Weird Clam Profile: The Heart Cockles

Corculum cardissa (from Wikipedia)

The heart cockle (Corculum cardissa) is so named because of its heart shaped shell. It is native to warm equatorial waters of the Indo-Pacific. While many bivalves sit with the their ventral valve facing down, the heart cockle sits on its side, with one side of both valves facing downward. The valves have adapted to resemble wings and are flat on the bottom, providing surface area that allows the bivalve to “raft” on the surface of soft sandy sediment and not sink. They may also sit embedded in little heart-shaped holes on the tops of corals.

Two heart cockles embedded in the top of a Porites coral. Source: Reefbuilders
A particularly green heart cockle from Singapore. Source: orientexpress on iNaturalist

Heart cockles are a member of a small club of bivalves which partner with symbiotic algae for nutrition created by photosynthesis. Most of the modern photosymbiotic bivalves are in the family Cardiidae, the cockles. The giant clams (Tridacninae) are also in this family and have a similar partnership with the same genus of Symbiodinium algae. This algae is also found in many species of coral.

The dark circles in these microscope images are Symbiodinium. The top is a view of giant clam body tissue. The cells are present throughout the tissue in giant clams. The bottom shows heart cockle “tubules” which contain their symbiotic algae. The algae are restricted to narrow tubes that run through the tissue of the cockle. Source: Farmer et al. 2001

So when you find a live heart cockle, it is often green in color, because of the presence of this algae near the surface of its tissue. Its shell has adapted to be “windowed” (semi-transparent) to allow in light for the algae to harness to make sugars. The algae are housed in networks of tubes within the soft tissue of the cockle. They trade sugars with their host in exchange for nitrogen and carbon from the clam.

As I’ve mentioned before regarding the giant clams, this is a very productive partnership and has evolved separately several times in the history of bivalves. However, we don’t know why almost all examples of modern bivalve photosymbiosis occur in the cockles. Why aren’t the heart cockles giant like the giant clams? What features are necessary to allow this symbiosis to develop? These are the kind of questions I hope to help answer in my next few years of work.

Oh, the seasons they grow! [research blog]

My latest clamuscript is published in Palaios, coauthored with my advisor Matthew Clapham! It’s the first chapter of my PhD thesis, and it’s titled “Identifying the Ticks of Bivalve Shell Clocks: Seasonal Growth in Relation to Temperature and Food Supply.” I thought I’d write a quick post describing why I tackled this project, what I did, what I found out, and what I think it means! Raw unformatted PDF of it here on my publication page.

Why I did this project:

I study the growth bands of bivalve (“clam”) shells. Bivalves create light and dark shell growth bands as they grow their shells, much like the rings of a tree. The light bands form during happy times for the clam, when it is growing quickly and putting down lots of carbonate. The dark bands appear during times of cessation, when the bivalve ceases growth during a hibernation-like period. This can happen in the cold months, or the hot months, or both, or neither, depending on the clam and where it lives. It turns out that there are a lot of potential explanations for why these annual cessations of growth happen. Different researchers have suggested through the years that temperature (high or low) is the biggest control on the seasons that bivalves grow, but others have suggested that food supply is more important. Others say it’s mostly a function of the season they reproduce, when they’re putting most of their energy into making sperm/eggs and not growing their bodies. I wanted to try to see if I could find trends across all of bivalves which would shed light on which factors are important in determining their season of growth.

Annual growth lines in the shell of a giant clam. The transparent spots are the times that it was growing more slowly and not happy. Was this because of temperatures? Or was it getting less to eat? I wanted to know.

What I did:

I read a ton of papers in the historical literature about bivalves. These were written by people in many fields: aquaculture, marine ecology, paleoclimate researchers (using the clams shells as a chemical record of temperature), and more. All of the papers were united by describing the seasons that the bivalves grew, and the seasons that they stopped growing. I ended up with nearly 300 observations of marine (saltwater) bivalve growth for dozens of species from all around the world. I had papers as old as the earliest 1910s, and some as new as last year.

A map of all the places the observation of bivalve growth came from. Blue means they shut down in the winter, while red means they do not.

