Halodule wrightii is a "seagrass" that may form carpet-like beds in warm, shallow waters from the southeastern United States to South America (seagrasses superficially resemble grasses, but are not technically grasses since they are not in the family Poaceae) (Haynes 2000).
In a study of Halodule wrightii in the Gulf of Mexico off Alabama (U.S.A.), the total biomass of H. wrightii generally increased through late summer, then began to decline. The proportion of that mass accounted for by roots and rhizomes generally declined from a high level in mid-spring as the proportion comprising leaves generally increased. The mass of below-ground structures was always greater than that of above-ground structures. (McGovern and Blankenhorn 2007)
Halodule wrightii is found in the intertidal zone of marine waters with sandy or muddy substrates at depth from 0 to 2 meters. Halodule wrightii occupies the shallowest waters in the Gulf of Mexico and is often exposed during low tides. (Haynes 2000)
In an assessment of light requirements of H. wrightii, Kenworthy and Fonseca (1996) found that the lower limit of depth distribution was controlled by light availability. Estimates of minimum light requirements ranged from about a quarter to a third of the light just beneath the water surface, much higher than light levels in the photic zone for many phytoplankton and macroalgae (typically just 1 to 5% of incident light).
Burd and Dunton (2001) used long-term data on the biomass of Halodule wrightii in the Upper Laguna Madre, Texas, to validate a model demonstrating the importance of underwater light intensity as a major abiotic factor regulating H. wrightii productivity.
Heck et al. (2006) assessed the individual and combined effects of removing large predators and enriching water column nutrients on Halodule wrightii meadows in Big Lagoon, Florida, U.S.A. To simulate the first-order effects of large predator reductions, the authors stocked enclosures with ~3 to 4 times natural densities of the omnivorous pinfish Lagodon rhomboides, the dominant
fish in local seagrass habitats, and supplemented nitrogen and phosphorus in the water column to nearly 3 times normal levels. Results showed both significant predator and nutrient effects, although there were fewer consumer effects and more negative nutrient effects on seagrasses than had been found in previous work, which had shown that mesograzers ameliorated the harmful effects of elevated nutrients on seagrasses. Epiphyte proliferation in nutrient enrichment treatments did not occur; thus, algal overgrowth could not explain the negative effects of nutrient loading on seagrass biomass. Instead, nutrient loading resulted in nitrogen-rich shoalgrass, and the authors suggest that this high-quality food stimulated pinfish herbivory, resulting in the decline of seagrass biomass in enrichment enclosures.
Armitage and Fourqurean (2006) transplanted H. wrightii sprigs into caged and uncaged plots in a Turtlegrass (Thalassia testudinum) bed near a patch reef. Nutrients (nitrogen and phosphorus) were added to half of the experimental plots. The authors recorded changes in seagrass shoot density, and after three months, measured above- and below-ground biomass and tissue
nutrient content of both Turtlegrass and H. wrightii. Herbivory immediately and strongly impacted H. wrightii. Within six days of transplantation, herbivory reduced the density of uncaged H. wrightii by over 80%, resulting in a decrease in above- and below ground biomass of nearly an order of magnitude. Turtlegrass shoot density and below-ground biomass were not affected by herbivory, but above-ground biomass and leaf surface area were higher within cages, suggesting that herbivory influenced both seagrass species, but that Turtlegrass was more resistant to herbivory pressure than was H. wrightii. Nutrient addition did not alter herbivory rates or the biomass of either species over the short-term duration of this study. In both species, nutrient addition had little effect on the tissue nutrient content of seagrass leaves, and the nitrogen-to-phosphorus ratio was near the 30:1 threshold ratio that suggests a balance in supply, indicating that neither of these elements appeared to be limiting growth. The authors note that the different impacts of grazing on these two seagrass species suggest that herbivory may be an important regulator of the distribution of multiple seagrass species near herbivore refuges like patch reefs in the Caribbean.
Halodule wrightii is the most common seagrass species in Brazil. This species approaches its southern distributional limit along the Rio de Janeiro state coast (Creed,1997). The northern limit of its range along the Atlantic coast of North America is North Carolina (Ferguson et al.1993).
The range of Halodule wrightii includes Alabama, Florida, Louisiana, Mississippi, and North Carolina (U.S.A.); eastern Mexico; the West Indies; Central America (Belize, Guatemala, Nicaragua); and South America (Venezuela) (Haynes 2000).
Like most seagrasses, Halodule wrightii is highly clonal, extending and maintaining perennial beds through the growth of underground rhizomes (McGovern and Blankenhorn 2007 and references therein).
Hall et al. (2006) found that vegetative fragments of Halodule wrightii can settle and re-root. Settlement of uprooted vegetative fragments may be a viable recruitment mechanism in some areas, especially where sexual reproduction (via flowering anfd fruiting) is not occurring or is rare. Fragments of the fast growing H. wrightii have the ability to remain viable for periods of time that allow for dispersal over large distances (kilometers). Morris & Virnstein (2004) observed a rapid recovery of H. wrightii in the northern Indian River Lagoon, Florida (U.S.A.), in quiescent, shallow water. This recovery occurred after complete demise of the existing seagrass bed. The initial recovery occurred as small patches, which may be indicative of recruitment by H. wrightii fragments. Hall et al. found that in their experiments H. wrightii was more successful at recruitment via fragments in spring than in fall. This species grows fastest throughout the spring and summer months and enters a period of dormancy during the late fall and winter months. Spring fragments of H. wrightii appear to have the potential to travel much greater distances in 4 weeks than do fall fragments, which lose viability by the second week. In addition to viability, of course, the distances the fragments can travel depend also on factors such as wind and tidal currents. (Hall et al. 2006)
In some parts of the range of H. wrightii, flowering and fruiting are rare, but in other areas these are apparently regular events (McGovern and Blankenhorn 2007 and references therein). In North Carolina, flowering has been reported from May to August and over a wide range of salinity (12 to 34 ppt). Flowers occurred at water depths from exposed to 1.5 meters deep at low tide. Halodule wrightii is dioecious (i.e., individual plants are either male or female rather than both). (Ferguson et al. 1993)
Seagrass beds may be disrupted by the activities of a variety of animals, including horseshoe crabs (Limulus polyphemus), Cownose Rays (Rhinoptera bonasus), and Southern Stingrays (Dasyatis sabina). One mode of foraging that has been reported for manatees (Trichechus manatus) involves the use of their forelimbs to uproot seagrass, leaving scattered roots and blades behind. These disturbances have generally been viewed as having a negative impact on seagrass beds. Hall et al. note, however, that if the fragments created by these animals are capable of settling and rooting elsewhere, the result may be the formation of new seagrass patches. (Hall et al. 2006)
Taplin et al. (2005) investigated interactions between H. wrightii and the macroalga Caulerpa prolifera. Their experiments indicated that the density and biomass of H. wrightii were negatively influenced by the presence of C. prolifera. Whether the nature of the interaction was a result of competition for space, nutrients or light could not be determined from their study. The performance of both species, however, differed between the two water depths studied, suggesting that the outcome of the interaction may be moderated in some way by light levels.
Evolution and Systematics
Systematics and Taxonomy
For many years Halodule along the North American coast were considered to be distinct from H. wrightii and referred to as H. beaudettei. Based on studies of variation in leaf tip shape in the northern Gulf of Mexico, however, along, along with a failure to identify genetic (isozyme) differences between plants with the different morphologies supposedly representing distinct species, Haynes concluded that H. beaudettei should be treated as a synonym of H. wrightii (Haynes 2000 and references therein)