ISSN 1063-0740, Russian Journal of Marine Biology, 2006, Vol. 32, No. 3, pp. 201–204. © Pleiades Publishing, Inc., 2006.
Original Russian Text © V.V. Laptikhovsky, 2006, published in Biologiya Morya.
The rule of Thorson–Rass, or simply Thorson’s rule,
as was ﬁrst referred to by Mileikovsky , is known
well by marine biologists.
Thorson [30–32] documented an increase in egg
size and in the frequency of occurrence of marine
organisms with nonpelagic development from the equa-
tor to the poles and from coastal waters to oceanic
depths, i.e., with a decrease in water temperature. He
gave an ecological explanation to this phenomenon and
linked it to the scarcity of food available to pelagic lar-
vae. Thorson worked with gastropods (mostly, proso-
branchs) from Thailand, Denmark, and Greenland. This
taxon reveals all modes of ontogeny: through pelagic
larva and direct development.
Thorson’s rule is generally valid with fairly heterog-
enous taxa and manifests itself as the difference in inci-
dence of r- and K-strategists in faunas of neighboring
areas with different temperature conditions. For exam-
ple, there are latitudinal gradients of reproductive
modes of echinoderms and bivalves in the southern
hemisphere because taxa with nonpelagic development
are predominant on the coast of Antarctica .
The formulation of Thorson’s rule has been some-
what amended, and it is currently treated rather in terms
of latitudinal gradients of planktotrophic and leci-
thotrophic modes among planktonic larvae. Further-
more, there are quite a number of exceptions to
Thorson’s rule. They can be found even among the gas-
tropods, in which the patterns described by Thorson are
sharply pronounced [18, 24]. The comparison of muri-
cid reproductive strategies showed that this rule is true
for assemblages of species inhabiting rocky bottoms
but it does not hold with inhabitants of sandy biotopes
. The study of reproductive biology patterns of
South American marine prosobranch gastropods sug-
gests that Thorson’s rule is valid on the Paciﬁc coast of
South America, whereas direct development is preva-
lent all along the Atlantic coast from the subtropics to
Antarctica. In the latter case (as in the case with muric-
ids), Thorson’s pattern is not followed because predom-
inantly sandy bottoms occurring along the entire Atlan-
tic coast do not favor the thriving of taxa with a pelagic
larval stage, which need rocky substrates for larval set-
To explain the numerous exceptions, Poulin and
Feral  proposed an interesting hypothesis assuming
that Thorson’s rule could have been in full validity at
the time of glaciation maxima, while now only its evo-
lutionary consequences are observed.
Reproductive strategies of species are more or less
isolated in a speciﬁc r–K-continuum. By a mathemati-
cal model of Vance [33, 34], only the extremes of the r-
and K-strategy are evolutionarily stable (chances of
surviving increase with a decrease in occurrence of a
strategy). Modes with intermediate characteristics are
rather ineffective for the reproduction of a population.
Vance’s hypothesis has been tested in recent decades.
Christiansen and Fenchel  re-examined the original
simpliﬁed model and eliminated its defects, replacing
the linear growth of larvae by sigmoidal and linking lar-
The Rule of Thorson–Rass:
One or Two Independent Phenomena?
V. V. Laptikhovsky
Falkland Islands Fisheries Department, 598 Stanley, FIQQ 1ZZ, Falkland Islands
Received December 7, 2005
—Two investigators, G. Thorson in the example of invertebrates and T.S. Rass in the example of
ﬁshes, simultaneously documented an increase in the egg size of aquatic organisms with a decrease in water
temperature. This pattern is related to two different phenomena, which are erroneously united into the well-
known rule of Thorson–Rass. Thorson’s rule describes ecological processes linked to the change in larval biol-
ogy and morphology that are caused by the natural selective pressure on different types of larval development.
Rass’ rule describes physiological processes and manifests itself in comparisons within populations, between
populations of the same species, and between closely related species, and it is not associated with the change
of type of reproductive strategy or with the biology of early development stages. Thorson’s rule describes the
relative abundance of both r- and K-strategists within an r-K-continuum, whereas Rass’ rule characterizes the
relative position of both groups within the continuum.
Thorson, Rass, egg size, reproductive strategy.