largest animal ever to have lived.

I was planning to make this an addendum, but it quickly got out of hand...

For information on sperm whale jaw oddities, see the post at Tetrapod Zoology


Gordon, Jonathan C. D. 1991. Evaluation of a method for determining the length of sperm whale (Physeter catodon) from their vocalizations. Journal of Zoology 224, 301-314

Gore, M. A. et al. 2007. Sperm whale, Physeter macrocephalus, stranding on the Pakistani coast. J. Mar. Biol. Ass. 87, 363–364

Wood, Gerald. 1982. Guinness book of Animal Facts and Feats. Guinness Superlatives, Middlesex.

Sunday, October 19, 2008

Tales of the Incredibly Outsized

Outsized animals have been a mainstay of this blog in the past, notable posts can be found here, here and here. While those posts appear to have been relatively popular*, I don't intend to write any more moderately fluffy posts rattling off trivia. I think it would be more worthwhile to determine if animals can reach the lengths claimed for them, see if there are widespread patterns in how outsized animals can be and suggest widespread implications. And I'll probably talk about a lot of other stuff, I never know where these things wind up.

*I guess this isn't saying much, the Nature Blog Network tells me this blog is abysmally unpopular. Oh well...

Unfortunately, there are a lot of issues that need to be brought up before we can start having fun (sic). I'm sure everyone is aware of scaling, but just for the heck of it: when Animal A is twice the length (2^1) of Animal B it will have four times the surface area (2^2) and eight times the mass (2^3). Muscle and bone strength depend on area, so we can anticipate animals being proportionally bulkier at larger sizes - among other allometric changes. Quickly and generally, larger animals exist at lower densities, have larger home ranges, are subjected to less predation, have lower metabolic rates, change temperatures more slowly, are more efficient and faster locomotors, et cetera (Peters 1983). It's an absolutely fundamental measurement and one source stated that "[b]ody size is the single most important axis of biodiversity" (Brown et al. 2007). This post isn't looking at body size in the context of biodiversity, but it seems reasonable to suggest that body size can be very important from an intraspecific perspective.

Despite size being such a useful, nay, pivotal measurement, "fish story" data plagues many sources that should have been substantially more critical. As I discussed previously, the wels catfish (Siluris glanis) is frequently stated to reach 5 meters and ~300 kg* (e.g. Stone 2007); in reality, specimens over 2 meters are noteworthy and the largest accurately measured/well supported specimen was 2.78 m and 144 kg (see here). Statements such as "the wels grows up to 5 m long" and "the wels averages 1.4-1.6 m long" (wiki) create vastly different impressions of the wels' size - the former should outweigh the latter by about 40 times, for one thing. In the past I have also discussed the size of the green anaconda (Eunectes murinus); I can't help but note that some sources state that the anaconda is 6-10 m (~10-33 feet) long (e.g. Burnie and Wilson 2001). A survey of 1000 anacondas did not turn up any snakes longer than 17 feet (5.18 m) and there is still a reward out for any snake longer than 30 feet (see article). How do you report extreme figures as if they are the norm of the species? Unless there is an extreme lack of data (e.g. some ziphiids) there's no reason to give a maximum length instead of the average adult length and mass.

* I'm not sure where this figure originated, it may have been rounded from a 4.6 m/327 kg claim made by Kessler in 1856. The weight is abnormally low (a wels that length should be over twice as heavy), which strongly indicates a fabrication. Alternately, the length could have come from a sturgeon (apparently they can be confused for wels - see here) and the weight could have been a crude estimate (perhaps a linear scaling).

So... why look at outsized animals at all? By definition they will not be numerous, but they may have exaggerated ecological impacts. Larger and older black rockfish (Sebastes melanops) produce greater numbers of larvae over broader periods of time than smaller individuals; their larvae will also be larger, faster growing and more starvation resistant (Birkeland 2005). This will probably not be the norm amongst fish species which reproduce only once or twice in their life or have high turnover rates (Birkeland 2005). Within species there may be significant ecological differences between size classes, e.g. large parrotfish will excavate substratum while smaller ones will not (Birkeland 2005). That's an extreme situation of course, but if an animal is much larger than average it will have to be doing something different in order to sustain itself. Outsized individuals will have the ability to feed on larger prey items and if some individuals get as massive as claimed (i.e. several times heavier than average) they may even be at a distinct trophic level. While the impacts of the average individual are important, looking at all the individuals on the bell curve can give a more complete understanding.

