Works about sea turtles commonly treat them from the perspectives of zoology, conservation technology, or resource economics. If you browse the Relative Index for "Sea turtles," you find entries for the topic in three different disciplines:
Sea turtles 597.928
Sea turtles--conservation technology 639.977928
Sea turtles--resource economics 333.957928
The number 597.928 appears directly opposite the term "Sea turtles" with no subheading—an indication that it is the interdisciplinary number.
Class works on the zoology of sea turtles and interdisciplinary works on sea turtles in 597.928 Chelonioidea (Sea turtles). In the upward hierarchy for 597.928 is 590 Animals (Zoology). An example of a work classed in 597.928 is Sea Turtles: An Extraordinary Natural History of Some Uncommon Turtles.
If one is familiar with the 590s, in which many numbers can be built following the add instructions under 592-599 Specific taxonomic groups of animals, one might expect to find sea turtles at a built number for turtles in the marine environment. In other words, one might expect to start with 597.92 Chelonia, follow the add footnote "Add as instructed under 592-599" and add 1 from 592-599: 1 General topics of natural history of animals, then follow the instruction to add "the numbers following 591 in 591.3-591.7" and add 7 from 591.7 Animal ecology, animals characteristic of specific environments, then follow the instruction at 591.71-591.78 Specific topics in animal ecology; specific environments to add "the numbers following 577 in 577.1-577.8" and add 7 from 577.7 Marine ecology, to build 597.92177 Marine turtles. However, the number 597[.92177] Marine turtles is bracketed (to show that it is invalid), with a do-not-use note: "Do not use; class in 597.928."
Class works that focus on the leatherback turtle in a subdivision of the broad number for sea turtles: 597.9289 Dermochelyidae (Leatherback turtle). An example of a work classed in 597.9289 is Leatherback Turtle: The World's Heaviest Reptile.
Class works that focus on the technology of saving sea turtles from extinction in 639.977928 Sea turtles—conservation technology. That number has 639.9 Conservation of biological resources and 600 Technology in its upward hierarchy. It appears already built in WebDewey: built with base number 639.97 Specific kinds of animals plus 7928 from 597.928 Chelonioidea (Sea turtles), following the add note at 639.971-639.978 Specific kinds of animals other than mammals. An example of a work classed in 639.977928 is Saving Sea Turtles: Extraordinary Stories from the Battle against Extinction.
Class broad works about sea turtles as a biological resource that include economic aspects in 333.957928 Sea turtles--resource economics. That number has 333.95 Biological resources and 330 Economics in the upward hierarchy. The number 333.957928 appears already built in WebDewey: built with base number 333.957 Reptiles and amphibians plus 928 from 597.928 Chelonioidea (Sea turtles), following the add note at 333.9578-333.9579 Amphibians, specific reptiles. More specific numbers can be built using the same add note, e.g., for Assessment of Leatherback Turtle (Dermochelys coriacea) Fishery and Non-Fishery Interactions in Atlantic Canadian Waters: 333.95792890916344 Leatherback turtle--resource economics--Gulf of Saint Lawrence, coastal waters of Newfoundland, coastal waters of eastern Nova Scotia. (The area number T2—16344 Gulf of Saint Lawrence, coastal waters of Newfoundland, coastal waters of eastern Nova Scotia best reflects the Canadian portion of the summer range of the leatherback turtle: see the map in the Canadian Geographic article "Leatherback Sea Turtle: Endangered Species.") Here is a summary of the number-building process in WebDewey: