Certainly, few underwater species are as intriguing, or as individualistic, as the horseshoe crab (Polyphemus occidentalis). Scientists now regard this chelicerate arthropod, with its odd, heavily-armored appearance, as an evolutionary descendant of both arachnids and the prehistoric sea scorpion. Both career fishermen and physicians prize the horseshoe for its intrinsic worth - the former for its ubiquitousness as fishing bait, the latter for the unique chemical properties of its blood in medicinal usage - but those twin forms of exploitation have pushed the species to the very brink of extinction and dramatically reduced the numbers of horseshoe crab eggs, which in turn threatens the sanctity of a predatory shorebird called the Red Knot. The PBS-produced Crash - A Tale of Two Species (an episode of the Nature documentary series) investigates the symbiotic relationship between the Red Knot and the Horseshoe Crab, and discusses the prospective ways in which human intervention could alternately bolster or destroy that connection. ~ Nathan Southern, All Movie Guide