Who writes this stuff?
I had to scroll through 2/3 of the article to find the (meager) definition of the rule being discussed.
High school writing assignments... Wait, no, grade school assignments taught to define the subject first, to make the remaining discussion meaningful.
This was clearly written for the average reader, not people within the community studying sharks (or other animals), so assuming people know this rule is a major oversight.
Why then, did they "bury the lede"? Because they had other points they wanted to make before providing the reader the necessary context.
Ten paragraphs in they state:
sharks surface area is proportional to volume raised to the power of 0.64. That's just 3% off the theoretical prediction of 0.67
So they still haven't explicitely stated this 2/3 rule, but I'll infer the rule predicts animals (all animals? Or just aquatic vertebrates?) body size is constrained by a rule that surface area is ~67% of body volume (they didn't even explain it that simply, instead using "raised to the power"). Can they be any more obfuscatory?
Why would you ever write it this way for non-scientist discussion?