We have mussels, oysters, scallops, clams, cockles, geoducks, giant clams, razor clams, quahogs, and more in the database. Bivalves that burrow. Bivalves that sit on the surface of the sediment. Bivalves that stick onto rocks. Bivalves that can swim. With each, I noted data that the researchers recorded. If they grew during a season, I coded it as a 1. If they didn’t, I coded it as a 0. So a bivalve growing in summer but not winter would be recorded as 1,0. I also recorded environmental data including temperature of the location in winter and summer in the location, as well as seasonal supply of chlorophyll (a measure of phytoplankton, which is the main source of food for most clams). It turned out that not enough of the studies recorded temperature or chlorophyll for their sites, so I wanted to back these up with an additional data source. I downloaded satellite-based temperature and chlorophyll data for each location, as well as additional studies which directly measured chlorophyll at each site. I wanted lots of redundant environmental data to ensure that any trend or lack of trend I observed in my analysis was not due to a weakness of the data.

I then compared the occurrence of shutdown by season with these environmental variables using a statistical technique called regression. Regression basically involves trying to relate a predictor variable (in this case, latitude, temperature and chlorophyll during a certain season) to the response variable (did the clam grow in that season or not?). We wanted to see which environmental variable relates most closely to whether or not the clam grows or not. Because our dependent variable was binary (0 or 1), we used a technique called logistic regression, which tries to model the “log odds” of an event occurring in response to the predictor variable. That log odds can then be back-calculated to probability of the event occurring.

What we found:


In a clamshell, we found that latitude (distance from the equator) is a very good predictor of whether or not a bivalve shuts down for the winter. As you’d expect, bivalves in the far north and far south of our planet are more likely to take a winter nap. However, bivalves at the equator mostly grow year round and are not likely to take a summer nap. In relation to temperature, the lower the winter temperature, the more likely the bivalve is to stop shell growth. High summer temperature is not as good a predictor for the occurrence of a summer shutdown, but the majority of summer shutdowns seem to occur at the low temperate latitudes, where the difference between the annual range of temperature is largest. Unlike at the equator, where bivalves likely can adapt to the hottest temperatures and be happy clams, they have to adapt to a huge range of temperatures in places like the American Gulf and Atlantic coasts, the Adriatic and Gulf of California. And if they are restricted at the northward end of their range, they may have no choice but to shut down in summer as there is nowhere cooler to migrate to.

GIF of the satellite data showing white as hotspots of phytoplankton ability. Notice that the food is more available in summer months for each hemisphere. We were trying to see if this relates back to when the bivalves grow in every place we had data for.

Food supply, on the other hand, is not a good predictor of when bivalves shut down. When we went into this project, we expected food to be a powerful control on seasonal growth because it is intuitive and well understood that the better fed a bivalve is, the larger it will grow overall. But the seasonal low amount of chlorophyll (and therefore the amount of photosynthesizing plankton) in the bivalves’ areas had no relationship to whether or not the bivalve shut down in a certain season. To double check that this wasn’t a weakness in my satellite data, I downloaded additional direct observations from the same places as many bivalve studies in the dataset, but I still couldn’t find the relationship. We propose that the seasonal supply of phytoplankton is not well related to seasonal growth of bivalves because: 1) phytoplankton supply isn’t very seasonal in nature in most of the sites we studied. There are peaks in multiple seasons rather than a clean up and down wave shape like temperature. 2) Bivalves are pretty flexible in what they eat. They also eat other types of plankton and suspended particles that are even less seasonal. It may be pretty difficult to find bivalves that are seasonally starving. One of the most probable places to find such starvation shutdowns might be the poles, where seasonal ranges of temperature are quite small but plankton does really have a seasonal pattern of availability. More research will be needed to describe the nature of polar bivalves and why they shut down growth.

What’s next?
This is the first chapter of my PhD. I have two more chapters I’m working on, both related to the geochemistry of bivalve shells. I am writing those manuscripts this summer and looking for postdoctoral fellowships in the fall related to geochemistry of marine organisms in the fossil record. I hope to pursue more projects looking at the season of growth in bivalves, switching to understanding the role that changing seasonal cycles in their environment and biology play in their evolution. Do bivalves that live closer together tend to reproduce at different times? Can we track season of reproduction in relation to temperature and food supply? There are a lot more clam stories to be told and I look forward to sharing them all with you. Until the next research blog,



When a clam gets an offer it can’t refuse

Tridacna maxima in Eilat, Israel

I study the giant clams, bivalves which can grow over three feet long and and are willingly “infected” by a symbiotic algae which they house in an altered stomach cavity. They provide their algae partners with nitrogen, a stable environment and even funnel light in their direction, and the algae happily share the fruit of their labor in the form of sugars. Imagine yourself swallowing algae, storing it in your gut and developing windows in your flesh to let light into your stomach. You’d never have to eat again. This is the growth hack that enables the giant clams to grow to unusual sizes. But it turns out that this lovely, beautiful partnership may not have started so peacefully. The algae may have made an offer the clam couldn’t refuse.

