What animals live in seashells? The treasure hunt for re-inhibited shells

You probably already have an idea of what animals live in seashells. Surely, you have seen the shells of mussels, oysters, scallops, pipis and clams. Washed out on the beach you will often also find pretty gastropod shells (from sea snails). But the animals that live in seashells are not only their original owners. You will be fascinated to know what critters inhibit empty shells!

 

Animals which live in seashells in the kelp forests of Southern California

Many divers started out as children who enjoyed walking along beaches in search of empty shells. In Southern California, the kelp forests are rich will shell bearing animals.

The Norris top snail has a smooth brown shell and bright orange foot. It makes a journey from the sea floor to the surface along stalks of kelp every day with the rise and fall of the sun. The wavy turban snail has a spiral shaped shell and grazes on kelp and other algae. It competes with spiny red, purple, and crowned urchins for food and hiding spaces on the rocky reefs. Both these creatures try to hide from predators in rock crevices when they can. The wavy turban snails, however, also rely on their armoured shell for protection. The urchins, on the other hand, use a more “proactive” defense, bristling with sharp spines.

Abalone are large, flat snails that hide by camouflage while feeding on kelp. Abalone are considered a delicacy by people and just about every animal in the ocean for their meat. They equally sought by people for the beautiful “mother of pearl” on the inside of their shells.

Rock scallops and mussels are “bi-valve” animals with two shells, that live their lives anchored to the reef. They open to filter-feed tiny organisms from the water, and squeeze closed when a predator comes near.

Kellet’s whelks are large, thick shelled snails. They are predators that feed on smaller snails including wavy turban snails and Norris top snails. All these creatures are part of the kelp forest’s circle of life. They survive by grazing, hunting, or hiding from predators. And if they fail at any one of these things, they can perish.

 

Examining a heap of trash

The slow moving, shell bearing animals often become a meal despite their armoured protection. Predators such as sea stars, marine mammals, or fish catch them. They drag them off the reef to the sand where they have no escape, and eat them. After a meal, empty shells are abandoned by the predators. A few feet off the reef, you can find these reminders of the oceans “eat or be eaten” way of life.

On most Southern California reefs, the oceans natural surge finishes the process by pushing the shells around until they settle into rows in the sand. Any diver who hasn’t examined these areas to see what animals are living inside the leftover shells might be shocked. Nature, after all, recycles. What first appears to be an underwater “trash heap” is teaming with life! 

Many small fish and invertebrates find refuge in these shells, taking advantage of the newly available real-estate.  Sometimes these secondary residents are difficult to spot. Because of this, divers should always take care when examining what appears to be an “empty” shell. Please remember never to touch sea-life! The best way to make a “shell dive” is to explore a shell pile with a camera or flashlight and discover new animals that you might have otherwise overlooked. 

 

What animals live in empty seashells?

Fish

In Southern California, one of the favourite shell-inhabitants is the sarcastic fringehead (Neoclinus blanchardi).  These ferocious little fish have an enormous mouth, jagged teeth, bulging eyes, and an aggressive attitude. Fortunately, they are only about six inches long! They prefer large shell piles set in sandy patches a few feet off a reef. 

In some places, every wavy turban snail shell on the bottom has a sarcastic fringehead poking its head out. The little fish back into the shell and sit with their head and mouth exposed. Like this they wait for an unwary crab or other meal to come by. They are fiercely territorial and watch closely to make sure no other fringehead’s get too close. These fish are named, in part, for the tassels that grow from the top of their heads and give them a perpetual bad hair day.  When diving for these fish in Southern California, for added fun, bring a small mirror (or a large camera dome-port). You will see them flare out their gills, open their mouths, or even rush out of the shell in a territorial display, when they see their own reflection!

Another common fish found near the rock/sand interface at the edge of most California reefs are black-eyed gobies (Rhinogobiops nicholsi). They will often take refuge inside or under an empty shell and watch the world go by from their protected space. You can catch them darting out to grab a bite when something interesting drifts by.

 

Crabs

There are two common species of hermit crabs found roaming the shell piles in their “mobile” homes.  The California hermit crab (Phimochirus californiensis) is the smaller of the two. It can be recognised by its smooth skin and red/white colouring. These relatively small hermit crabs are shy and will quickly retreat inside their shells. They are experts in testing the patience of most divers before finally emerging again. 

