B16 Course  
 

Science B-16: The History of Life
Mini course 2: Ecologic Diversity of the Bivalves


  
 
The bivalve’s calcium carbonate shell serves many functions.  It provides protection from predators and adverse environmental conditions, but it is also one of the animal’s most important tools for interacting with the environment.  Bivalves live in many different habitats, and the bivalve shell has evolved into a variety of shapes and functions.  Some bivalves live attached to rocks or seagrass, others burrow in sand or mud, some bore into wood or rock, and a few can swim.  Because form and function are intimately connected, we can often interpret a bivalve’s mode of life from the morphology of its shell.  Bivalves provide a fascinating case study in evolution—they demonstrate how a single body plan can be modified to allow inhabitation of many different environments.


Basic Layout of the Bivalve Shell
The two valves are mirror images of each other.  One covers the left side of the animal, and the other covers the right.  The two valves are connected by a ligament on the dorsal (or back) side; the opposite side is the ventral (or belly) side.  The position of the mouth defines the anterior (front) end, and the anus is in the posterior (rear).  The mouth and anus do not leave marks on the shell, but by studying living bivalves we know which end of the shell is which.  Two oval marks are visible—these are the adductor muscle scars, where the adductor muscles attach to the shell.  The bivalve uses the adductors to close the shell.  The organic ligament pops the shell open whenever the adductors relax (this is why clams open when they are cooked).



Burrowers vs. Non-Burrowers
The bivalve foot is a muscular appendage that the animal extends from either the ventral or anterior end of the shell (see picture on page 5 to see the extended foot).  Many bivalves use their foot to burrow into sand or mud.  The bivalve pushes its foot into the sediment and inflates the end.  It then closes the shell, which forces blood into the foot and causes it to further inflate.  The expanded foot acts as an anchor, and the bivalve pulls its body downwards, towards the foot.  This burrowing behavior requires two adductor muscles (one on either side of the foot).  Bivalves that live above the sediment surface, attached to a rock or reclining, do not need two adductors.  A single, centrally placed muscle will close the shell just as effectively.  Therefore, among surface dwellers, one of the adductors is often greatly reduced in size or lost entirely.

The adductor muscle scars therefore provide powerful insight into the mode of life of a bivalve.  If a shell has two adductor scars that are similar in size, then it probably belongs to a burrowing animal.  If it has one adductor, or two that are greatly different in size, it is likely to be a surface dweller.  Compare the oyster with the disk shell (Dosinia).  Based on the muscle scars, which is the surface dweller and which is the burrower?









Byssal Attachment
Most bivalves are suspension feeders, sucking water into their shell and filtering out small particles of food.  Thus, they do not need to move to eat.  Many use byssal threads to remain attached and immobile their entire adult life.  Byssal threads are strong fibers grown by a gland on the foot.  They protrude through an opening between the valves.  Most bivalves have a swimming or floating larva that settles to the bottom and metamorphoses into an adult.  The larvae use byssal threads to grab onto the bottom when they settle; some bivalves have evolved such that they retain the byssus into adulthood.

Epibyssate Bivalves—Mytilus edulis, Anomia simplex, and Ambonychia radiata
Epibyssate bivalves attach to surfaces.  They have been around since the early Paleozoic (the Ambonychia is from the Ordovician).  Mussels, such as the common Mytilus edulis, are familiar bivalves that use byssal threads to attach to hard surfaces on rocky shorelines.  The opening for the byssus is a thin slit between the valves; on the small mussel from Chesapeake, MD, you can see the byssal threads.  Why would mussels need a strong attachment mechanism?









Look at the jingle shells, Anomia simplex.  Only one valve is present, but the other is illustrated on the bookmarked page in the shell guide.  The other valve of Anomia has a large byssal opening and is very flat.  In fact, the side of an epibyssate bivalve that presses against the hard substrate is often flattened.  How is the flattening different in Anomia and Mytilus?   What does the flattening accomplish?







