Fab four Phyla part one: Echinoderms

When we take groups out tidepooling they are going to see a lot of new and interesting animals and 80% of these animals fall into one of four different phyla.  Phyla are on of the levels of classification scientists use to  where the broadest group is a Kingdom and it increases in specificity until you reach the species level.  All together you have Kingdom, Phylum, Class, Order, Family, Genus, Species.  We rarely use scientific names when referring to animals but we like people to know the four main phyla that we see on our beaches.  Our fab four phyla are Cnidaria (anemones and jellies), Arthropoda (crabs, shrimp, barnacles), Echinodermata (urchins, stars, cucumbers) and Mollusca (clams, octopus, snails).

As the name implies, Brittle Stars lose their arms very easily

The first phyla I'd like to talk about is Echinodermata.  Echino means spiny (think of an echidna) and dermata refers to skin (as in a dermatologist) so Echinoderm translates to "spiny skin".  Sea urchins are the eponymous animal for this phyla and don't outwardly resemble their cousins, sea stars and sea cucumbers.  However, all Echinoderms share the distinct characteristics of pentaradial symmetry, the ability to regenerate, and water vascular system that enables them to move.  

A beautiful Rose star

For our groups, Sea stars are the most popular invertebrates and people are always excited to see them.  This works out since there are a lot of cool things we can show people about them.  Sea stars have a dot on their dorsal side called a madreporite or mother pore which serves as a hydraulic intake valve.  By pulling in water through this pore they are able to move their tube feet which act as tiny suction cups.  They use this hydraulic action to pull open mussels and clams and eat them.  Once they have a mussel shell open even a millimeter, the sea star will slide its stomach out of it's body and inside the mussel shell in order to eat it.  Sea stars also have tiny pinchers called pedicilleria which allow them to protect the gills that cover their dorsal side.  They don't want kelp or barnacles to settle on them and the pedicilleria defend this vulnerable spot.  These pinchers are strong enough to tear out arm hair (as I often demonstrate) or they can also be used to attach a sea star to a kid's fleece.

Stimpson's Sun Star

Urchins, the next member or this group, are not nearly as dangerous here as they are are in tropical waters.  Our urchins don't have poisonous spines and are completely safe to handle.  Urchins eat algae using their five teeth and their tube feet are extra long so they can extend past their spines.  Urchins are also capable of seeing color, though this ability is not well understood by scientists.  They have proven in experiments that urchins are able to associate color with food and that urchins can see the entire color spectrum but they are unable to figure out how the urchins can see.  

 

A True Star nestled amongst Green Urchins

The last member of the Echinoderms is the sea cucumber.  Although not readily apparent, sea cucumbers also have penta radial symmetry if you stand them on their ends like a cylinder.  They also have tube feet which they use to hang onto the rocks and mud they bury themselves in.  The cool fact about cucumbers that we quote endlessly to our groups is their ability to eviscerate themselves to distract predators.  They will spit out their guts as an offering/distraction if something is harassing them enough and then they will regrow their insides if given enough time.  Sea cucumbers eat by pushing their mouth parts out in a feathery fan which they can use to collect plankton and other detritus out of the water or mud.  

Sunflower Star

The largest member of the Echinoderms that we find in our tide pools is the sunflower star.  These stars can often reach three feet in diameter and have up to twenty four arms.  Compared to the other sea stars, sunflower stars are extremely soft because they are holding more water.  They use this extra water to operate more tube feet (over 15,000 of them!) and can move much faster than other sea stars because of this.  They are extremely voracious predators and will often eat other sea stars when given the chance.  Because their body is so soft, these creatures prefer the lower inter-tidal zone and cannot be out of water for very long.

Sunflower Star's tube feet