Top left: normal mussel. Top right: heavily infected L-shaped shell opening. Bottom: view of an algae-infected mussel, including close up of pearls. From Zuykov et al. 2018

A team from University of Quebec recently discussed what such a fresh infection looks like in mussels and it ain’t pretty. The mussels basically have their shells and bodies overgrown by parasitic Coccomyxa algae, leaving its flesh bright green and transforming its shell from the classic elongated, acute angled margin typical of Mytilus mussels into a strange L-shaped overhang. The more algae are present in the mussel, the more extreme this deformity becomes. The researchers propose that this is no accident, but that as they move in, the algae also manipulates the biochemical pathway that the mussel uses to create its shell.

Mussels, like all bivalves, create their shells by laying down calcium carbonate in layers at the outer edge of the shell. The calcium is sourced from salts in the water column and the carbon primarily comes from carbonate ions also available in the water. This reaction is easier when the pH of the clam’s internal fluid is higher (less acidic), and that is exactly what the algae may assist with. Algae like all plants take in carbon dioxide to use in photosynthesis, and in doing so they increase the pH of the mussel’s body fluid,

The authors note that the region of shell which experiences abnormal thickening in the infected mussels is also the most exposed to light. The Coccomyxa algae may be causing runaway calcification of shell in the regions that they infect, and even may be directly assisting with the calcification in an additional way through the action of an enzyme called carbonic anhydrase, which is used in both their photosynthesis and in shell production (I won’t get into the nitty gritty of that reaction here). But the calcification of the mussels does appear to be in overdrive, as infected mussels were also observed to make pearls!

The algae’s photosynthesis may be assisting the mussel’s shell formation, though overall these are still quite unhealthy organisms of lower weight than their uninfected brethren. Still, Coccomyxa is known to form symbioses with lichens and mosses, so it could be that with enough generations of collaboration and a bit of evolution, the harmful algal infection could become a much more mutually beneficial partnership. It’s not so far fetched to imagine that an ancestor of today’s giant clams got a bad case of gastritis and decided to make the best of a bad situation. Making a deal with their invaders, they became greater than the sum of their parts and evolved to be the giant hyper-calcifiers we know today.

Revenge of the Clams

Lampsilis showing off its convincing fish-like lure. Photo: Chris Barnhart, Missouri State.

Clams are traditionally the victims of the aquatic realm. With some exceptions, clams are generally not predatory in nature, preferring to passively filter feed. When they are attacked, their defenses center around their protective shell, or swimming away, or just living in a place that is difficult for predators to reach. They are picked at by crabs, crushed in the jaws of fish, and pried apart by sea stars. But some clams are sick of being the victims. They have big dreams and places to be. For these clams, the rest of the tree of life is a ticket to bigger and better things. These clams have evolved to live inside of other living things.

Pocketbook mussels, for example, have a unique problem. They like to live inland along streams but their microscopic larvae would not be able to swim against the current to get upstream. The mussels have adapted a clever and evil strategy to solve this problem: they hitch a ride in the gills of fish. The mother mussel develops a lure that resembles a small fish, complete with a little fake eyespot, and invitingly wiggles it to attract the attention of a passing fish. When the foolish fish falls for the trick and bites the mussel’s lure, it explodes into a cloud of larvae which then flap up to attach to the gill tissue of the fish like little binder clips. They then encyst themselves in that tissue and feed on the fish’s blood, all the while hopefully hitching a ride further upstream, where they release and settle down to a more traditional clammy life of filter-feeding stuck in the sediment.