Endobyssate Bivalves—Pinna and Tridacna
Some bivalves sit partially buried in soft sediments and use their byssal threads to anchor themselves to the sediment.  These include the delicate pin shell (Pinna) and the giant clam (Tridacna).  The pin shell is a “mud sticker;” it lives with the pointy end inserted into the sediment.  Can you think of other mud stickers that we’ve seen, in the bivalves or in another group?

Pinna, showing
byssal threads

Cementers—Ostrea virginica
Some bivalves cement to a hard surface, secreting calcium carbonate that bonds their shell to a rock or shell.  Look at Ostrea, and notice how the large, complete valve has been torn from the rock to which it cemented.  Also look at the cut valves to see the growth lines.  How many adductor muscle scars does each valve have?  Is this consistent with its cementing lifestyle?







Recliners
During the Mesozoic, many bivalves simply laid on the sediment surface.  Often, they had a curved lower valve and a flat upper valve.  The edge of the shell was thus raised above the sediment surface, which helped to keep sediment out of the shell, while the shell did not stick too far up into the water column, thus avoiding currents.  After the Mesozoic, this lifestyle declined dramatically.  Why?  (Remember earlier labs.)







Many recliners, such as these large Mesozoic oysters, developed extraordinarily thick lower valves.  What function did this serve?









Swimmers—Pecten, Atrina, and Amusium
Scallops are the major group of swimming bivalves.  When threatened, they will swim short distances to escape.  They clap their valves, and water shoots out the dorsal side, to either side of the ligament.  Scallops are often confused with brachiopods by budding paleontologists because a plane of symmetry can almost be passed down the center of each valve.  Why might this symmetry be beneficial to a scallop?  Why might the scallops typically have thin shells?








Burrowing Bivalves
Most bivalves burrow into soft sediment.  This mode of life has certain benefits, such as added protection from predators.  It also has disadvantages.  Since most bivalves are suspension feeders, they must maintain a steady flow of water into their shell.  Most suck water into their shell using siphons, tubes that reach from the posterior of the animal up to the sediment-water interface.  Molluscivorous humans often refer to the siphons as “necks.”  Most siphonate bivalves have two siphons, one to suck in water and one to expel it, but some only have one siphon.

Many siphonate bivalves have a pallial sinus, an indentation in the pallial line at the posterior end of the shell (see picture on page 1).  The pallial line is where the mantle attaches to the shell; the pallial sinus is an infolding of the mantle that provides room for the siphons to be drawn into the shell.  The presence of a pallial sinus thus indicates a siphonate, burrowing bivalve, although the lack of a sinus does not rule out burrowing as a possible lifestyle.  The size of the pallial sinus can be a rough guide to the depth of burrowing, as a larger sinus can accommodate longer siphons (all things being equal).  However, different species have siphons of varying widths, and longer siphons can be fit into the same space if they are narrower.  Therefore, the relationship is only approximate.

 
 

 

Rapid Burrowers—Ensis
Rapid burrowers generally have smooth, elongate, streamlined shells.  This razor clam represents the extreme case.  How does a smooth shell enhance burrowing speed?







Shallow burrowers—Venericardia, Mercenaria campechiensis and Venus (Antigona)
Many shallow burrowers have extensive surface ornamentation (ridges, spines, etc.).  For the most part, this ornamentation actually slows them down when they burrow.  Once they are emplaced in the sediment, these ridges anchor them in place, preventing currents that scour into the sediment from ripping them loose.  Often these bivalves also have thick shells.  How would a thick shell be useful for these animals (list at least two ideas)?







Deep burrowers—Panopea and Lutraria
Some bivalves live tens of centimeters below the surface, and some live a meter or more deep.  Such animals need very long siphons.  If you touch the ends of the siphons at the sediment surface, the animal will quickly retract them downwards.  However, the siphons of some species are so long that they cannot fit into the shell, even when fully retracted.  The posterior edges of the valves do not come into contact; a gape is left for the extra-long siphons.  The posterior gape is often a sign of deep burrowing, as is a deep pallial sinus.  The foot is also large, and a gape is often left at the anterior end.  Look at the specimens of Panopea, the geoduck (“gooey duck”), and note the anterior and posterior gapes.  Why do deep burrowers survive even though their shell does not fully protect their soft parts?


