We surveyed eight offshore islands and shoals in the Philippine Kalayaan Island Group. The aims were to (1) map the distribution, (2) evaluate the demographic status, (3) elucidate the vegetative growth dynamics, and (4) quantify the sexual reproductive capacity of Thalassia hemprichii meadows in the region. We combined age reconstruction techniques and in situ estimations to attain such goals.

We found T. hemprichii meadows only in Pag-asa Island. These meadows were generally young (mean age <1 year) and rapidly expanding (net recruitment ∼0.8 ln units per year). This was consistent with the horizontal rhizome apices being dense (162–334 apex m⁻²) and fast-spreading (36–56 cm per apex per year). The results for in situ measurements of plastochron interval (9.2 days), and relative leaf (3.7 cm² cm⁻² per shoot per day) and areal growth rates (2.3–2.7 cm² per shoot per day) were similar to those of previous studies. The variation in the vertical internodal lengths showed a unimodal annual trend more distinctly in the south than in the north station, suggesting local, small-scale differences in environmental conditions between stations. Flower production was clearly seasonal, peaking around January 1999. Fruiting (128–134 m⁻²) and seedling recruitment (22–32 m⁻²) values were high, indicating a substantial contribution of sexual reproduction to the rapid expansion or meadow maintenance. The apparent disjunct occurrence of such young and highly reproductive meadows in the region merits further investigation.