5 Phyla to Identify in a Tide Pool

Yeah so this is basically a hodgepodge of my notes from that zoology course I took back in undergrad.

Tide pools are perfect places to explore when one is steeped in the taxonomy and physiological nuances of a zoology course. It is here that the diversity of the Animal Kingdom comes alive! (Although I also threw some algae in here . . . )


Chitons and barnacles surrounded by black encrusting sponge and assortions of red and green algae. A limpet lives near the etches left by two other limpets who previously lived here.

The arthropods are the largest phylum on the planet. The largest class within arthropods is insects – and the largest order within insects is coleoptera (cue a JBS Haldane quote, something about God having an ‘inordinate fondness’ of beetles.)

But we are at a tide pool, not exploring under rocks in a forest. Hence our arthropods include the sedentary barnacle and nimble crabs.

In the splash zone, you can find crabs (in the Bay Area, there are lots of purple-clawed species as well as chevron-patterned green ones) as well as the tiny rock louse (Ligia occidentalis). Both belong to the order malacostraca – the rock louse is an isopod and the crabs, decapods (also including lobsters, shrimp, and hermit crabs). The malacostracans have body types with high segmentation, allowing for this high diversity in exoskeletons.

Barnacles are also malacostracans, and the arthropod animal lives inside the outer barnacle formation. When the tide is high, the barnacle opens and the sends out its long cirri to filter feed plankton out of the seawater.

A sand louse.

Most animals are divided by their embryonic development patterns as protostomes or deuterostomes. Arthropods are protostomes, meaning their embryos:

  1. Begin with spiral cleavage, in which each cell’s position determines what kind of cell it will become
  2. Develop their coelom (body cavity) from mesoderm tissue, with the mouth forming first
  3. And finally, their namesake: the mouth forms first (“protostome” is literally “first mouth”).

Protostomes are further divided into lophotrochozoans or ecdysozoans, and arthropods are in clade ecdysozoa. The most familiar defining feature of this clade is that these animals molt their exoskeletons.


I most often find sponges in the encrusting sponge version found hugging the rock, a pink, white, or black creature almost resembling a terrestrial lichen. Sponges of the more three-dimensional variety can sometimes be found washed up on the beach. They are filter-feeders and thus major players in their habitats, keeping the water clean.

Sponges are neither protostomes nor deuterostomes. They are cellular animals with no tissues, but collections of choanocytes that filter feed seawater. They have no symmetry but consist of prosopyles (pores) and dermal ostia (cannals) that flush water through – many cleaning up to 1500L/day.


Your pink and red algae – coralline algae, Corallina, and Porphyra – are rhodophytes, species within the algae kingdom that is not quite Kingdom Plantae.

Seaweeds, including rhodophytes, are not classified as plants because they lack organized tissues. Plants have three tissue types: roots, stems, and leaves. While seaweed blades resemble leaves and a holdfast resembles roots, there is no difference in cell types throughout the algae.

Taxonomy among photosynthetic organisms is always changing; seaweeds have switched in between algae and plant kingdoms before, and currently they are placed into an obscurely named clade within Eukaryota domain. As such, seaweeds are about as taxonomically esoteric as you can get.

Although chlorophyll A and B are still used by rhodophytes, dominant pigments are the red-absorbing phycobilins. The phycobilosomes (light-absorbing proteins of photosystem II, used in photosynthesis) are packed with chromophores (light-capturing pigments) like phycocyanobilin (a blue pigment), phycoerythrobilin (red), and phycourobilin (orange).


Starburst anemone. Anemones are another kind of cnidarian, though their nematocysts are harmless to people.

Such beautiful creatures! Named for their stinging nematocysts (a type of cnidocyte, or harpoon-like cells that inject venom into whatever contacts them), cnidarians include jellies, sea anemones, and hydras.

True jellies are in class scyphozoa, and you may find them washed up on the beach (among other squishy things that end up there). In the tide pools, you’ll find anthozoans – in California, that may include speckled multicolored starburst anemones, or mesmerizing pink-tipped giant green anemones. Corals, a colonial animal, are also anthozoans.

There are only two cnidarian body types: medusae, and polyps. Many life cycles transfer between the two; in jellies, sexually produced plankton find a spot on the seafloor and grow in polyp form, sometimes developing asexually produced strobilates of polyps before releasing them all as mature jellies. Sea anemones, on the other hand, are polyps only and know no medusa stage.

A jellyfish washed up among the seaweeds.

Corals use their nematocysts to catch zooplankton, but jellies and sea anemones can catch and kill fish, crustaceans, sea snails – the sea anemone doesn’t just eat its prey, but covers itself with the shells of its meals (for camouflage and to keep from drying out).

It’s mildly terrifying, if you think of it, that animals almost entirely water – animals that evaporate on the beach if left a moment too long – are capable of destroying something as hard and stony as a mollusk.

Cnidarians are also outside of the protostome-deuterostome dichotomy that divides much of zoology.


It can be hard to spot some of these long and thin animals – you have to look closely, maybe check under rocks, and perhaps reach the tide pool at a very low tide.

I speak of course of the vermiform creatures known as worms, though any zoologist knows that ‘worm’ is not specific enough – wormy phyla include nematodes, platyhelminths, and annelids. You can certainly find platyhelminthes representatives (flatworms) in tide pools, perhaps by sneaking a look under a wet rock. Annelida (segmented worms) include earthwormy-types and worms with a bit more décor.

A worm found crawling near some mussels and a sea star.

These annelid classes are polychaetes and clitellata. The clitellatans are your familiar earthworms, as well as the long, thing creature we see here in a Californian tide pool. They are named for their clitella – their collar, which stores eggs.

Polychaetes are also in tide pools. In the littoral zones, you can find honeycomb worms (Sabellaria alveolata), which almost resemble barnacles in their aggregate housing formations. But these are worms, not arthropods, living inside. And unlike barnacles, they do not grow their shells – they collect them, building them out of sand and shell bits.

Another polychaete is the clam worm. In this creature you can see the namesake of its class – they are bristle worms. The appendages you see along this segmented worm are not legs, but bristled parapodia. Where do they sit taxonomically? Annelids are protostomes. Rather than the hard-bodied ecdysozoans (such as arthropods), they are soft and squishy lophotrochozoans. They might have a complete gut, a blind gut or none at all.

Zoology is a class with a lot of taxonomy and a lot of terminology – but it certainly made me much more attentive to the beauty and diversity of a tide pool.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: