Journal of Applied Phycology 11: 293–299, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 293 Organismic determinants and their effect on growth and regeneration in Gracilaria gracilis Albertus J. Smit 1 & John J. Bolton Botany Department, University of Cape Town, PO Box Rondebosch, 7700, Cape Town, South Africa 1 Present Address: Department of Botany, The University of Western Australia, Nedlands, Western Australia 6907; E-mail: asmit@cyllene.uwa.edu.au Received 1 November 1998; revised 1 April 1999; accepted 13 April 1999 Key words: organismic determinants, Gracilaria gracilis, Rhodophyta, regeneration, growth, mariculture Abstract The growth of Gracilaria gracilis (Stackhouse) Steentoft, Irvine et Farnham was examined by studying the effect of organismic determinants such as thallus length, position along the thallus and branching. Knowledge of these factors is essential in order to increase production from suspended seaweed rafts seeded with vegetative G. gracilis fragments. Seeding netlons with seaweed material freshly collected from subtidal populations provided up to 30% higher relative growth rates than seaweed maintained on the netlons for successive months. Initial seedstock length greatly affected growth rates and yields such that 30-cm thalli fragments resulted in growth rates 14% higher than for 10-cm fragments. This difference is suggested to be due to the higher contribution to overall biomass by growth of lateral branches. Comparisons of the growth of apical and basal fragments suggest that growth takes place over the entire length of the thallus, but that the apex contributes more to overall elongation than does the proximal part. The removal of apical meristems resulted in an enhanced branching frequency with production of four times as many branches as intact fragments. Evidence is also provided for extensive morphological differentiation following long periods of rapid growth. These thalli have very high frequency of branching, are hollow due to the disintegration of medullary cells and are considered to be completely senescent. These factors have implications for the successful cultivation of G. gracilis on commercial mariculture systems. Introduction Gracilarioid seaweeds contribute to more than half the world’s agarophyte production (Fletcher, 1995). They are cultivated in countries such as Chile, China, Taiwan, Namibia and South Africa (Dawes, 1995; Friedlander & Levy, 1995; Smit et al., 1997). Charac- teristics of Gracilaria spp. that make them desirable for cultivation are fast growth rate, good agar yield and quality and relative ease of growth (Buschmann et al., 1995). The most important attribute is that al- most all cultivated species reproduce solely through fragmentation, leading to a high regenerative capacity (Hurtado-Ponce, 1990; Santelices & Varela, 1995). This, along with the fact that Gracilaria species are morphologically plastic (Dawes, 1993; Meneses, 1996), has initiated many studies focusing on selection of strains with good gel characteristics and fast growth rates (Levy & Friedlander, 1990). Several studies on Gracilaria have been done in southern Africa since World War II. Most of these have dealt with the ecology of Gracilaria (Isaac, 1956; Si- mons, 1977; Anderson et al., 1993; Molloy & Bolton, 1995; Anderson et al., 1996b). Several other studies have looked at ecophysiological aspects of Gracilaria cultivation (Engledow & Bolton, 1992; Anderson et al., 1996a; Rebello et al., 1996; Smit et al., 1997; Anderson et al., 1998; Smit, 1998) or dealt with com- mercial aspects of its utilisation (Rotmann, 1990). The recent interest in seaweed mariculture in southern Africa however, has necessitated research into factors inherent in Gracilaria itself, which affect growth and regrowth performance. Santelices and Varela (1995) termed these factors ‘organismic determinants’. Since