Otherworldly

On 4 April 2012, Rebecca found an amazing pelagic amphipod washed ashore on Doran Beach.

An amphipod is a shrimp-like crustacean (think about the beach hoppers in previous posts), but this species is only found in the open ocean and is very unusual in that it lives in a transparent "barrel." Their appearance is otherworldly and a bit bizarre.

Because they modify other pelagic animals (e.g., salps and pyrosomes) into barrels, these amphipods are sometimes known as "marine coopers", or tonneliers de la mer.  And because they hold onto the barrel, deposit their young inside of it, and propel it by beating their pleopods, they are informally called "pram bug amphipods." (Although you may think of a pram as a type of boat, it's also a British term for baby carriage.)

Here's Rebecca's amphipod living inside a gelatinous barrel (probably a salp).  The amphipod is approximately 30 mm long.  The cream-colored band near the bottom is actually a cluster of juveniles (her offspring — more about them later).  Meet Phronima sedenteria!

 

Below is a dorsal view.  Although Phronima is mostly transparent, look at the interesting maroon-colored pigment spots (chromatophores).  Phronima is not attached to the barrel.  She can adjust her position within it (including somersaulting to reverse direction) and can leave through one of the openings and return.

 

And here's a side view.  Note the way she uses the front appendages to hold on to the barrel, her very elongate head, and the large claw at the end of the fifth thoracic appendage.

 

The next photos are close-ups of the claw and the tail.  (The claw is apparently not used for capturing prey, but may be utilized when defending the barrel.)

 

 

Her eyes are one of her most impressive features.  She has four of them — two lateral and two median eyes.  

Here's a close-up of her face and a diagram to help illustrate the eyes.

 

 

Figure adapted from Ball, E.E.  1977.  Fine structure of the compound eyes of the midwater amphipod Phronima in relation to behavior and habitat.  Tissue & Cell 9: 521-536.

The red spots are retinas.  The outermost or lateral eyes are small and easy to see.  The inner or median eyes are conical and very elongate, extending from the curved surface at the top down to the retina below.  The cone cells are connected to the retina by long (~5 mm) light-guides (the converging lines in the diagram).

From above, the large median eyes look somewhat like the eyes of a fly (see below).  They appear to be faceted, but you're actually seeing the individual cone cells beneath a smooth transparent surface.  There are ~400 cone cells in each eye.

These unusual eyes are apparently adapted for living inside a barrel (with a narrow field of view), seeing small objects against uniform backgrounds in low light conditions, and minimizing visibility to predators (such as albacore and other tuna).  Phronima is generally found at depths between 80-1100 meters below the surface.

One more curious fact about Phronima: they practice demarsupiation, or the deposition of progeny (juveniles) on the barrel wall (see below).  Most amphipods maintain their young in a brood pouch, or marsupium.  Phronima begins that way, but then transfers the tiny juveniles (up to 600) to the barrel wall where they continue to develop (they feed on the barrel and on food she captures and brings to them).

Phronima means "clever", and perhaps refers to their ability to fashion a barrel out of another animal and then use it for shelter and a nursery (and sometimes food).

I'm still working out the best way to photograph transparent animals.  Click on these links to see superb photographs and video footage of this remarkable amphipod.

Phronima is found throughout the world's oceans, but it's unusual to see them on shore.  Good find, Rebecca!