Reproductive Biology of the Tropical-Subtropical Seagrasses of the Southeastern United States
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Reproductive Biology of the Tropical-Subtropical Seagrasses of the Southeastern United States Mark D. Moffler and Michael J. Durako Florida Department of Natural Resources Bureau of Marine Research 100 Eighth Ave., S.E. St. Petersburg, Florida 33701 ABSTRACT Studiesof reproductivebiology in seagrassesof the southeasternUnited States have addressed descriptive morphologyand anatomy,reproductive physiology, seed occurrence,and germination.Halodale wrightii Aschers.,Halophila engelmannii Aschers., Syringodium filiforme Kutz., and Thalassiatestudiaum Banks ex Konig are dioecious;Halophila decipiens Ostenfeld and Ruppiamaritima L. are monoecious.In Halophila johrtsoaii Eiseman, only fernale flowers are known. With the exception of R, maritima, which has hydroanemophilouspollination, these species have hydrophilous pollination. Recent reproductive-ecology studiessuggest that reproductivepatterns are due to phenoplasticresponses and/or geneticadaptation to physico-chemicalenvironmental conditions. Laboratory and field investigationsindicate that reproductive periodicityis temperaturecontrolled, but proposedmechanisms are disputed.Water temperature appears to influencefloral developmentand maybe importantin determiningsubsequent flower densities and fruit/seed production.Flowering under continuouslight in vitro suggeststhat photoperiodplays a limitedrole in floral induction.Flower expression and anthesis,however, may be influencedby photoperiod.Floral morpho- ontogeneticstudies of T. testudinumfield populationsdemonstrated the presenceof early-stageinflorescences during short- and long-dayphotoperiods, further suggestingday neutrality in this species.High initial reproductiveefforts, annual variation in male sex expression,secondary sex characters,and possible interaction of ramet agewith sex expressionhave also been detected. INTRODUCTION TABLE 1. BREEDING SYSTEMSFOR SEAGRASSSPECIES OCCURRING IN S.E. UNITED STATES, Seagrassesexhibit two modesof propagation vegetativeand sexual.The mostcommon method of SPECIES DIOECIOUS MONOECIUS propagationis vegetative,but sexualreproduction allowsgenetic recombination and dispersalof pro- Halodule u rigMi geny. Flowering plants, including the seagrasses, Halophito decipieos Kalophila eogelmanniui + can generallybe placedinto one or more of three +'t major reproductive categories hermaphroditic, Holophito johosonii monoecious,or dioecious.Three to four percent of Ruppia marihma all flowering plants are dioecious;however, nine of the twelve seagrassgenera are dioecious Pettitt et Syringodiwn filiforme al, 1981!.Table 1 showsbreeding systems for the Thaloeeio testuCiouoi seagrassspecies of the southeasternUnited States. 78 FLORIDA MARINE RESEARCH PUBLICATIONS Seagrasses have apparently descended from The first filamentous-pollen-development terrestrial flowering plants. Although their vegetative studies were conducted by Bornet 864! on organs demonstrate adaptation of form and anatomy, Cyrnodoceae aeguorea Konig C. nod0sa Ucria! the reproductive organs are quite similar to those of Aschers.!. In his studies, he observed anther related land plants Sculthorpe, 1967!, and the elongation and rapid extension of the peduncle basic sequence of seagrass reproductive events is before anthesis. He also documented longitudinal not discernibly different from that of flowering land splitting of stamens containing cottonwood-like plants. Seagrasses, like their terrestrial relatives, pollen. Clavaud 878! observed pollen tubes formed have floral adaptations allowing pollination, regula- from one end of the Zostera marina pollen grain, tion of outbreeding, nurturing of the embryo, and which subsequently forced their way into the stigma. efficient seed dispersal Pettitt et aL, 1981!. He also noted that any water movement dislodged The first studies of seagrass reproductive and dispersed the pollen from the stigmas. biology can be traced back to the 4th century B.C., Prior to the 1960's, studies of seagrass repro- when the father of botany, Theophrastus, observed ductive biology principally concerned basic descrip- that Zostera ocean ca Posidonia oceanica L Delile! tive morphology Addy, 1947; Black, 1913; Bo~ produced fruits similar to oak acorns Ducker et aL, 1916, 1922; Dahlgren, 1939; Dudly, 1893; Graves, 1978!. Subsequent early studies were principally 1908; Murbeck, 1902; Rosenberg, 1901; Rydberg, taxonomic in nature, presenting descriptive mor- 1909; Setchell, 1929; Svedelius, 1904; Taylor, phology of flowers and pollen. Ducker et aL 978! 