A deep burrower that cannot fully retract its siphons.


 Borers
A few groups of bivalves bore into wood, rock, or coral, creating a tube-like home.  They secrete chemicals to weaken the wood or rock, then use their shell to rasp away the substrate.  Their shells seem surprisingly thin given their function.  Borers generally have long siphons to reach the end of their hole.

Wood borers—Teredo navalis
This is the dreaded “shipworm” that bores into wooden boats, docks, etc.  Its shell is quite reduced and does not cover the soft parts.  How does the shipworm survive with a shell that provides such poor protection?







Rock borers—Petricola pholadiformis
Several groups of bivalves can bore into limestone, coral, hard mud, and peat.

Photosymbiosis in Bivalves
Photosymbiosis, a symbiotic association between algae and a larger organism, is quite common, and the bivalves are no exception.  A number of bivalves have photosynthetic algae living in their tissues, just like modern corals.  The bivalve provides a stable environment for the algae, and the algae provide food for the bivalve.  Photosymbiosis requires exposure to sunlight, which requires living at or above the sediment-water interface.  Many photosymbiotic bivalves evolved from burrowing ancestors; the infamous paleontologist Adolf Seilacher has suggested that photosymbiosis is “the only feasible trade-off that could have lured a bivalve out of a more protected life within the sediment” (Seilacher 1990).

Tridacna
The most famous photosymbiotic bivalve is Tridacna, which can grow to be 6 feet long (see specimen in endobyssate section).  Some species live on the surface and others live partially buried in sediments; all are attached by a byssus (find the byssal opening).  Their tissues are so full of algae that they appear green.









Corculum
If you had to guess based on shell shape, what is the lifestyle of this bivalve (burrowing, byssate, reclining, boring, etc.)?  Why?  How is it morphologically different from other bivalves adapted to this lifestyle?







A related species, Corculum cardissa, houses symbiotic algae deep within its tissues.  It exposes them to sunlight without having to open the shell—the shell structure of the upper side is modified to form little “windows” or “lenses.”   These windows allow light to pass through the shell and focus it towards the algae-bearing tissues.  What benefits do these morphologic modifications provide?







Rudists—Hippurites
It has also been suggested that the Mesozoic rudist bivalves housed algal symbionts.

Deposit feeders
Not all bivalves are suspension feeders—a few are deposit feeders, consuming particles from the sediment.

Nucula
This type of bivalve has persisted from the early Paleozoic to the present.  They burrow shallowly (up to 1 cm) and have no siphons.

Tellina radiata
Tellinids burrow horizontally in the sediment and extend their long, skinny siphons to the surface to suck up particles (see picture, page 5).  What evidence can you find that Tellina has long siphons?  What evidence can you find that it burrowed rapidly?







Bivalve Ecology Questions

1)      Mercenaria mercenaria is a common New England bivalve known as the quahog.  All living Mercenaria species and all their relatives are shallow burrowers.  A Pliocene relative, Mercenaria tridacnoides, has developed an irregular, wavy commissure (the joining between the valves).  It can also have massively thickened valves.  Adolf Seilacher has suggested that such a non-streamlined shell would be incapable of burrowing.  What might have induced M. tridacnoides to abandon its infaunal habit?  Why do you make this suggestion?







2)      What is the mode of life of this bivalve?  What features led you to this diagnosis?  Be as specific as possible.







3)      What is the mode of life of this bivalve?  What features led you to this diagnosis?  Be as specific as possible.







4)      Which of these probably burrows more deeply?  What morphological feature(s) allow you to make this diagnosis?  How do these features relate to the soft parts of the animal?






5)      Siphon-feeding bivalves were almost completely absent in the Paleozoic.  During the Mesozoic, there was a radiation of siphonate bivalve groups.  It could be argued that the expansion occurred during the Mesozoic because siphons did not evolve until then, and if they had evolved earlier, then siphonate groups would have radiated earlier.  However, the invasion of the subsurface by the bivalves coincides with other events, such as the advent of burrowing echinoids.  Why might these groups have simultaneously evolved burrowing forms?  Can you think of evidence from other groups that we have seen that would support this hypothesis?