More exciting is the hairy hermit crab (Pagurus hirsutiusculus). These thick bodied brutes grow large enough that they can be found inhabiting the empty shells of whelks or wavy turban snails. They are easily recognised by the thick fuzz growing off their legs. Best of all, they are less shy than their smaller cousins. So even divers who startle them into hiding are apt to see them again soon. 

 

Octopus

Inspecting “empty” shells will provide plenty of discoveries for divers. But the best find for most divers is probably an octopus. The two-spot octopus (Octopus bimaculoides), red octopus (Octopus rubescens) and the smaller pygmy octopus (Octopus microprysus) scour the shell beds for crabs, their favourite prey. In a bit of irony, an octopus can easily extract a hermit crab from a shell. And so it leaves the shell behind to be re-inhabited by another hermit crab later. 

Sometimes the octopus itself will move into the shell. Small two-spot octopi will coil into wavy turban snail shells. Red octopi will squeeze into Norris top snail shells, poking their eyes up like twin periscopes.  Shells provide protection for small octopi, which are a favourite food of many larger fish and marine mammals.  The octopi could also be waiting in a shell to ambush an unsuspecting hermit crab intent on moving into what it thinks is an empty shell!  Shell-searching on dives can be tricky. A pygmy octopus no bigger than a thumbnail might be blending into the enamel on the inside of an old scallop shell!

 

Other critters that live in seashells

In addition to several octopus species and the sarcastic fringeheads, there are other fish and invertebrates that call these underwater “graveyards” home. Coon-stripe shrimp, also known as dock shrimp, (Pandalus danae) are common finds among discarded shells. As tiny organisms like hydroids and sponges settle onto the substrate offered by the shells, you might even come across some colourful nudibranchs cruising around for a meal. 

When learning to scuba dive, responsible divers are taught to be careful when examining “empty” shells on the sea floor. Many shells have been re-inhabited by other animals. Those first scuba “check out” dives, kneeling in the shallow sand with a scuba class, allow divers the opportunity to see what critters actually live inside those shells. As divers move beyond those first discovery dives, they explore underwater reefs full of colour and life. But sometimes they forget the sandy edges of these reefs.

Taking a closer look at these areas, however, can be a “grown up” way to experience those same childhood thrills of searching for shells along beaches. In the kelp forests of Southern California, the species described here are quite common. “Shell Dives”, however, can be done on any reef, wherever people dive. On your next dive, take a few kicks away from the reef and look for empty shells lying nearby in the sand. At first glance it may look like an underwater graveyard, but the discoveries of what animals live in seashells might just leave you shell shocked!

 

About the author

Douglas Klug is a scuba instructor, underwater photographer, and diving writer based in Santa Barbara, California. He took his first underwater breaths on certification dives at the Monterey breakwater over 35 years ago. Since then, “Diver” Doug has continued to explore California’s waters as well as many tropical destinations. His favourite destinations are the kelp forests of Southern California’s Channel Islands. But his adventures have taken him to the sea turtles of the Hawaiian Islands, the great white sharks of Guadalupe Island, the stunning walls of the Cayman Islands, the coral reefs of Fiji, and even the humpback whales of Moorea.  

Doug’s photo-essays have been published in print world-wide, including articles in California Diving News, DAN Alert Diver, and Dive Training. His images have been used in numerous print and digital formats including use by the US National Park Service, US National Marine Sanctuary, and the Monterey Bay Aquarium, Google, and Microsoft. 

Doug’s experience includes being a recreational scuba instructor as well as diving with Nitrox and Rebreathers. He holds professional level certifications from NAUI, SSI, PADI, NASDS, and TDI. Doug also participates in volunteer scientific diving for organizations like Reefcheck. 

He is a former dive-shop owner and scuba repair technician. Doug still teaches private scuba lessons to interested new divers in Southern California. He also teaches underwater photography skills to more seasoned divers, and even offers an on-line course on Southern California nudibranch identification. 

Doug now splits his underwater time between taking photographs and showing new divers the underwater world. You can view his underwater work on his Instagram feed or on Flickr.