1957a, b; Tepper, 1882a, b!, and only a few studies presented an excellent historical review of the early concerned reproductive ecology or population biology scientific studies of seagrasspollen. Kausik, 1941; Kausik and Rao, 1942; Pascasio and Santos, 1930!. These works were confined primarily Cavolini 806! conducted the first major to qualitative observations. study on seagrasspollen and developed an ingenious Interest in seagrass biology was renewed after method of directly growing and studying Posidonia Phillips' 960! pioneer treatise on seagrasses in submerged containers. Konig 806! added to Table 2!. Since that time, the number of published Cavolini's observations by noting the filamentous papers concerning seagrass reproduction has in- pollen of Posidonia and Cymodoceae,as well as the creased steadily Table 2!. Work covering the south- dioecious nature of Cyrnodoceae.Gaudichaud 826, eastern United States has also increased consider- cited in Ducker et al., 1978! provided the first ably, with an 86% increase in publications over the evidence that the pollen of Arnphibolis antarctica last 25 years, 70% of these occurring since 1970. was thread-like and was released from the anthers in Seventy-one percent of all seagrass papers have rope-like strands. been published since 1960. Much of the seagrass- Fristche 837! found that the pollen of many reproductive-biology literature published since 1960 submerged plants is mucilaginous and that the has centered on the temperate Amphibolis, Phyl- pollen-surrounding mucous has the property of lospadix, and Zostera species Churchill, 1983; absorbing and swelling in an aqueous medium, a Churchill and Riner, 1978; DeCock, 1978, 1980, feature not observed in aquatic plants with aerial 1981a, b, c; Ducker and Knox, 1976; Ducker et aL, flowers or in land plants. 1977; Ducker et al., 1978; Gagnon et al., 1980; TABLE 2. PUBLICATIONS CONCERNING SEAGRASS REPRODUCTION. NUMBER OF PUBLICATIONS TIME PERIOD ALL SP. SP, S.E,U,S, 1860-1959 38 5 1960-1969 10 6 1970-1979 38 12 1980-1985 JULY! 41 14 TOTAL 127 PERCENT SINCE 1960 71% 86% NUMBER 42 79 Harada, 1948, 1949; Harrison, 1979; Jacobs, 1982; Subsequent sections of this paper present a Jacobs and Pierson, 1981; Keddy and Patriquin, suinmary of available information concerning the 1978; Lsmounette, 1977; McConchie et aL, 1982; reproductive biology of the five genera of seagrasses McMillan, 1983b; Nozawa, 1970; Orth and Moore, in the southeast United States. 1983; Pettitt et aL, 1983; Phillips, 1972; Phillips et aL, 1983; Riggs and Fralick, 1975, Robertson and HALODULZ WMGHT1T Mann, 1984; Silberhourn et aL, 1983; Stewart and Rudenberg, 1980; Yamashita, 1973!, and on the tropical Indo-Pacific-Australian species Thalasso- Floral Morphology dendron ciliatum, Cymodocea spp., Syringodium isoett'folium, Thalassia hemprichii, Enhalus acoroides, This species is dioecious, with inconspicuous Halophila spp., and Zostera capensis Brock, 1982; flowers enclosed in a vegetative shoot resembling a Ducker et aL, 1978; Ducker and Knox, 1976; perianth. In the staminate flowers, anthers are borne Harada, 1951; Issac, 1969, Kausik, 1940a, b; Kay, on a 12-23 inm long filament. Anthers are slender, 1971; Kirkman, 1975; Lakshamanan, 1963; Lipkin, 3.5-5 mm long, and white when immature. Mature, 1975, McMillan, 1980a; Pettitt, 1976, 1981; Pettitt pollen-containing anthers are usually green, turning et aL, 1981; Sachet and Fosberg, 1973; Swamy and brown after anther dehiscence Johnson and Lakshinanan, 1962; Verhoeven, 1979!. Principally, Wiliimns, 1982!, However, inature anther color can these studies have concerned descriptive morphology be variable, ranging from cream to red Den Hartog, and cursory physiology. 1970; Phillips et al., 1974!. Pistillate flowers are Seven species of seagrasses occur in the sub- comprised of two ovules; the ovaries are ellipsoid, tropical-tropical coastal waters of the southeastern ovoid, or globose, 1.5-2 mm long with 10-28 mm United States. Four species, Thalassia testudinum long style. The fruit is ovoid or slightly compressed, Banks ex K'bnig, Halophda decipiensOstenfeld, H. 1.5-3 mm long, with a short subterminal or lateral engebnannii Aschers., and H. johnsonnii Eiseman, rostrum It is black, rough textured, and has a hard represent the Hydrocharitaceae family, and three coat McMillan, 1981!. The fruit-seed coat separates species, Halodule wrightii Aschers,, Syringodium into nearly equal halves at germination, ftliforme Kutzing, and Ruppia maritima L., repre- sent the Potomagetonaceae family. Little informa- Reproductive Ecology tion exists concerning reproductive processes in six of the seven species Table 3!. Most research has Published literature includes little information centered on the dominant species, Thalassia testu- concerningthe reproductive dynamicsof H. wrightii dinurn. However, our recent observations Durako field populations. Phillips 960! reported that and Moffler, 1981, 1985a, b, in press; Moffler et aL, sexual reproduction in Diplanthera Halodule! is 1981! suggestthat