As Kramer from Seinfeld demonstrated, sponging off of other people can be a successful life strategy. The same is true for many members of the Animal Kingdom. In a new study published in Biology Letters, researchers Sara Weinstein and Armand Kuris from the University of California-Santa Barbara show that parasitism independently evolved many more times than originally thought.
To conduct their analysis, Weinstein and Kuris examined how often parasitism evolved from non-parasitic ancestors. They concluded that it evolved at least 223 times, far more than the previous estimate of 60. As shown below, parasitism arose more times in certain phyla (e.g., arthropods, nematodes, flatworms, and mollusks) than in others.
Parasitism evolved on at least 143 occasions in the arthropods, not because there is something unique about them but most likely because this phylum includes a very large number of species. Indeed, arthropods comprise ocean critters (e.g., crabs and shrimp) as well as land-dwelling creepy-crawlies (e.g., scorpions, spiders, ticks, centipedes, and insects).