Human exploitation tends to "favor" large individuals and this can cause the average size of an organism to decrease. If there is good historical data of individuals much larger than average, then it may indicate that modern individuals aren't at previous levels of diversity. This is assuming that outsized individuals have a genetic basis since humans with pituitary gigantism tend to have many other medical issues and rather short lifespans. Anyways, let's look at sperm whales (Physeter macrocephalus), which I mentioned in the past.

Wood (1982) states that the average adult bull* is 14.93 m (49 feet) in length, but another source (mentioned here) states that the average length is 16 m (52.5 feet). The difference doesn't look like much, but a 107% difference in length means at least a 123% difference in weight. That's probably a difference of thousands of tons of prey annually. Photographic measurement of 13 bachelor males off New Zealand ranged from 11.7-15.8 m (med. 14.2 m) and 7 males off California ranged from 14.7-18.2 m (med. 16.4) (Jaquet 2006). The sample size is too small to make a fuss over the figures (probably slight overestimates) but there do appear to be regional differences in sperm whale size (and behavior, proportions, markings, etc) (Jaquet 2006). This complicates things since the largest male may not have been as outsized as predicted if it came from a rather large population - a rough estimate will have to do. The record was a 20.7 m (67'11") bull caught off South Georgia in the '48-'49 season, by the way (Wood 1982). Taking the high road (16 m avg.), the record specimen was was about 129% longer than average and at least 217% more massive. So how massive are sperm whales? Wood (1982) estimates his average bull to weigh 36 tonnes (~40 tons) (I have no idea where this figure derives from) and mentions large bulls which were weighed in full at 18 m/53 tonnes (59 feet/58 tons) and 16 m/40 tonnes (52'6"/44 tons). Using these figures, the record bull would weigh at least 96, 81 or 87 tonnes. Wood (1982) also mentioned an 18 m bull which weighed out piecemeal at 53.37 tonnes, hinting that the whale was proportionally heavier in life (maybe 10% due to fluid loss ~ 58 tonnes/64 tons) given an estimate of 81-88 tonnes. Wood's estimate of 80 tonnes for the record bull may be a bit light and I'm not entirely sure how he arrived at this figure in the first place**.

*I should point out that the total length of a whale is taken from the snout to the notch in the tail fluke, some less than scrupulous authorities in the past got larger figures by measuring the flukes or even taking a measurement along the curve of the body.
**The largest sperm whale in the southern hemisphere was 19.5 m, and if you scale up Wood's 14.93 m/36 tonne animal to this size the estimated weight is 80.2 tonnes.

There's a world of difference between a ~40 tonne 16 m whale and a ~90 tonne 19.5 m whale, and incredibly much larger animals have been reported in the past. People are generally full of crap when it comes to extreme animal sizes and I'm extremely distrustful of any claims before the era of photography, but the case for giant sperm whales is unique. Wood (1982) mentions that the British Museum has a mandible measuring 5 m (16'4.75") in length, however there is apparently a 5.5 m (18') mandible in the Nantucket Whaling Museum (see here and here). The sperm whale jaw seems rather short when viewed externally (see here) but it seems much longer proportionally when the animal is viewed as a skeleton (see here). Why bring thus up? Well, Wood (1982) cites a source that claims a mandible:body ratio of 1:6.2 in a 14.7 m whale and a 1:5.4 ratio in an old 16.28 m whale. These may be measurements of the externally visible jaw, but I'm in desperate need of a peer-reviewed information on sperm whale mandible:body proportion. Wood (1982) used a 1:5.1 proportion for the animal to derive a figure of, gulp, 25.5 meters (83'8") and a 1:5 estimate for a 5.5 m jaw gives a length of 27.5 meters (90 feet). So let's say that Wood's estimate for sperm whale mandible:body length proportion is wrong and it is something like 1:4 - in this case the 5.5 meter mandible would still belong to a 22 m (72 foot) whale 106% longer and 120% heavier than the record specimen and close to triple the weight of the average sperm whale at something like 110 tonnes/120 tons. The record sperm whale was caught in 1950 and the mandibles are probably from much earlier (the 5 m one was from 1851) so it could be possible that the average shrank substantially since then. Wood's 25.5 m (83'8") sperm whale would be something like 170 tonnes/185 tons and if there was ever a 27.5 m sperm whale (90 feet) it would weigh something like 210 tonnes/230 tons. I'm not going to seriously entertain the notion that sperm whales rivaling blue whales for the largest animal actually existed, but it does seem like the average Physeter macrocephalus was considerably larger before heavy exploitation.

