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I was reading Wikipedia regarding deep-sea gigantism -- the fact that deep-sea species are often much larger than their shallow-dwelling counterparts. The article said,

Decreasing temperature is thought to result in increased cell size and increased life span... both of which lead to an increase in maximum body size.

The citation is: S F Timofeev Izv Akad Nauk Ser Biol. 2001 Nov-Dec:(6):764-8. [Bergmann's principle and deep-water gigantism in marine crustaceans]

I haven't tried to retrieve that article because I can't read Russian. Following up on "Bergmann's principle" doesn't help; it's just identifying ecographic patterns in gigantism.

From a surface area to volume ratio perspective, I can see how smaller cells are adaptive to cold temperatures. A huge amount of cellular processes are diffusion-driven. All of them, except for some potential electron or proton tunnelling processes, all mediated with thermal movements.

So let's say the transport of glucose into a given cell is driven by the concentration gradient (diffusion). At lower temperatures the diffusion rate is lower, so a cell of a given volume gets a reduced import of glucose. So it seems that increased surface-to-volume ratio (a smaller volume) would allow for a cell to compensate, and to accomplish the same rate of import as the warmer cell. This would also apply to some intracellular processes.

So assuming my thinking about diffusion processes is irrelevant to the real-life biology, as it predicts for colder temperatures that smaller cells are adaptive, is the point that metabolic processes are slower at lower temperatures? Then, a reduced rate of transport is not necessarily the limiting factor. Maybe the cellular volume can be scaled up until the total metabolic flux is comparable to the warmer-living cell? If there's more dissolved oxygen at lower temperatures, and if oxygen equilibrates well-enough, maybe no other nutrient or signalling diffusion is relevant?

This thinking does not at all seem consonant with this story by Curtis Deutsch and coworkers: Impact of warming on aquatic body sizes explained by metabolic scaling from microbes to macrofauna.

The model reproduces three key aspects of the observed patterns of intergenerational size reductions measured in laboratory warming experiments of diverse aquatic ectotherms (i.e., the "temperature-size rule" [TSR]). First, the interspecific mean and variability of the TSR is predicted from species' temperature sensitivities of hypoxia tolerance, whose nonlinearity with temperature also explains the second TSR pattern-its amplification as temperatures rise. Third, as body size increases across the tree of life, the impact of growth on O2 demand declines while its benefit to O2 supply rises, decreasing the size dependence of hypoxia tolerance and requiring larger animals to contract by a larger fraction to compensate for a thermally driven rise in metabolism. Together our results support O2 limitation as the mechanism underlying the TSR, and they provide a physiological basis for projecting ectotherm body size responses to climate change from microbes to macrofauna.

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    $\begingroup$ are there any deep sea animals limited by a skeleton or exoskeloton? that wouldn't be related to cell size. another consideration is whether the cell size difference is average or median. I know there are a few single celled organisms that are like the size of the basketball that live on the sea floor. most of the sea floor is deep, so if you have a few species down there like that, they could dramatically increase the "average" cell size without affecting the median. then again, maybe Caulerpa taxifolia is proof that actually the largest average cell size is actually not in the deep sea. $\endgroup$ Mar 16 at 23:02
  • $\begingroup$ I didn't know about how large the cells can be in C. taxifolia. That does seem to place tensions on any theory that emphasizes deep sea conditions and doesn't include oxygen availability as a factor. Since photosynthesizers generate their own local oxygen, they would be able to flaunt any biophysical constraints on cell size for ectotherms mentioned below in @Eonema's answer. $\endgroup$
    – Ryan
    Mar 20 at 0:53

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Woods (1999) proposed an explanation for the negative effect of temperature on ectotherm cell size based on the rates of transport and metabolism of oxygen - both of which you considered in your question. The diffusion of oxygen decreases with increasing temperature. However, this effect is small compared to how much the consumption of oxygen increases with increasing temperature. Therefore, in a warmer environment, despite having greater oxygen supply, a cell is still more oxygen-limited because of the relatively greater increase in the oxygen demand. In a colder environment, a cell can be larger because it is less oxygen-limited and can afford to have a lower surface area:volume ratio. As Woods puts it:

The critical observation is that oxygen supply (determined jointly by [the surface concentration and diffusion coefficient]) rises slowly with temperature, but that oxygen consumption rises rapidly with temperature. Thus, at higher temperatures, the oxygen gradient within a metabolizing sphere will be steeper, and the radius at which the oxygen concentration at the center falls to zero will be smaller.

