The arithmetic that makes this matter to anyone counting tonnage sits in one example. Clams in the Baltic are projected to lose 16.2 percent of average length and 41 percent of average mass [7]. Cube 0.838 and you get 0.588, so mass in that case is tracking almost exactly the third power of length [16]. Push the same exponent through the two groups projected to gain and the cephalopods' 2.9 percent length increase in the Arctic works out near 9 percent more mass, the tusk shells' 3.3 percent near 10 percent [17]. That extrapolation is mine rather than the authors', and it holds only if the same shape-preserving scaling carries across groups and basins.
What carries the weight in this study is the direction of change; magnitude is secondary. The relationships were fitted against temperature, oxygen and ocean productivity [4], and none of those three moves in step everywhere, which is the reason Trindade-Santos gives for the same group shrinking in one basin and growing in another [10]. Biology sorts the rest of it. Cephalopods carry high oxygen demand but pair it with efficient circulation and fast life cycles, which the authors offer as grounds for holding or adding size under warming, while the heavy shell-builders among bivalves and gastropods come out consistently vulnerable [9].
Five groups across ten basins gives a 50-cell grid, which is the number of combinations reported [14]. Two-thirds of that grid is roughly 33 cells under high emissions; the low-emissions run leaves fewer than half that many, and the count of projected increases barely moves [11][15].
This study leaves unresolved what is doing the shrinking. It is a correlative projection, built from 2.79 million occurrence records and a body-size database assembled out of decades of natural history collections [4][5], not an experiment in which any animal was warmed. The reported output is projected mean body size per group and basin [6], and the account does not separate individuals growing smaller from a change in which species occupy the basin at all. Those are different problems for a stock assessment, with different lead times and different remedies. Nor does the analysis reach landings, prices or farm sites; it stops at body size [3].
So the disagreement in sign is the finding worth acting on; the decimal places matter less. A single shrinkage coefficient applied ocean-wide will point the wrong way somewhere on that grid, and McClain's own framing is that the rule of thumb was too simple a story to begin with [12]. The magnitudes are best treated as hypotheses to test against the length data that surveys and monitoring programs have already been collecting for years.</body_markdown> </invoke>
