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Functional morphology of hydrozoan stolons: stolonal growth, contractility and hydroplasmic movement in Gonothyraea loveni (Allman, 1859)

ABSTRACT In many hydroids the movement of hydroplasm inside the colony is generated by the pulsation of coenosarc walls. We hypothesized that the stolon coenosarc is not only morphologically but also functionally non-homogeneous, and tested this hypothesis using time-lapse video microscopy to study rhythmic patterns – transverse pulsations, longitudinal pulsations and hydroplasmic flows – in the five distal internodes of the stolon in Gonothyraea loveni, which is commonly used in laboratories to study growth and morphogenesis. Growth (apical) and transverse (lateral) pulsations of the coenosarc are rhythmic to varying degrees. Growth pulsations and longitudinal oscillations of the distal part of the growing tip (GT) have the most regular rhythm. In the proximal part of the GT and in the coenosarc of the terminal internode behind the GT, transverse pulsations have a different period, about double that of growth pulsations. The highest amplitude of transverse pulsations is observed in the terminal segment of the stolon and the following segment between the 1st and 2nd upright stems. Longitudinal oscillations of the coenosarc tube are clearly discernible throughout all five internodes, even though they subside with distance from the GT. Thus, the zone of intercalary growth spreads over several internodes of the stolon and is not limited to its distal segment. Hydroplasmic flows are rhythmical. Their velocity generally increases with distance from the GT, but can differ significantly between adjacent internodes, which points to the important role of local subcurrents. There are six morphofunctional zones in the hydroid stolon.