Species species of the Week week #4 OR Hiphopopotamus

Post previously published at Rah Rah Radula.
Hippopus hippopus

(The Horse's Hoof Clam OR
The Strawberry Clam OR
The Bear Paw Clam)

Figure 1. What a wacky looking clam!!
Photo Credit: Mehmet Atatur

Figure 2. Yup, wacky from this angle too!

Right, so the radness of the shell alone (Figs 1 & 2) is pretty much enough coolness for this tautonym to stand on, but there is so much more to Hippopus hippopus (Disambiguation: Did you mean "Hiphopopotamus"?) that makes it cool. And for that we need to see a picture of it in situ, up close:

Figure 3. Open live H. hippopus, looking even wackier

Go closer...

Figure 4. Siphon and wacky mantle tissue.
Photo Credit: Artesub.com

Closer still...

Figure 5. Extreme close-up. Reflective proteins in mantle tissue of Tridacna.
Micrograph Credit: Griffiths et al. 1992

Too close, a little too close!

Figure 6. Dinoflagellates (non-algae algae) of the genus Symbiodinium.
They live in giant clam flesh!

There! Living right in the mantle tissues of these Giant Clams are dinoflagellates(1), phytoplankton of the genus Symbiodinium. The genus name kind of says it all. These wiggly bulbous photophiles have a symbiotic relationship with the clams whereby they use sunlight to produce carbohydrates, which the clam then eats, and in return, they get a stable habitat in which to live and drink up the sun. Free-living dinoflagellates, like all plankton, are at the mercy of the tides and currents and can get forced down to a depth beyond which they can obtain sufficient light to photosynthesize (= bitty dinoflagellates, wasting away). By paying a bit of rent in the form of sugar to the clam, symbiotic dinoflagellates are assured that they will not be sucked into the deep.

Ok, so that's cool enough, but not unique. This is roughly the same symbiosis as corals and our own local Anthopleura anemones (see really, really ... really cool research at Western on climate change and symbiosis).

But! What I learned at a talk by Alison Sweeney(2) was that not only do the clams have algae all up in their skin, but they apparently also modify their own mantle tissue feed more dinoflagellates, and thereby get more sugar. They're farmers! The iridescence you see in the mantle (Figure 4) is generated by iridocytes, cells with organized proteins all stacked up (Figure 5). What are the iridocytes for? There are a number of ideas that have been considered, including protecting the dinoflagellates from exposure to too much sun, which can cause chemical stress on both dinos and clams.

However, modelling and experiments done by Dr. Sweeney and friends indicate that proteins in iridocytes forward-scatter wavelengths of light that are useful in photosynthesis, deeper into the clam than it would otherwise go. This is good news for the dinoflagellates and the clam (more light -> more photosynthesis for more dinoflagellates -> more candy for the clams!). However, the iridocytes backscatter the other wavelengths (like green), so we see green iridescence. So, it's possible that the clam uses iridocytes to improve their dinoflagellate husbandry.

Dang, clam, you pretty and smart!

References and miscellany: (1) Pedantic note about pronunciation of this word: Nigh on a decade ago, it was STRONGLY emphasized to me by Greg Teegarden who learned it from his Academic forefathers/mothers that "Dinoflagellates" is pronounced "DEE-no" NOT "DYE-no". The former is derived from Greek meaning "rotation", referring to the transverse flagellum that causes these guys to spin, while the latter signifies the Greek word "deinos", meaning "terrible" - like Dinosaur = "terrible lizard". It seems to me that these are spinning, but not terrible, flagellates.
(2) Most official website I could find for Dr. Sweeney, this is from Duke where she did her Ph.D. Currently at UCSB Institute for Collaborative Biotechnologies, headed to Penn Physics this fall. She has also done lots of work on coloring and iridesence and optic stuff in cephalopods.

-- Emily Grason

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