Very tiny Mytilus edulis living in the gills of a crab (Poulter et al, 2017)
The tiny 2.5 mm long Mimichlamys varia, living on the leg of a crab (Albano and Favero, 2011)



Clams live in the gills of all sorts of organisms. Because they broadcast spawn, any passing animal may breathe in clam larvae which find the gills a perfectly hospitable place to settle. Sure, it’s a bit cramped, but it’s safe, well oxygenated by definition and there is plenty of food available. They also may just settle on the bodies of other organisms. Most of these gill-dwelling clams are commensal: that means that their impact on the host organism is fairly neutral. They may cause some localized necrosis in the spot they’re living, but they’re mostly sucking up food particles which the host doesn’t really care about. In addition, in crabs and other arthropods, these clams will get shed off periodically when the crab molts away its exoskeleton, so they don’t build up too heavily.

Top: Kurtiella attached among the eggs of the mole crab. Bottom: aberrant Kurtiella living within the tissue of the crab (Bhaduri et al, 2018)

While being a parasite is often denigrated as taking the easy way out, it is actually quite challenging to pursue this unusual lifestyle. Parasitism has evolved a couple hundred times in 15 different phyla, but it is rare to find some organism midway in the process of becoming a true parasite. One team of researchers just published their observations of a commensal clam, Kurtiella pedroana, which may be flirting with true parasitism. These tiny clams normally live in the gill chambers of sand crabs on the Pacific coasts of the Americas. They attach their anchoring byssal threads to the insides of the chambers and live a comfortable life until the crabs molt, when they are shed away. The crabs mostly are unaffected by their presence, but the researchers noticed that some of the clams had actually burrowed into the gill tissue itself. This is an interesting development, because the clams would not be able to filter feed in such a location, so they must have been feeding on the crab’s hemocoel (internal blood). These unusual parasitic individuals are currently a “dead end” as they haven’t figured out how to get back out to reproduce, but if they ever do, they could potentially pass on this trait and become a new type of parasitic clam species. The researchers have potentially observed a rare example of an animal turning to the dark parasitic side of life, with some living in a neutral commensal way and other innovative individuals seeking a bit more out of their non-consensual relationship with their host crabs.  Considering the irritation that other bivalves suffer at the claws of pesky parasitic crabs, this seems a particularly sweet revenge.





Hard shells aren’t actually that hard to make (yet)

One of the Antarctic bivalve species featured in this study. Source

Like all organisms, bivalves have a limited budget governing all aspects of their metabolism. If they put more energy into feeding (filtering the water), they can bring in a bit more food and therefore fuel more growth, but sucking in water takes energy as well, particularly if there isn’t enough food to be filtered out. Bivalves also periodically have to grow gonadal material and eggs for reproduction, expand their body tissue (somatic growth) and of course, grow their shells (made of of a mineral called carbonate). All of these expenditures are items in a budget determined by the amount of energy the bivalve can bring in, as well as how efficiently they can digest and metabolize that energy.

If a bivalve is placed under stress, their scope for growth (the max amount of size increase per unit time) will be decreased. Because they’re cold-blooded, bivalves are limited by the temperature of their environment. If temperatures are low, they simply can’t sustain the chemical reactions required for life at the same rate that endotherms like us can. They also may have to shut their shells and stop feeding if they’re exposed by the tide, or are tossed around by a violent storm, or attacked by predators or toxins from the algae that they feed on.

When their budget is lower, they have to make painful cuts, much like a company lays off employees if their revenues are lower. The question is which biological processes get cut, and when? My first chapter (submitted and in review) has settled temperature being the primary control on seasonal shell growth. Bivalves at high latitudes undergo annual winter shutdowns in growth, which create the growth bands I use to figure out their age, growth rate, etc. We’d be a lot closer to accurately predicting when bivalves suffer from “growth shutdowns” if we had hard numbers on how much energy they actually invest in their shells. A new study from a team led by Sue-Ann Watson of James Cook University attempts to do just that.

Diagram relating the growth bands of Antarctic soft-shelled clam with a chart showing the widths of those bands. Source

Collecting a database of widths for the annual growth rings of bivalve and gastropod (snail) species from many latitudes, Watson and her team were able to get a global view of how fast different molluscs grow from the equator to the poles. Because the unit cost of creating carbonate is determined by well-understood chemistry, they were able to create an equation which would determine the exact number of Joules of energy used for every bivalve to grow their shells.

