Abstract: The macrophyte Gelidium corneum (Hudson) J. V. Lamouroux, 1813 plays an important role as an ecosystem engineer on temperate rocky shores on the northeastern Atlantic coast. However, its cover and population biomass have declined in recent decades within the southern Bay of Biscay. This study aimed to identify the environmental thresholds, with respect to seawater temperature and irradiance, that influence the development capacity of newly formed individuals of G. corneum through vegetative reproduction. Therefore, an in situ experiment was conducted at two depths within the range of distribution of this species (5 and 12 m) and at two coastal sites on the north coast of Spain (east and west coasts of the Cantabria region, Spain). Our results revealed that G. corneum apical fragments undergoing vegetative propagation developed many new shoots over a period of sixteen weeks. The temperature and irradiance seemed to contribute to the length reached by the new fronds, whereas the number of recruits remained constant throughout the duration of the experiment. Given the slow growth rate characteristic of this species, the total shoot length in this study reached 2.97 cm, which confirms the gradual and restrained developmental pattern during the early stages of its life. We also quantified bite marks at the tips of the new plants, and more than three-quarters of them presented these signs by the final month of the study. Our study provides valuable insights into the growth process of the endangered species G. corneum through vegetative propagation and elucidates the impact that abiotic and biotic factors can have on its growth.

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