Abyssal Gigantism

Dec 24, 2002
A few months ago, I took a trip to the Monterey Bay Aquarium, and I managed to speak with one of the research institute's scientists about size and growth rate in deep water animals.

He said that there is a growing (no pun intended) interest in what is being known as "abyssal gigantism". Does anyone know if such work is being done and how such research is being focused?

What I was wondering is about the physiological constraints to growth in say, giant species of squid, versus any evolutionary, histological, or physiological processes that would select for gigantic size in deep-water animals? In other words, how could this be, and how is it accomplished?

Thanks for any replies!


"Keep on Rockin' in the Free World." - Neil Young
John, the deep is the realm of the near inconceivably bizarre, with many animals thought to be living fossils or simple relicts of a time gone by (a snapshot into the past).

Gigantism is a thing of the past (for what reason I don't know); but historically large species ruled, even larger than Recent counterparts (so that physiological limitations probably do not dictate the upper size of Recent animals; something else is responsible).

I don't believe many massive giants remain undiscovered (as in 20-foot-long animals), but 'giant' is a relative term, so the discovery of 'giant' sea spiders, sponges, whelks, slugs, worms etc. is a possibility. Perhaps 'real giants' (of the 20-foot kind) survive in the deep, and have done for millions of years, as this environment has remained Earth's most stable in time and space (until recently, with the advent of deep-sea trawling). Therefore it might not be selective pressure that accounts for their size; rather temporal stability and the persistence of relictual 'giant' taxa. It's a stretch - I'm just playing Devil's advocate. (Less variation in habitat than their shallow-water or terrestrial counterparts, and accordingly less speciation (if habitat variation drives speciation).)

But here's another thought. There are many instances of speciation where taxa rapidly double or halve in size, rather than acquiring new characters or character states and gradually morphing in time and space. (Punctuated Equilibria, or whatever the term is they use today; like species the terminology also evolves.) Perhaps shallow-water environments do limit size (it would be hard to hide a giant squid in a rock pool, beneath a slab of rock or coral, or manoeuvre around the coast; Sperm Whales frequently strand close to shore ... a dead end in the old gene pool; the same applies to squid). Big things die when they hit something or strand; little things are less likely to fall into this trap; the gene pool perpetuates in the latter. Could the question really be why do we experience 'gigantism in open habitat', rather than 'gigantism in the abyss'? Really, 500 metres, the typical habitat depth of giant squid, isn't abyssal, and the juveniles still occur in the upper few metres, and subadults (1-2 metres) probably occur in the upper 30m. During the next interglacial, with elevating temperatures and melting of the ice caps, and the increase in ocean size and depth, might we experience even larger marine forms (and smaller terrestrial forms).

Perhaps it's a bit of all - temporal stability, relictual taxa and unobstructed habitat. Then again, perhaps there is some selective pressure, some physiological adapatation to being large (or attaining a large size not being a lethal character state for a deep-sea species - as in don't think of it as being an adaptation - think of it as being a non-lethal condition). Cooler temperatures are known to result in development of larger forms/individuals, taking more time to mature, and cooler temperatures are experienced at depth (of course we're verging on Lamarckism here - the acquisition of acquired characters/character states .... but I'm ok with that). There are a number of papers out there on the effect of temperature on size (particularly for the Antarctic).

A really good question; I haven't got a clue!
I don't believe many massive giants remain undiscovered

If you compare the mean body size of fishes, there is a gradual increase until 200m depth, followed by a decrease downslope to around 1000m. Below this there are numerous examples of gigantism (e.g. purple ghost shark, sleeper shark, Lepidion cod etc). IMHO there are probably between two and five undiscovered "giant" species (ie. 3m+, all taxa) out there.
Leptocephalus giganteus (an eel-like larvae) still hasn't been identified with an adult - if the arvae/adult ratio is the same as for known species, the adult would be ca.20ft long.
M, that's pretty interesting stuff. I've a couple of questions as a consequence:
1) Is there any latitudinal variation in this (@ ~ 1000m)
2) Is there a pronounced fish-faunal discontinuity at 200 and 1000m depth to account for the differences in size? Do you have another @ ~ 650m, or some other intermediate depth?
3) Are you talking benthopelagic fish, mid-water or those found throughout the water column?
4) If a faunal discontinuity exists, and intuitively it does, is this discontinuity at the generic level, or higher (entirely new suite of families or greater)?
5) If I was to ask the difficult question, and I'm going to, at what depths do you recognise megafaunal (fish) discontinuities for say NE, NW, central and SE regions of the 'landmass' upon which you reside? (I'm not sure what's been done/what the collections are like to the SW, and this area is a little confusing anyway, being steep and otherwise quite unusual; I know you've got some interesting shallow things).

Sorry for the barrage; will send a pm to explain what we're up to, though it will be a couple of weeks away (synergy).
Ahh, Dr O, so many questions..so little time.
These comments are merely based on my observations of the sort of specimens we see from research & commercial trawls that get to here.....

1. latitudinal variation

None apparent. The limited sampling suggests that at least in temperate to Antarctic waters the trends are similar. I’m not aware of much fishing at suitable depths to ~1000 m in more tropical latitudes. The limited samples from more northern latitudes suggest the same occurs (but the deep water fauna probably extends well north as the temp etc at those depths is not all that different latitudinally)

2) fish-faunal discontinuity

Fish biomass decreases downslope from 200 to around 600 m, then rises again to peak around 1000 m. This is mainly due to the presence of orange roughy at the lower depths. Mean fish size follows a similar trend, with what I regard as true gigantism occurring at depths below 1000m, without any distinct depth change (gigantism species can and do turn up as shallow as 600m)

3) Are you talking benthopelagic fish, mid-water or those found throughout the water column?

Mainly Benthopelagic/demersal, although similar trends may occur in the epi/mesopelagic zones: too little sampling with self-closing nets has been done to get an accurate assessment. Giant midwater Ceratias seem to only occur at depths of c.1000m

4) If a faunal discontinuity exists, and intuitively it does, is this discontinuity at the generic level, or higher (entirely new suite of families or greater)?

at the generic level. At greater depths there may be a new suite of families, but below 1200m there has been insufficient sampling – certainly no sampling with large trawls – mostly it has been dredges/beam trawls.

5) at what depths do you recognise megafaunal (fish) discontinuities for say NE, NW, central and SE regions

The discontinuities are at similar depths but different suites of species.
You are a most kind and noble soul! (or should I say sole?) I promise to deliver you flounders one day.
When it comes to 'M', Matt, we know zip about fish! And when we think we've caught up to 'M', M's surpassed M's previous appreciation/understanding of fish, and we know relatively zip again. We'll never catch up. So, the secret is to drain M's brain now, and to develop the world's first e-straw.

Need new emoticon.
Over in the Fossil Forum I believe I read that there were gigantic squid known to have inhabited the shallow seaway on the east side of the proto-Rocky Mountains. So perhaps there are more factors associated with gigantism than depth, if gigantic animals also flourish in a shallow sea of the past?

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