Mar 01, 2024

Species Highlight: Lesser Known Tidepool Species on the Central Coast

Between the high and low tide lines on our rocky coastal beaches you can find a wide variety of marine life. In the rocky intertidal zone, low tides expose the tidepools, giving us a view into this fascinating ecosystem. Most people are familiar with larger charismatic species such as sea stars, sea urchins, crabs, and snails. However there are many more species that go unnoticed or misidentified in the tidepools. This blog post explores three of these lesser known invertebrate species that you may spot on a day out tidepooling along the Central Coast. 


Tunicates are a marine invertebrate (creatures without a backbone) with about 3,000 species found all over the globe. The name tunicate refers to their tunic-like, cellulose-rich outer covering that surrounds the body. Adult tunicates have a simple, sac-like body structure with two openings known as siphons. They feed and respirate by pulling water in one siphon and pushing it out the other. 

Pacific sea pork (Aplidium californicum) are a common sight in Central Coast tidepools. This colonial tunicate can grow several inches or more in width. Some tunicates are solitary, while others are colonial. Colonial tunicates are made up of individuals known as zooids that all share one “tunic” covering.

There are three living classes of tunicates, the most well-known being the class Ascidiacea. The species in this class are commonly known as sea squirts, sea tulips, or sea pork. Adult ascidians are sessile (non-moving) and may attach to natural elements such as coral, seaweed, or rocks, or to man-made structures such as boat hulls or dock pilings. Ascidians,commonly seen in rocky tidepools, are often misidentified due to their small size or unusual appearance. 

The long-stalked sea squirt (Styela montereyensis) is a solitary ascidian tunicate found in local tidepools.

On the SLO County coast, over 40 species of tunicates have been observed and recorded with the iNaturalist app. Some of the most commonly observed tidepool tunicates are the Pacific sea pork (Aplidium californicum), the chain tunicate (Botrylloides violaceus), and the long-stalked sea squirt (Styela montereyensis). Many other tunicates, including free-swimming salps and pyrosomes, have been observed by divers just off the coast or washed up on the beach. 


Hydrozoans (meaning “water animal” in Greek) are closely related to animals such as jellyfish, sea anemones, and corals. can be solitary or colonial. In colonial species, the individuals are known as polyps, and they are connected through a tube-like structure called a hydrocaulus that allows them to share nutrients. 

Although there are about 3,800 known species of hydrozoans worldwide, there are only a few commonly found in and around the Morro Bay area. The ostrich plume hydroid (Aglaophenia sp.) is commonly seen in local tidepools. Due to its brown coloration, branched body, and large size, it is commonly mistaken for algae or kelp. 

Ostrich-plume hydroids (genus Aglaophenia) are colonial hydrozoans frequently seen in local tidepools. The polyps of these species are very small and grow along the “branches” of the colony.

Although not a tidepool creature, another common hydrozoan on the Central Coast is the by-the-wind sailor (Velella velella). Despite its resemblance to a jellyfish, it is actually a colonial hydrozoan made up of many individual polyps. We typically see them in the springtime when winds push the colonies onto beaches, causing mass strandings. 

You can read more about Velella velella and their presence on the Central Coast in our blog post from last year.


Bryozoans (meaning “moss animal” in Greek) can come in a variety of shapes ranging from white single-layer encrusting sheets a few centimeters wide to a brown kelp-like shape several inches long. Today there are about 5,800 species of bryozoans worldwide, but looking at the fossil record, about 15,000 more species existed going back to at least 480 million years ago. Bryozoan species can be found in salt water, fresh water, and brackish water. 

Bryozoans are distinguished by their lophophore, a “crown” of tentacles around the mouth that aids in filter feeding. Most bryozoans exist as colonies, although a few are solitary. A bryozoan colony is made up of individuals known as “zooids.” Each colony has specialized zooids known as autozooids which serve functions such as feeding, excretion, and nutrient distribution. 

In SLO County, sixteen species of bryozoans have been observed and recorded using the iNaturalist app. The most commonly observed species are the leather bryozoan (Flustrellidra corniculata), the kelp lace bryozoan (Membranipora membranacea), the red bryozoan (Integripelta bibabiata – pictured above), and the red-rust bryozoan (Watersipora subtorquata).

Species Identification

Tidepool species can be difficult to identify, especially given the wide diversity of marine life you might encounter. Apps such as Seek or iNaturalist can be useful when trying to identify species. iNaturalist is a community science-based app where users upload geotagged photos of wildlife. The app then provides its best guess of a species identification based on the photo and location. Once the identification has been confirmed by another user, it is considered “research grade” and can be used by scientists around the globe. By using this app, you are not only learning more about local species but you are also contributing to biodiversity databases and research around the world. 


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