Gamete Expulsion and Release in Sargassum muticum

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Abstract
Both in the sea and in culture the brown seaweed S. muticum (Yendo) Fensholt expels eggs in a number of pulses, at intervals of several days. In nature, most of the receptacles on an individual plant exude eggs simultanously. Isolated branches cultured under the same conditions initially expel their eggs in synchronous pulses, although this synchrony and the discreetness of the pulses eventually breaks down. The very 1st expulsion invariably occurs about 2 days after a full or new moon, but the timing of subsequent pulses depends on the culture conditions. In the absence of lunar entrainment, higher water temperatures shorten the interval between successive emissions. The rhythm of gamete emission continues normally for some time in continuous darkness. It may be inhibited in continuous illumination, although it can be restored by a single dark period. The eggs are not released immediately following extrusion. Even in moderately turbulent conditions they are retained for at least 24-48 h, clinging tenaciously to the parent receptacle. After this period the dissolution of their anchoring mechanism facilitates their release. Retention of the ova on the receptacle surface not only allows fertilization to take place in situ, but means that the zygotes can be incubated before release. Over 75% of the propagules are not liberated until they are well-developed germlings with the attachment rhizoids either just beginning to protrude or about to do so. These large germlings sink rapidly and adhere to the substratum on contact.