So are there patterns as to how large the largest outsized animals can be? Maurice Burton apparently stated that outsized specimens are 68% longer than the average size and 40% longer than the unusual large (Heuvelmans 1968 - from Burton's Living Fossils?). An animal 168% longer than average would also weigh around 5 times more - it would be a marvel for the bauplan of a species to expand to such extravagant sizes and work well enough for the animal to survive in nature. I'm not really a big believer in any self-proclaimed zoological "laws" or "rules" (I guess "Zoological Loose Guideline" doesn't have the same ring), but it is worth noting out that a population of sperm whales with 16 m long bulls could theoretically have an outsized specimen 26.88 m long. This pattern does not emerge with record-sized land mammals - the largest bush elephant (Loxodonta africana) was ~125% taller than average (~4 m vs. 3.2 m) and the largest Masai giraffe (Giraffa tippelskirchi) was a mere 111% taller than average (5.87 m vs. 5.3 m) assuming it was measured correctly (i.e. without the horns) (data from Wood 1982). Perhaps aquatic animals can tolerate outsized specimens more - the aforementioned record Wels catfish (2.78 m) was about 185% longer than average (~1.5 m).

Topics such as this one are certainly not going to be resolved in one blog post, so I'll end it here to get on to other things. There is a great deal of data out there, particularly on state record fish, that might be interesting to go through, but I think the rough picture here is sufficient. While incredible size may offer advantages such as resistance to predation and increased offspring health, tradeoffs will exist. Increased food acquisition comes to mind and there could be morphological issues as well. There are reasons that the outsize isn't the norm, but if situations change it could be adopted as a new norm. Australian giant feral cats are a possible example - emphasis on possible. Outsized animals aren't critical to understanding evolution and ecology but they can make for a potentially very interesting topic.


Birkeland, Charles and Dayton, Paul K. 2005. The importance in fishery management of leaving the big ones. TRENDS in Ecology and Evolution 20, 356-358

Brown, James H. et al. 2007. The metabolic theory of ecology and the role of body size in marine and freshwater ecosystems. In:I Hildrew, A. G. et al. 2007 (Ed.) Body Size: The Structure and Function of Aquatic Ecosystems. Cambridge University Press. pp. 1-15.

Burnie, David and Wilson, Don E. 2001. Animal: The Definitive Visual Guide. Dorling Kindersley, London and New York.

Heuvelmans, Bernard. 1968. In the Wake of the Sea-Serpents. Hill and Wang, New York.

Hutchings, Peter A. 2005. Life history consequences of overexploitation to population recovery in Northwest Atlantic cod (Gadus morhua). Can. J. Fish. Aquat. Sci. 62, 824–832

Jaquet, Nathalie. 2006. A simple photogrammetric technique to measure sperm whales at sea. Marine Mammal Science 22, 862-879.

Peters, Robert Henry. 1983. The Ecological Implications of Body Size. Cambridge studies in Ecology.

Stone, Richard. 2007. The Last of the Leviathans. Science 316, 1684-1688

Wood, Gerald. 1982. Guinness book of Animal Facts and Feats. Guinness Superlatives, Middlesex.

Sunday, October 5, 2008

Anomalous Cephalopod Appendages

Relative to other cephalopods, octopuses are reported with abnormal appendages* much more frequently. Symmetry and streamlining do not appear to be issues for animals that jet around infrequently and spend most of their time as benthic crawlers. It isn't fair to assume that every octopus is a benthic crawler, but as far as I can tell no pelagic octopuses were reported with abnormalities. Octopuses also seem capable of coping with arm morphologies quite divergent from the norm, one author reported seeing octopuses in the wild with the greater portion of seven arms missing (Toll and Binger 1991).

* The appendages are known as arms, regardless of how they're used. One recent study, which has not yet been peer reviewed as far as I can tell, reports that octopuses use arm pair IV as "legs" - but they primarily studied Octopus vulgaris and it is perfectly conceivable that different species have specialized their arms for different purposes.

