Flowering of the seagrass Thalassia testudinum (Hydrocharitaceae) in the Tampa Bay, Florida area

Aquatic Botany, 5 (1978) 251--259 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

251

FLOWERING OF THE SEAGRASS THALASSIA TESTUDINUM ( H Y D R O C H A R I T A C E A E ) IN THE TAMPA BAY, F L O R I D A A R E A

WILLIAM F. GREY and MARK D. MOFFLER Marine Research Laboratory, Florida Department of Natural Resources, St. Petersburg, Florida 33701 (U.S.A.) (Received 15 November 1977)

ABSTRACT Grey, W.F. and Moffler, M.D., 1978. Flowering of the seagrass Thalassia testudinum (Hydrocharitaceae) in the Tampa Bay, Florida area. Aquat. Bot., 5: 251--259. Nine sites in the Tampa Bay area were sampled during June 1976 to document flowering occurrence and obtain baseline data for the study of flowering populations of Thalassia testudinum Banks ex Kbnig. Flowering occurred at all sites. Qualitative information was obtained on three distinct floral stages: bud, anthesis, and fruit. Comparison between two sites showed that frequency of sexual reproduction varied from 35 to 56%. Average densities of reproductive short-shoots ranged from 8 to 19 m -~. Sex ratios from individual sites indicated general predominance of female over male short-shoots, with an average overall ratio of 3:1. Field observations and transect data demonstrated patchiness in the spatial pattern of reproductive short-shoots. Limited evidence provided additional information indicating that short-shoots on a common rhizome (i.e. an individual Thalassia plant) are dioecious.

INTRODUCTION The marine a n g i o s p e r m Thalassia testudinurn Banks ex KSnig is d i s t r i b u t e d t h r o u g h o u t the Gulf o f Mexico, Caribbean, and in t h e western n o r t h A t l a n t i c along the Florida east coast, the B a h a m a Banks and B e r m u d a (Moore, 1 9 6 3 ; den H a r t o g , 1970). Thalassia m e a d o w s are ecologically i m p o r t a n t in this region, providing s e d i m e n t stabilization, silt e n t r a p m e n t , animal habitats, n u r s e r y grounds, and substrate for e p i b i o t a (Phillips, 1 9 6 0 ; H u m m , 1 9 7 3 ; O d u m , 1 9 7 4 ; D a r o v e c et al., 1 9 7 5 ; T h a y e r et al., 1975). T w o m o d e s o f p r o p a g a t i o n , vegetative and sexual, are t y p i c a l o f Thalassia and o t h e r seagrasses. Vegetative g r o w t h allows significant increase in p l a n t biomass and h a b i t a t cover, whereas sexual r e p r o d u c t i o n is an i m p o r t a n t m e c h a n i s m w h i c h provides a d a p t a b i l i t y and c o m p e t i t i v e ability t h r o u g h outbreeding. T h e latter m o d e o f r e p r o d u c t i o n can t h e r e f o r e be c o n s i d e r e d a basic r e q u i r e m e n t f o r genetic plasticity and successful g r o w t h o f these seagrasses in variable coastal e n v i r o n m e n t s .

