POLLINATION ECOLOGY OF A SEAGRASS, THALASSIA TESTUDINUM (HYDROCHARITACEAE), IN ST. CROIX

The reproductive ecology of a dioecious Caribbean seagrass, Thalassia testudinum was studied in St. Croix, U.S. Virgin Islands. Flowering in Thalassia coincided with spring tides with anthesis and anther dehiscence in staminate plants occurring at night. As predicted by search theory, pollen is dispersed in negatively buoyant rafts of pollen grains (that superficially resemble strings of frog spawn) which are bound by a slime of apparent thecal origin. Dispersal of pollen is submarine and approximately two-dimensional along the plane of the substratum. The stigmas on pistillate plants are linear, stiff, and densely papillate; they protrude from the mouth of the subtending leaf. Floral ratios of staminate to pistillate flowers were 60 to I with an average distance of 4.0 cm from pistillate flower to the closest staminate flowers. Populations included a high proportion of short shoots with numerous (> 100) leaf scars indicating ages for these axes of up to 3-4 years. Nonflowering shoots can be sexed and frequency of flowering can be assessed from scars of old inflorescences. Preliminary results suggest that there is no regular pattem of flowering, since the interval between flowering events is variable. THE GENUS Thalassia Banks ex Konig. consists of two tropical species of seagrass, Thalassia hemprichii (Ehrenb.) Aschers., which occurs in the Indian and Pacific Oceans and Thalassia testudinum Banks ex Kbnig (Hydrocharitaceae), commonly known as "turtle grass," which is distributed throughout the shallow intertidal regions of the Caribbean, Gulf of Mexico, and Atlantic Coast from Bermuda to the Bahamas (Den Hartog, 1970). In the Island of St. Croix, T. testudinum forms dense meadows restricted to shallow areas behind reefs on the eastern and southwestern portions of the island (Ogden, 1974; Johnson and Williams, 1982). Although T. testudinum is a major component of the seagrass community, providing food for conchs, urchins, sea turtles and fishes (Ogden, 1974), as well as habitat for a heavy and diverse population of epiphytes, little has been known concerning its pollination ecology. In an attempt to relate previous information on the floral morphology and anatomy of T. testudinum (Tomlinson, 1969, 1982) to the possible ecological function of the floral structures, a field study of T. testudinum was designed and carried out in St. Croix in April 1987. Information on flowering times for Thalassia in St. Croix is given by Johnson and Williams (1 982). In 1975 the flowering period was recorded from March to May, with flowers less abundant than in other seagrass genera (Halodule, Halophila, I Received for publication 15 June 1987; revision accepted 28 August 1987. This study was supported by NSF Grant BSR-8452090. and Syringodium). One population of Thalassia, however, was reported as "60% of bed in flower." Phillips, McMillan, and Bridges (1981) recorded dates of initial anthesis in Tague Bay, St. Croix, as 18 May and into late June for other Caribbean sites. The morphology of Thalassia has been described by Tomlinson and Vargo (1966). Axes are dimorphic (cf. Tomlinson, 1974) with scaly horizontal rhizomes (long shoots) giving rise to leafy erect shoots (short shoots) at regular intervals by a distinctive method of leaf-opposed branching that simulates an equal dichotomy (Tomlinson and Bailey, 1972). Scale leaves on rhizomes are separated by intemodes of 1-1.5 cm, foliage leaves on erect shoots are about 1 mm apart. In contrast to rhizomes, which branch at very regular intervals separated by 9, 1 1, or 13 nodes (Tomlinson and Vargo, 1966) vegetative branching of short shoots is rare and irregular with the branches always rhizomes. Flowers are restricted to the axils of leaves on short shoots. Female flowers are solitary, male flowers usually occur in twos or threes (Tomlinson, 1969; Durako and Moffler, 1985). The flowers of each triplet usually develop in sequence rather than simultaneously. MATERIALS AND METHODS -Two populations of Thalassia testudinum at a depth of about 1.5 m were studied in St. Croix -one at Robin Bay on the southwest coast ofthe island, and one at Salt River Bay on the north coast. All underwater manipulations and observations on these populations were performed us-