They still needed a total energy budget for each species, in order to the percent of the energy budget that each bivalve was investing in their shells. They drew on a previous paper which had calculated the standard metabolic rates for each species by carefully measuring their oxygen consumption. We could do the same for you if you sat in a sealed box for an extended period of time while we measured the exact amount of oxygen going in and CO2 going out. Dividing the amount of energy needed to grow the shell by the total amount of energy used in the organism’s metabolism would give us a percent of total energy that the bivalve dedicates to adding growth layers to its shell.

That number is…not very large. None of the bivalves or gastropods they looked at put more than 10% of their energy into shell growth, and bivalves were the lowest, with less than 4% of their energy going into their shell. Low-latitude (more equatorward) bivalves have the easiest time, putting less than 1% of their energy into growth but getting way more payoff for that small expenditure. High-latitude polar bivalves have to work harder, because the lower temperatures they experience mean the reactions needed to create their shells are more expensive. In addition, most of that energy is going into the protein-based “scaffolding” that is used to make the shell, rather than the crystals of carbonate themselves. Organisms right now don’t have to put a whole lot of effort into making their protective shells, which could explain why so many organisms use shells for protection. That is good, because if shells were  already breaking the bank when it came to the bivalves’ growth budget, they wouldn’t have a lot of room to invest more energy in the face of climate change. Unfortunately, as the authors note, these budgets may need to change in the face of climate change, particularly for bivalves at the poles. As the oceans grow more acidic due to human CO2 emissions, growing their shells will start to take up more of their energy, which is currently not a major part of their budget.

A cold-water ecosystem dominated by Antarctic scallops. Source

Right now, the cold waters of the poles are refuges for organisms that don’t deal well with shell crushing predators. As polar regions warm, such predators will begin to colonize these unfamiliar waters. Polar bivalves may encounter the double whammy of needing to spend more energy to make the same amount of shell, but also find that it is no longer enough to protect them from predators that easily crack open their protective coverings.

I found this study to be an elegant and thoughtful attempt to fill in a gap in our current understanding of how organisms grow and how energy budgets are influenced by environmental variables like temperature. I instantly downloaded the paper because it answered a question that has long been on my mind. Maybe can sneak its way into my manuscript during the review process!

A hinged shell does not a clam make (QUIZ)

Bivalves are so named for their two hard shell valves made of carbonate, linked by a soft ligament acting as a hinge. They use a strong adductor muscle to close their shell, and the relaxation of the muscle allows the springy ligament to reopen (you might be familiar with adductor muscles as the edible tasty part of a scallop). In deference to the bivalves, laptops and flip-phones are called “clamshell” designs. That satisfying snap into place when you spring the ligamen… I mean, hinge of a flip phone is an example of human design imitating the ingenuity of evolution. But it turns out that plenty of other members of the tree of life have also stumbled upon the durable idea of a hinged two-valve shell. On the other hand, plenty of bivalves have given up on the classic clamshell look. In fact, the ancestor of all bivalves had a one-part shell, and the hinge evolved later.

Test your knowledge by trying to identify which which pictures are bivalves and which aren’t. Answers and picture sources at the bottom!

A. brachiopod-semenov.jpg



















A. This is not even a mollusk, never mind a clam! It’s a different benthic (bottom-dwelling) invertebrate called a brachiopod, which make up their own phylum. To put that in perspective, brachiopods are as far from bivalves on the tree of life as you are (you’re in phylum Chordata)! Yet they evolved a similar look through a process called convergent evolution. If environmental needs are the same, organisms may come to the same solution multiple times. Much like wings for flight evolved independently in insects, birds and bats, bivalves and brachiopods both evolved a hinged shell as a form of protection from predation.

B. These are indeed bivalves: rock scallops commonly found off the coast of California (picture by me of an exhibit at Monterey Bay Aquarium). So similar to the brachiopods in their ridged, hinged shells. Like picture A, these guys specialize in living on hard, rocky nearshore bottoms. Some cultures do apparently eat brachiopods (I have not), but I have little doubt that rock scallops are tastier.

C. These are also bivalves: windowpane oysters. Also known as capiz shell, they are commonly used for decoration and art due to their beautiful, thin semi-transparent shells. A large industry harvests them off the shores of the Philippines, where they unfortunately are growing scarce due to overexploitation.

D. These are not bivalves! They are crustaceans called clam shrimp. They have little legs poking out of a hard hinged shell, and have been found in some of the harshest environments on earth, where they wait in extended hibernation, sometimes years, between bouts of rainfall.