Octopuses with subnumerary arm counts unrelated to natural losses have been reported. An article cited by Toll and Binger (1991) (Gleadall 1989 - see further reading) documented an Octopus sp. with seven arms*. Earlier this year it was claimed that the first "hexapus" was discovered, but somebody didn't bother checking the literature because I quickly found a description of a hexapodous specimen by Toll and Binger (1991). The aberrant Pteroctopus tetracirrhus entirely lacked arm pair II and there was a single adoral (near mouth) sucker located on the web between arms LI and LIII (Toll and Binger 1991). Arm pair I was observed with several other abnormalities: LI was narrower and shorter than RI, three of the first five suckers on LI were abnormally small, in the first ten rows of suckers on RI one third of them were abnormally small and there was widespread misalignment - some areas were even roughly tri-serial (Toll and Binger 1991). The authors reported that otherwise the specimen was normal and the abnormal appendages did not show signs of regeneration.

* Not to be confused with male Haliphron atlanticus

Octopuses with abnormally high counts of arms have also been reported. The Pink Tentacle posted on an octopus with 9 arms, which either has an extra RI or LIV (where's a hectocotylus when you need one?). The limb may be "split" off from another, I'll discuss this below. Kumph 1960 cited an earlier study (Parona 1900 - see further reading) which described an O. vulgaris with two thin arms in the place of LI and an Eledone cirrhosa* with an arm in between RII and RIII. Toll and Binger (1991) described the first decapodous octopus**, a lab-raised O. briareus which had a repeated arm pair IV. All of the arms were evenly spaced around the branchial crown, but the octopus still had a couple of other abnormalities. Male octopuses have a specialized arm for spermatophore transfer (RIII*** - the hectocotylized arm) and the Toll and Binger specimen had a supernumerary spermatophoric groove on LIII with no hectocotylus. A supernumerary diverticulum of the penis was also present.

* Toll and Binger 1991 cite this specimen as E. moschata, describes it as E. aldrovandi. Gonzalez and Guerra note that the species name has been changed.
** I can't help but mention the fact that there are octopodous decapods. Theirs' is a natural condition, however.
*** Some species have a hectocotylized LIII (Palacio 1973).

Abnormal hectocotylization is another abnormality recorded in octopuses. Palacio (1973) discussed some prior cases: hectocotylization was reported on RIII and LIII from an Eledone cirrosa (Appellof 1892 - see further reading) and on RIII and LII from Octopus briareus (Robson 1929 - see further reading). Palacio (1973) reported an O. vulgaris with hectocotylized RIII and RIV and an O. selene with hectocotylized RII and RIII. The extra hectocotylized arms all manifest with roughly the same morphology, they are not diminished in size and have proportional differences compared to the normal arms (Palacio 1973). Unlike the Toll and Binger decapodous octopus, none of these four specimens exhibited abnormalities with the genitalia (Palacio 1973). Palacio (1973) speculated that the extra hectocotylized arms were functional and were likely the result of mutations in sex-linked genes. It seems really odd that the mutation would occur on different arms each time, but how the hectocotylus wound up on RIII (for the most part) also seems rather odd. Another paper (Jereb et al. 1989) has discussed bilateral hectocotylization, but not exactly in a language that I can read.

So far I've only mildly alluded to the commonest and most bizarre condition that occurs in cephalopod arms - splitting. Bifurcation has been observed recently in the tentacle clubs of Onykia (= Moroteuthis) ingens by Gonzalez and Guerra (2007); polyfurcation was observed not-so-recently in the arms of a cuttlefish (Okada 1937 - see further reading). As can be predicted, octopuses are far more prone to demonstrating this trait - 7 were captured off Japan from 1884 to 1964 (Okada 1965a). Kumph (1960) described a bifurcation in LIII from O. briareus, both of the new arms had a biserial series of suckers and there was some webbing in between them. Kumph rejected the idea that it was the result of injury and interpreted it as a mutation or developmental oddity. This is a rather mild defect compared to some.