252

Flowering in Thalassia has been d o c u m e n t e d for Florida populations. It is known to occur sporadically from April through August with apparent peak inflorescence p r o d u c t i o n during June (Phillips, 1960; Orpurt and Boral, 1964; Marmelstein et al., 1968; Tomlinson, 1969; Moffler, 1976). However, there are few available quantitative data on Thalassia flowering and fruiting. Limited flowering o f Thalassia in the T am pa Bay area was observed in the summer of 1975, but observations in the summer of 1976 revealed abundant flowering. In an a t t e m p t to gain additional information about sexual reproduct i on, we made observations of flowering occurrence, densities, frequencies, and sex ratios at selected sites. This information can be i m port ant in the assessment o f methods for restoration of destroyed Thalassia meadows involving the use o f local seed stock. METHODS AND MATERIALS Locations of study sites were selected to include well-established Thalassia beds and to provide an even distribution of sites t h r o u g h o u t the T a m p a Bay region. Nine sites were visited, ranging from North Anclote Key to Mullet Key (Fig. 1). Observations were made between 8 and 30 June 1976. T w o sites, Anclote and Mullet keys, were visited more than once All sites were examined for the occurrence of visible stages of sexual reproduction: bud, flower, or post-anthesis. F o u r stages in the developmental sequence (bud, flower, fruit and seedling) are shown in Fig. 2. Thalassia beds were observed using snorkel and mask at each site for a p p r o x i m a t e l y one hour. The size of the area scanned at a site was generally d e p e n d e n t upon the e x t e n t of its Thalassia vegetation. At some sites there were extensive meadows of Thalassia, whereas at other sites patches occurred among ot her seagrasses or in bare areas. Reproductive material was collected in order to estimate relative numbers of floral stages and to obtain data on sex ratios at each site. Thalassia sex determination was based on floral m o r p h o l o g y as described by Tomlinson (1969). Quantitative samples were taken at two sites: North Anclote Key (site 1) and Mullet Key (site 9). A combination of transect and quadrat sampling was used to determine the frequency of occurrence, densities, and sex ratios of reproductive short-shoots. The short-shoot was used as a vegetation sample unit since it was n o t practical to sample entire plants. F r e q u e n c y of occurrence was determined by a modified poi nt - - quadr a t m e t h o d as described by MuellerDombois and Ellenberg (1974, p. 84). A 100 m transect cord, marked at 5 m intervals, was stretched through an area dominated by Thalassia plants. A metre stick was centered perpendicular to the transect at each 5 m interval. Observations were then made for reproductive short-shoots in a small area no greater than 0.5 m 2 at each end of the metre stick. A total of 20 intervals along the transect provided 40 f r e que nc y sample points. Three 1 m 2 quadrats were sampled along the f r e que nc y transect. Quadrats 1 and 3 were always at each end of the transect. The position of quadrat 2 was established after fre-

253

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Fig. 1. Location of sites used for observation of flowering Thalassia t e s t u d i n u m . 1, North Anclote Key; 2, Indian Bluff; 3, Crystal Beach; 4, Lower Gandy Flat; 5, shoal, Old Tampa Bay; 6, Lassing Park; 7, Coquina Key; 8, Cockroach Bay; 9, Mullet Key.

254

Fig. 2. Four stages in the developmental sequence of Thalassia t e s t u d i n u m . A, Bud; B, anthesis; C, fruit; D, seedling. quency observations were made, which allowed intentional placement in an area with reproductive short-shoots, if quadrats 1 and 3 yielded no flowering data. An ex cep tio n to these methods occurred at site 1 on 10 June 1976, when data were obtained from 2 blind quadrat casts but the transect was n o t traversed. Densities were determined for the total n u m b e r of short-shoots and for the n u m b e r of reproductive short-shoots in each quadrat. All reproductive short-shoots were collected for the determination of sex ratios, inflorescence n u m b e r and stages of development. Reproductive material was preserved as dry m o u n t or 5% formalin/sea-

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water wet stack voucher specimens and catalogued into the Florida Department of Natural Resources Marine Research Laboratory herbarium collection (STPE). Thalassia fruits were collected along beach strands and in seagrass drift in the Tampa Bay area during July. RESULTS

Reproductive short-shoots were observed at all nine locations (Table I). Samples containing bud, flower, and fruit stages were collected on 8 June 1976 at site 9 (Mullet Key); bud stages were still occurring at this site on 29 June. Thalassia can have more than one flower per short-shoot. Therefore, values representing floral stages listed in Table I should n o t be compared with numbers of short-shoots listed in Table II. Mean inflorescence number per female short-shoot was 1 + 0.38 (range = 1--2; n = 93), and per male shorts h o o t was 2 + 0.84 (range = 1--3; n = 29). Sex ratios were based on shortshoots as the reproductive units. The female : male ratio varied from 2 1 : 0 TABLE I Floral stages of Thalassia testudinum in the Tampa Bay area, June 1976 Site

Date

Sex

Floral stages Bud

1 1

10/6/76 17/6/76

2

30/6/76

3 4

30/6/76 18/6/76

5

18/6/76

~ ~ d 9

Flower

Fruit

13 5 12 2

1

12

2

3

1

2

6

Post-anthesis (6 's) 7 6

5 No data*

1 2

6

14/6/76

7

21/6/76

8

21/6/76

9

8/6/76

9

23/6/76

d 9 6 9 6 9 6 9 6 9

9

29/6/76

2 1 3 3 2 23 3

4

19

6 ~

* Single incomplete specimen.

