Sunday, February 8, 2009

Another giant from the tropics: Superlucina
As the blogosphere buzzes about Titanoboa I’m going to review another recent paper that hasn’t received the same degree of publicity but describes another tropical giant that is as equally interesting to me. I’m talking about Superlucina: a new generic named coined for an old species ‘Lucina’ megameris named in 1901. Superlucina megameris is a giant bivalve from the Eocene of Jamaica that was revised and interpreted by Taylor and Glover in the latest issue of Palaeontology. Yes that’s right, a paper about the proverbial Eocene clams. This one is for you Mike ;-)

That’s a big cockle. The giant bivalve Superlucina megameris. From Taylor and Glover 2009.

All joking aside, bivalve molluscs have a reputation for being simple, dull filterfeeders with little interest anyone except perhaps those few crazy taxonomists that specialize on them. Even Chris Taylor, who appears to have a boundless love for the systematic s of all biota confessed that it was hard to get excited about bivalves.
It is a tribute Taylor and Glover that they have produced such a fascinating and readable paper on these maligned creatures. However some of this credit has to go to the organisms themselves, which are fascinating once you look below their dull, clammish exterior.
Superlucina megameris belongs to the family Lucinidae which lead an unusual lifestyle. They are chemosymbiotic sulfide miners that inhabit the interface between the oxic and anoxic zones of marine sediments. By using an elongate muscular foot they build a tunnel up to the sediment surface in order to bring down oxygenated water. They also push holes down into the anoxic sediment below to bring up water with dissolved sulfides. These sulfides are oxidized by bacteria held symbiotically in the bivalve’s tissues and provide much of the nutrition that the bivalve requires.
To understand some of the unusual adaptations of S. megameris and the lucinids I first need to give a quick primer in bivalve anatomy. Bivalves are shell-bearing molluscs so that surround their body with a skirt-like fringe of tissue known as the mantle. The mantle secretes the shell, which in bivalves is divided into left and right valves that are joined dorsally along the hinge. The space between the mantle and the body forms a chamber into which the gills (called ctenidia) protrude. To ventilate the gills a water current needs to be drawn into the mantle cavity, passed over the gills and then expelled. To help with these currents many bivalves have evolved inhalant and exhalent siphons (which are modifications of the mantle. The mantle has a series of flap like, medially directed folds that partially enclose the mantle cavity. The inner fold is controlled by pallial retractor muscles which leave a long linear scar on the inner surface of the shell. This line is called the pallial line. At each end of the pallial line are two shell-closing muscles known as adductor muscles (which also leave prominent scars). Ventrally there is a muscular organ known as the foot. That will do for now.

A schematic , grossly simplified, diagram of bivalve anatomy in cross-section. Drawn hurredly by myself last night.

Ok, now to the interesting stuff. First of all lucinids house their symbiotic bacteria in the tissues of the ctenidia which makes breathing a little difficult, especially since the bacteria need to be supplied with anoxic, sulfide-rich water. In order to compensate lucinids use the anterior end of the mantle folds, as respiratory surfaces. In some large lucinids the anterior end of the inner mantle fold is thickened and pleated with complex folds that act as mantle gills. To keep the oxygenated water separate from the anoxic water, the mantle cavity is partially divided. To help with this division the anterior adductor muscle of many lucinids becomes highly elongate and extends posteroventrally, thus creating a channel between it and the mantle gills. Concomitant with this adaptation lucinids lack the posterior inhalant siphon that many bivalves have and take water in at the anterior end of the animal. In S. megameris the elongation of the anterior adductor muscle is more extreme than in other lucinid. A pustulose channel runs between the anterior adductor scar and the pallial line on internal moulds of S. megameris seems to mark the presence of a unique respiratory channel in this species that was longer than in any other lucinid.

An internal mould of S. megameris from Taylor and Glover 2009. Scars from several anatomical features impressed upon the internal surface of the shell are replicated in the mould. I have colourised these: blue – anterior adductor muscle; red – respiratory channel; yellow – pallial line; green posterior adductor muscle.

A reconstruction of the internal anatomy of S. megameris from Taylor and Glover 2009 showing the division of water inflow. Colours follow the figure above.

S. megameris also differs from other lucinids in its great size. It is the largest lucinid known and is one of the largest burrowing bivalves of all time (epifaunal bivalves like giant clams get even larger).
S. megameris inhabited a transitional zone between an open shelf region and a shallow lagoon filled with seagrass meadows. The presence of seagrass is important because it is the decaying grass that provides the sulfide that fuels the bacteria in their bodies. Although the great size of S. megameris is impressive by itself it is all the more impressive when one compares it to modern shallow water lucinids. These rarely reach a height of 10 cm (less than one third the height of S. megameris) while the vast majority range in height from 0.5 cm to 3 cm. Some chemosymbiotic bivalves inhabiting sulfide rich deep-water cold-seeps reach similar impressive sizes (as indeed did some extinct lucinids from cold seep deposits) and it has been suggested that it was the cold-seep environment that allowed for gigantism in chemosymbiotic bivalves. However the paleoenvironment of S. megameris was definitely not a deep-water cold seep. So why did S. megameris get so big? At this stage we do not know.
Lastly it is interesting to consider the relationships of Superlucina. The genus presents no particularly close similarity with any other but seems most closely related to the genera Miltha, Pseudomiltha and Eomiltha. These also tend to have the most elongate anterior adductor muscles and n some cases toothless hinge lines (like Superlucina). This potential clade was diverse in the past but is now reduced to a handful of mostly rare species, apparently being replaced by more advanced lucinids that have broader shorter anterior adductor muscles, instead using a septum to effectively divide the mantle cavity and have highly plicated mantle gills to efficiently extract oxygen. This takeover appears to have occurred rather recently, I have collected sizeable Miltha specimens from the latest Pliocene or earliest Pleistocene of southern Australia where none now live.



Christopher Taylor said...

Even Chris Taylor, who appears to have a boundless love for the systematic s of all biota confessed that it was hard to get excited about bivalves.

And boy, did I ever cop it from the bivalve enthusiasts for that little faux pas.

Mike Taylor said...

Hi, Adam. I got behind with my blog-reading, and have only just caught up with this one. OK, OK, I admit it -- you made an Eocene clam interesting. But only because it's what passes for a "giant" :-)

I had trouble following the anatomic directions in this post, though. Am I right in thinking that the direction you stated out referring to as dorsal later became anterior?