15.2: A Quick Lesson in Classification

For centuries, taxonomists relied on rank-based classification for describing the diversity of organisms. The development of cladistics spurred a new nomenclature and a reshuffling of evolutionary relationships. These reshufflings—some of which are controversial—have resulted in a Game of Thrones–type battle between various groups of taxonomists. (Let’s just hope it ends better than the series did.) Both approaches have merit: traditional classification is easier to remember, but cladistics is more accurate. We briefly review both here.

The traditional Linnaean rank-based system relies on placement of organisms into different taxa, the different levels in the hierarchical system. The most common taxa—in order from the most specific to the most general—are species, genus, family, order, class, phylum, kingdom, and domain. Modified groupings also exist (e.g., subphyla, superfamily, infraorder), but we’ll stick with the standard eight taxa here. You can recall the order of ranks with this helpful mnemonic: “Didn’t Know Popeyes Chicken Offered Free Gizzard Strips.” There are vegan alternatives, too, if you prefer. (See mnemonic-device.com 2022.) As one example, the largest animal to have ever lived on Earth—yes, bigger than any dinosaur found so far—goes by the common name blue whale—a filter-feeding baleen whale named for the blue hue of its skin when underwater. Its scientific name, Balaenoptera musculus, makes it part of the genus Balaenoptera, a genus with seven other species, including minke, Bryde’s, fin, and sei whales. At the family level, these eight join the lone humpback whale species (Megaptera novaeangliae) in the Balaenopteridae (e.g., Fordyce and Perrin 2023).

At lower levels it gets trickier. And here’s where classification using cladistics influences rank-based classification. When I learned whale taxonomy, baleen whales (suborder Mysticetes) joined toothed whales (suborder Odontocetes) in the order Cetacea. New fossil finds and molecular evidence joined the baleen and toothed whales with—wait for it—cows, camels, hippopotamuses, and deer, among others (e.g., Poor 2023). The combined group, known as the order Cetartiodactyla or Artiodactyla (depending on the taxonimists), now includes the Cetacea, Mysticeti, and Odontoceti but their ranks remain unclear (e.g., Fordyce and Perrin 2023). Dolphins and porpoises join this group, along with all other even-toed ungulates. These groupings are more accurate, but quite the tongue twisters.

To the layperson, this may seem like pencil straightening. But the discovery that whales and hippos have a common ancestor is truly remarkable and almost makes sense, given how hippos swim. The Royal Ontario Museum calls them “unlikely cousins” (Hirt 2017). Fortunately, at higher levels, blue whale classification is a bit more stable. They are members of the class Mammalia (hey, that’s our class!), the phylum Chordata (our class!), the kingdom Animalia (our kingdom!), and the domain Eukarya (yep, ours). Hopefully, this brief example illustrates the level of detail inherent in the classification of organisms and, more important, the understanding that it brings to the natural world.

Be that as it may, the hierarchical rank-based system proves very useful for making sense of the world before your eyes. A trip to the rocky intertidal zone reveals a cornucopia of life-forms and ecological roles. Green, red, and brown seaweeds—the phyla (the plural of phylum) Chlorophyta, Rhodophyta, and Phaeophyta, respectively—form bands of color arranged by tide height. Tidepools reveal organisms that look like flowers—the sea anemones—the most abundant of which is commonly called the aggregating anemone, or Anthopleura elegantissima. If you snorkel out a ways (or visit a marine aquarium), you might see their gelatinous relatives, the jellies (not jellyfish because they’re not a fish). Sea anemones and corals make up the stinging phylum, the Cnidaria (pronounced nye-DARE-ee-uh).

A superficially similar gelatinous form—the comb jellies, or phylum Ctenophora—may be the oldest animals on Earth, a position they currently compete for with sponges, the phylum Porifera (see Neilsen 2019; Li et al. 2021; Redmond and McLysaght 2021). If you look closely, you may see a few purple patches with tiny, chimney-like openings spread across the underside of a ledge. These are the purple encrusting sponge, Haliclona—a genus with many representatives around the world. They belong to the demosponges (class Demospongiae), the most diverse class (some 7,000 species) among the more than 9,000 species of sponges (Morrow and Cárdenas 2015; WoRMS Editoral Board 2023). Occasionally, a segmented bristle worm, a polychaete, will crawl across the rockscape. Members of the phylum Annelida—the same phylum as earthworms—polychaetes inhabit nearly every ocean environment, from free-swimming forms in surface waters to benthic forms on abyssal plains, hydrothermal vents, and oceanic trenches.

You’re also sure to see a mollusk, phylum Mollusca, arguably the most successful phylum in the world ocean. Mollusks brought us seashells, one of nature’s most beautiful forms. The phylum also includes the octopus, one of the most intelligent invertebrates—animals lacking a backbone. The phylum also includes animals we eat, namely, clams, mussels, and squid. The byssal threads by which mussels attach themselves to rocks, are among the strongest materials on Earth (e.g., Bell and Gosline 1996).

Of course, the phylum Arthropoda, the jointed appendage animals, which include the insects, are the most successful terrestrial phylum. Nevertheless, they are well represented in the ocean as species of crabs, shrimps, lobsters, barnacles, krill, sea spiders, and the flea-like copepods, possibly the most numerous animals in the ocean. Their movements are hypnotic, like tiny dancers beneath the sea.

And while there are many, many more varieties of incredible marine life-forms that I could talk about for days, I’ll leave you with one of the most iconic forms, the sea star (again, not a fish). Their five arms, spiny skeleton, and legions of water-powered tube feet illustrate the major characteristics of the phylum Echinodermata, to which they belong. Along with the prickly sea urchins (purple and red ones in Southern California), the soft-bodied sea cucumbers (whose internal muscles are a delicacy in some parts of the world), the wavy-armed brittle stars (often found beneath rocks), and the less familiar crinoids (whose arms resemble the headpieces seen in Las Vegas shows), the echinoderms represent the largest ocean-only phylum and the most abundant phylum on the abyssal seafloor. Plastic sea stars make wonderful toppers on a Christmas tree. (Just don’t use the real thing; they belong in the ocean . . . alive.)