Around 8 years ago I found some diminutive quasi-cryptozoology book in a library (the downtown Naperville library) and I recall being extra dubious of the claim that octopuses have been found from extra "tentacles". Besides Toll and Binger (1991), the genesis of this post was a link my sister sent me to a post from Pink Tentacle which showed octopuses with up to 96 branching "armlets". The pattern of branching is quite complex: bifurcations have a continuous series of suckers, trifurcations have side branches with independently developed sucker series, then there are dorsal bifurcations, dorso-lateral mixed branching, dorsal branching and subcutaneous branching (with no suckers) (Okada 1965a, 1965b). Toll and Binger (1991) suggested that Okada's data indicates that the branching is not a random event. All of the specimens that Okada looked at were Octopus vulgaris from Japan, but it could be possible that this is due to sampling bias. If there is a regional tendency to be crazily polyfurcated I have no idea why.

Figures 4 and 5 from Okada 1965b. These are from the 1884 Uraga specimen with 90 branches total. Incredibly, RII did not have any branches. The left arm is LIII with five primary and thirteen secondary branches (21 total). The right arm is RIV with four primary and eighteen secondary branches (30 total). Ventral arms (III and IV) tend to have more furcations.

So what could possible cause these anomalous appendages? Gonzalez and Guerra (2007) note that in limb regeneration, an Apical Epidermal Crest covers the severed organ and Hox genes are activated to reproduce a limb as it was in the embryo. In their Onykia specimen, the tentacles appeared to have been regenerated but with clubs only (and bifurcate ones at that) - as such it had to get by with arms only (Gonzalez and Guerra 2007). Toll and Binger (1991) brought up amphibian regenerations with (surgically induced) supernumerary distal limb development but didn't reach firm conclusions on its relevance. These appendage abnormalities are likely due to developmental defects and/or regenerative difficulties, but the literature still is scant and I'm sure there's a lot more interesting work to be done.

Before I go, I should mention that five pairs of arms appears to be the ancestral conditions for cephalopods, including Nautilus. Pairs I and II turn into parts of the hood (!) and III-V turn into digital tentacles. I should probably mention the ocular and buccal tentacles in Nautilus which are not of homologous origin to these. Since there appears to have been a polyfurcation event in the past (albeit a less haphazard looking one), this could support the idea of some appendage anomalies being wholly genetic.


González, Ángel F. and Guerra, Ángel. First observation of a double tentacle bifurcation in cephalopods. JMBA Biodiversity Records - Published Online

Kumph, H. E. 1960. Arm abnormality in Octopus. Nature 185, 334-335.

Okada, Y. K. 1965a. On Japanese octopuses with branched arms, with special reference to their captures from 1884 to 1964. Proc Jap Acad 41, 618-623. Available

Okada, Y. K. 1965b. Rules of arm-branching in Japanese octopuses with branched arms. Proc Jap Acad 41, 624-629. Available

Palacio, F. J. 1973. On the Double Hectocolyziation of Octopods. The Nautilus 87, 99-102. Available (staring on page 124).

Shigeno, Shuichi et al. 2007. Evolution of the Cephalopod Head Complex by Assembly of Multiple Molluscan Body Parts: Evidence from Nautilus Embryonic Development. Journal of Morphology 269, 1-17

Toll, Richard B. and Binger, Lynetta C. 1991. Arm anomalies: cases of supernumerary development and bilateral angensis of arms pairs in Octopoda (Mollusca, Cephalopods). Zoomorphology 110, 313-316

Further Reading:
For those with better access to materials than me

Appellof, A. 1892. Teuthologische Beitrage IV. Uber einem Fall von doppelseitiger Hectokotylisation bei Eledone cirrosa (Lam.) d'Orb. Bergs Mus Aarsb, 14-15

Gleadall, Ian G. 1989. An octopus with only seven arms: anatomical details. Journal of Molluscan Studies 55, 479-487.

Jereb, P. et al. 1989. Sue due esemplari anomali di Scaergus unicirrhus (Mollusca, Cephalopoda). Oebalia 15, 807–809.

Okada, Y. K. 1937. An occurrence of branched arms in the decapod cephalopod, Sepia esculenta Hoyle. Annot Zool Japon 17, 93-94

Parona, C. 1900. Sulla dichotomia delle braccia nei Cefalopodi. Boll Mus Zool Anat Comp Univ Genova, vol 4 (No 96), 1-7.

Robson, G. C. 1929b. On a case of bilateral hectocotylization in Octopus rugosus. Pro Zool Soc Lond. 95-97.

Oddly enough, I have a somewhat relevant shirt featuring an octadecapodous octopus. I got it from yonder (I feel contractually obliged). I also feel obliged to say that this was the best of several pictures, yeesh.