Orientation and righting behavior of the sand‐dwelling bryozoan Conescharellina catella

Most bryozoan colonies live firmly attached to hard surfaces, but some species live on sandy or muddy bottoms, anchored to the substratum by one or more rhizoids. The small, conical colonies of members of the genus Conescharellina live attached to the substratum by several rhizoids, each produced by a modified, non‐feeding zooid (kenozooid). It was previously thought that these colonies lived prone on the substratum, anchored but not supported by the rhizoids. In this study, I observed the behavior of a living colony of Conescharellina catella collected from the East China Sea and maintained in the laboratory for 2 months. The colony lived apex down, suspended above the substratum by several rhizoids arising in the apical two‐fifths of the colony, to a total height of nearly five times colony height. Starting from an uprooted position lying on the substratum, the colony reattached and righted itself to the erect position using existing or newly produced rhizoids. After I removed all rhizoids, the colony was able to right itself only after it regenerated rhizoids. In each of several series of observations, righting behavior involved similar steps and followed a similar time course, and it can thus be considered a stereotypical behavior. Histological sections showed that the rhizoids lack muscles. I speculate that righting is effected by differential turgor (internal pressure) and differential growth of rhizoids originating from kenozooids in different positions around the central colony axis, although it is unclear how the colony senses orientation and coordinates adjustments to achieve the preferred orientation.