Re: metabolism in autonomous arms of Linckia multifora

Hello again, Jon!

I apologize for taking so long to address your follow-up questions. They proved to be much more difficult to answer than I had anticipated. It seems that nobody has actually documented in the recent scientific literature the exact information that would answer your questions -- and believe me, I looked. However, given what I have been able to find about regeneration in seastars, and my own observations and understanding of these animals, I can tell you what I think happens when a Linckia arm goes on to regenerate a whole animal.

Question 1: If each arm has its own stomach, doesn't the food therein contained fall out when the arm's disconnected?

Yes, each arm does contain extensions of the gut, which I called "pyloric caeca" in my previous answer and are labelled "digestive glands" in the diagram below; both terms are correct. The caeca make up most of the volume of the digestive system, as the central stomachs themselves are comparatively small. See below:

When Linckia casts off an arm, the food does not fall out of the caeca, because the connection between the caeca and the stomach is a narrow duct (not much stuff can leak out of a small tube, compared with a large one). There may also be some constriction of the caeca at the site of the "injury" that also prevents loss of food.

Question 2: Can the arm (or any other part of the starfish body) metabolize itself? That is, if an arm is disconnected and insufficient food is present in the gut, can the arm shrink and grow a disc (and other arms)?

I wouldn't say that an autotomized arm can metabolize itself, but there is a certain amount of tissue reorganization as the wounds heal and regeneration begins. However, I wouldn't expect to see a large-scale reduction in size. In the only case of severe damage and regeneration that I've been able to watch over time, one of my pet ochre stars was attacked by a sunflower star and lost two of its arms. The wound was closed after several days, and within a month or so I could see the tiny buds of new arms beginning to form. Now, about two years after the original injury, the star has almost completely regrown the missing arms, and as far as I can tell is about the same size. It was able to feed on pieces of fish the entire time it was regenerating.

Here's what Libbie Hyman, author of a 6-volume treatise called The Invertebrates, has to say about regeneration in the genusLinckia:

The wound closes over, leaving an opening that becomes the mouth, and after several weeks a crescentic ridge of new growth appears at the proximal end of the arm. This gradually grows out into four projections, each with an aboral groove continuous with the mouth. Even after a year these incipient arms are still very short and the comet form thus produced evidently lasts a long time.