Inconclusive evidence of sexual reproduction of invasive Halophila stipulacea: a new field guide to encourage investigation of flower and fruit production throughout its invasive range

The dioecious seagrass species Halophila stipulacea reproduces mainly through fast clonal growth, underlying its invasive behavior. Here, we provide morphological evidence to show that the first findings of fruits in the Caribbean were misidentified. Consequently, H. stipulacea reproduction is likely still only asexual in the Caribbean. Therefore, we introduce an identification key of H. stipulacea reproductive structures to encourage careful identification and quantification throughout its invasive range. Until large-scale seed production in invaded habitats is reported, the apparent low rate of sexual reproduction needs to be considered in current studies investigating the invasion capacity of this species.

Native to the Red Sea and Western Indo-Pacific, the seagrass Halophila stipulacea (Forsk.) Ascherson, was reported to have invaded the Mediterranean Sea in 1894 (Lipkin 1975) and the Caribbean in 2002 (Ruiz and Ballantine 2004), after which it spread successfully in both regions (Winters et al. 2020). Similar to many invasive macrophytes, H. stipulacea expands mainly through asexual clonal growth and fragmentation (Lipkin 1975; Smulders et al. 2017), a feature believed to characterize the colonization of multiple Caribbean islands (Willette et al. 2014).

Sexual reproduction increases the genetic variability and dispersal potential of seagrasses, which is important for long-term stability of populations under dynamic change (Ackerman 2006). In its native range, Halophila stipulacea flowers predictably with both female and male flower production, followed by seed formation within 1–2 months after fertilization (Dural 2020; Malm 2006; Nguyen et al. 2018 see Figure 1A and B). Male flowering has commonly been reported throughout its invasive range; both in the Mediterranean as well as in the Caribbean (Chiquillo et al. 2019; Dural et al. 2020; Gambi et al. 2018; Procaccini et al. 1999; Vera et al. 2014). In contrast, documentation of female flowers and fruits is rare, and was limited to four studies in the Mediterranean (Dural et al. 2020; Gerakaris and Tsiamis 2015; Lipkin 1975; Nguyen et al. 2018), until a recent report of fruits in the US Virgin Islands (Chiquillo et al. 2019).