Zur funktion des afterschornsteinschleuderns bei diademseeigeln

In some diadematid sea urchins, the anus opens at the top of the anal cone, i.e., a chimney-like elevation of the periproctal membrane. With this anal cone, Diadema setosum can perform a rotatory movement which often lasts more than 1 h. During rotation, the anal cone inclines towards the rotation axis which yields a device similar in function to a centrifugal pump that can throw water out of the rectum. The calculation of the hydrodynamic mechanism pertinent to the parameters given for D. setosum shows that anal-cone rotation is sufficiently effective to empty the rectum and a great part of the hind gut within a short time. This allows a more rapid water change in the second intestinal circuit, which could provide several advantages: (1) increased rinsing results in better evacuation of soluble metabolic end products which, in turn, promotes their excretion by the hind gut; (2) provision of oxygen-rich water to the intestine improves the oxygen supply to the perivisceral cavity in cases of increased oxygen demand; (3) faecal pellets are expelled into the free water and do not sink to the aboral body surface, where they would be difficult to remove because of the lack of tridentate pedicellaria. The rotating anal cone — in D. setosum encireled by an orange iris-like ring — probably resembles a rolling eye, which may deter predators.

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Mit Unterstützung der Gesellschaft der Freunde der Zoologischen Staatssammlung München e. V.