Note that while this hypothesis makes theoretical sense, it's not clear that the effect of temperature on cell size actually drives differences in body size or even occurs in nature. The study on temperature and cell size (Van Voorhies, 1996) that was cited by the Timofeev (2001) paper was solidly refuted by Partridge and Coyne (1996) for looking at genetically identical individuals and failing to consider latitudinal genetic variation, along with other sources of faulty inference. There are a number of plausible competing hypotheses to explain deep-sea gigantism and Bergmann's rule, not all relating to cell size.

Nevertheless, this same process proposed by Woods could also apply to organisms as a whole, rather than individual cells. Deutsch et al. (2022) consider the same mechanism as Woods, except they considered whole organisms. Therefore, their equations are somewhat abstracted from the biophysical processes that Woods deals with, but it's basically the same mechanism.

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    $\begingroup$ Bergmann's Rule is ecographic, but that's because higher latitudes tend to have lower temperatures. "The rule derives from the relationship between size in linear dimensions meaning that both height and volume will increase in colder environments" from the wiki article. $\endgroup$
    – Rich
    Mar 19 at 19:58
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First, some corrections to assumptions in your question. As I pointed out in my comment; Bergmann's Rule is ecographic, but that's because higher latitudes tend to have lower temperatures. "The rule derives from the relationship between size in linear dimensions meaning that both height and volume will increase in colder environments." Also a careful reading shows that rather than contradicting you, Deutsch and his collaborators’ article fully supports your hypothesis; “Maybe the cellular volume can be scaled up until the total metabolic flux is comparable to the warmer-living cell?” Not only that but increasing temperature increases the metabolic rate, further exacerbating the problem of oxygen uptake for the warmer cell. From the Woods article quoted by Eonema: “rates of reaction are more sensitive to temperature than are physical processes such as diffusion and electrical conductivity.” And Eonema correctly states that in larger cells this increased rate of metabolism prevents the diffusion of sufficient oxygen to the center of such cells.

From the Deutsch article; “Among aquatic ectotherms, the TSR has been ascribed to limitation by dissolved O2. According to this hypothesis, a thermally driven increase in O2 demand outpaces any rise in O2 supply, a balance that can be restored by confining growth to a smaller size, resulting in a larger ratio of respiratory surface area to body volume.”
And from your excerpt: “Third, as body size increases across the tree of life, the impact of growth on O2 demand declines while its benefit to O2 supply rises, decreasing the size dependence of hypoxia tolerance and requiring larger animals to contract by a larger fraction to compensate for a thermally driven rise in metabolism.” The Deutsch full article shows that it is the oxygen concentration decreasing with increasing temperature, rather than the temperature increase itself, that is the real driver of TSR.

The paper introduces several different models and experimental results to compare the effects of increasing temperature and decreasing oxygen concentration across a wide range of fauna, from bacteria on up.

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This relates resting oxygen demand rate to size.

Whilst the Deutsch paper does not directly address increasing cell size, it does show the same effect in bacteria and other microbes, indicating that this probably applies to individual cells in larger ectotherms.

So, in conclusion, it is the decreased metabolic rate, coupled with the higher oxygen concentration in cold water that enables and encourages larger cell and animal sizes.

This answer has not addressed the possibility of longer lifetimes on increased size.

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  • $\begingroup$ Besides the assumption that my considerations of temperature-dependent metabolism were different from the model of Deutsch and others (thanks for explaining how my picture is closer to theirs than I thought), what assumptions needed correction? I wasn't saying anything about Bergmann's Rule (other than correctly labelling it as ecographic). My question is focusing entirely on deep sea gigantism and the role of low temperatures in supporting it on a cellular level. $\endgroup$
    – Ryan
    Mar 22 at 14:18
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    $\begingroup$ It was just that you said " Following up on "Bergmann's principle" doesn't help; it's just identifying ecographic patterns in gigantism." I was only pointing out that Timofeev's paper refers to Bergmann's principle as related to temperature rather than location. "An increase in the geographic latitude and depth of crustaceans habitat (correlating mainly with lower temperatures) leads to an increased cell size" $\endgroup$
    – Rich
    Mar 22 at 14:47

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