E. Not a bivalve. These are another kind of crustacean called ostracods. Like clam shrimp, ostracods live in a hinged shell and swim around with the help of tiny legs, filter-feeding in the water column. Ostracods are everywhere in the oceans and in freshwater, but have undergone an extreme process of miniaturization from their ancestral form, and are now represent some of the smallest complex multicellular life known.

F. These are fossil ostracods. You can see why they are sometimes mistaken for bivalves! The givaway is that one valve is overhanging the other. Most bivalves have symmetry between the two halves of their shell, but ostracods and brachiopods do not.

G. This is a snail, so it’s a close molluskan cousin of bivalves. Some snails feature a hinged lid at their shell opening called an operculum. This operculum can be closed to protect from predators and also seal in water to help land snails from drying out between rains.

H. This is a bizarre bivalve called a rudist. They were common during the time of the dinosaurs but went extinct during the same extinction, 66 million years ago. While they come in many bizarre shapes, this elevator form (or as I prefer to call them, trash-can form) would have been stuck in the sediment with its small lid poking out at the surface. They could open and close the lid to filter-feed.

I. This is a giant clam, Tridacna crocea! Its shell is hidden, embedded in the coral that has grown to surround it on all sides. Only the mantle (soft “lips”) are exposed, and are brilliantly colored by the symbiotic algae in its tissue. It harvests the sugars made by the algae for food. Despite being embedded in the coral, the clam does have enough room to close and pull back its mantle if a predator approaches.

J. This is a one of the weirdest modern bivalves, called a hammer oyster. These guys are found in the tropics, and the hammerhead part of their shell is actually their hinge, extended at both sides. The hinge provides the surface area needed to “snow-shoe” on top of the soft sandy bottom where they live. Other bivalves sometimes take advantage and live on the oyster like a raft!

K. This is a different brachiopod. Notice the lack of symmetry between the valves which gives it away.

L. This is by far the weirdest modern bivalve, a shipworm. These guys live buried deep within wood and are the number-one killer of wooden ships. They secrete a long tube of carbonate and have largely given up the hinged lifestyle, looking more like worms than mollusks.

When a clam has a stowaway

My mussel contained a tiny half-eaten crab! - Imgur
Source: jeredjeya on Reddit

Bivalves put a lot of energy into their shells. These hardened, hinged sheaths of carbonate are an effective defense against many predators looking to get at the squishy clam’s body encased inside. Parasitic pea crabs have evolved to free-ride on the bivalves’ hard work.


(video courtesy Dana Shultz)

Pea crabs are small (pea-sized), very specialized parasites which live in the mantle cavity of many bivalve groups including oysters, mussels, clams and more. The mantle is the wall encasing the soft body of the bivalve, and the cavity is the space between this soft gooey tissue and the shell itself.

For a pea crab, there is no better place to be than this tiny, claustrophobic space. In fact, they can’t live anywhere else, though some species have been found in other unusual places such as inside the anuses and respiratory tracts of sea cucumbers (link SFW, fortunately, unless you’re a sea cucumber). In a bivalve host, the crab is protected from predation by the shell, and the bivalve provides a constant buffet of food as it sucks in suspended particles with its gills. The crab steals some of this food from itself before the bivalve can digest it.

As you might imagine, having a crab living in you taking your food and pinching at your gills is not an ideal arrangement for the bivalve. Pea crabs damage their hosts’ gills with their constant picking, and bivalves infected with crabs suffer slower growth than uninfected individuals, particularly for those unlucky enough to play host to the larger female pea crabs. At a certain point, the males will sneak out of their hosts and find a bivalve with a female crab inside. At this point, they mate inside the host’s shell, adding great insult to injury. The female releases her larvae, which swim out to infect new hapless bivalves and start the cycle over again.

Aww, she’s expecting! The papers refer to this as being “with berry” which I find amusing for some reason. (photo from Dana Shultz)


You might think that commercial oysters with crab parasites would be thrown out, but to the contrary, finding a pea crab or its close relative the oyster crab with your meal is a cause for celebration in some areas, such as the Cheasapeake Bay. The crabs are eaten whole and often raw, and are said to have a texture akin to shrimp, with notes of sweetness and umami. Personally I prefer surprises in my Kinder eggs rather than in my shellfish, but to each their own.

A pea crab serves as a nice side dish for this lunching sea otter. Source: Brocken Inaglory on Wikipedia