1 1 3 1

5 7 9

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TABLE II Sex ratios of Thalassia t e s t u d i n u m short-shoots from the Tampa Bay area Site

Date

Female

Male

Ratio

9 9 9 1 1 4 5 6 7 8 2

8/6/76 23/6]76 29/6/76 10/6]76 17/6/76 18]6/76 18/6/76 14/6/76 21/6/76 21/6/76 30/6/76

21 4 10 17 7 2 1 3 3 19 9

0 1 6 9 9 1 1 0 0 2 0

21:0 4:1 2:1 2:1 1:1 2:1 1:1 3:0 3:0 10:1 9:0

Total number 96 Average 9 : 6 sex ratio

29 3 :1

TABLE III Density and frequency of sexually reproductive Thalassia t e s t u d i n u m short-shoots Site

Date

Frequency of occurrence t

Total no. of short-shoots m -2.

Reproductive short-shoots m -2.

% Reproductive short-shoots m -2

1 1 9

10/6/76 17/6/76 29/6/76

N.D.** 35% 56%

500(352--648) 346(264--418) 234(199--299)

19(14--23) 8(4--11) 16(6--22)

3.8 2.3 6.8

*Mean value and (range). **N.D. = data not available. t Reproductive short-shoots.

t o 1 : 1 . A v e r a g e f e m a l e : m a l e r a t i o f o r all s i t e s c o m b i n e d w a s 3 : 1 ( T a b l e I I ) . M e a n r e p r o d u c t i v e s h o r t - s h o o t d e n s i t i e s r a n g e d f r o m 8 t o 1 9 m -2 ( T a b l e I I I ) . T h i s is e x p r e s s e d a l s o as p e r c e n t r e p r o d u c t i v e s h o r t - s h o o t s m -2. I n t h e t w o s i t e s f o r w h i c h d a t a a r e a v a i l a b l e (1 a n d 9), a n i n c r e a s e in r e p r o d u c tive short-shoot frequency appears to correspond with an increase in reproductive short-shoot density. Areal patterns of flowering along transects indicated patchiness. DISCUSSION Tomlinson (1969) provided general information on the range and average number of flowers found on short-shoots, which agrees well with our data.

257

He also suggested that plants were apparently dioecious, based on his analysis of individual flowering short-shoots. He did not determine, however, whether the sex of short-shoots was consistent on one rhizome of an individual plant. From a limited number of specimens, we confirmed flowering in several short-shoots from one plant. A portion of a plant was collected at site 7 which had four female short-shoots. At site 9, two flowering shortshoots were also taken from one plant and were identified as females. A section of another plant was collected at this site in July 1977 which had five female short-shoots. This reinforces the suggestion that Thalassia is dioecious, though more extensive sampling of entire plants for sex determination is needed. A few speculative conclusions are offered for the pattern observed in sex ratios of these flowering populations. Many of the sex ratios from single sites are admittedly based on small sample sizes. However, most samples indicate a predominance of female over male short-shoots. All visible reproductive stages in both sexes were considered. When all sample data are combined (Table II), an average female : male sex ratio of 3:1 is obtained. A possible explanation is the short duration of the male floral structure. Post-anthesis condition of male flowers deteriorates rapidly; therefore, males are essentially not recognized as part of the flowering population when they are in postanthesis. This may be an important factor in the apparent dominance of females over m~es in our data. Tomlinson (1969) observed an average male : female short-shoot sex ratio of 4:1 from a Biscayne Bay, Florida, population. He did not state, however, whether all visible stages of flowering were included in determination of sex ratio. Information presented by Lot-Helgueras (1977) coincides with Tomlinson's data by showing a greater number of males over females at Veracruz, Mexico. He restricted his sex ratio determination only to the time when both male and female flowers were open. Differences between our results and those of Tomlinson and of Lot-Helgueras may be due to geographical variation in sex ratios. This cannot be confirmed, however, because of our limited data and differences in other investigators' sampling methods. Standardization of sampling methods would lead to a better understanding of sex ratios. Limited data from our quadrat measurements seem to indicate that flowering density varies somewhat independently from short-shoot density. Our subjective m e t h o d of quadrat placement, however, could influence any statistical treatment of this data and should be interpreted with caution. LotHelgueras (1977) found that the percentage of flowering in Veracruz, Mexico, increased with what he termed "foliar group density". Our observations indicate an apparent uniformity of sexual reproduction in the Tampa Bay area during the summer of 1976, though the pattern of reproduction within individual sample sites is not well understood. Patchiness of sexual reproduction in Thalassia meadows has been noticed by other researchers (Phillips, 1960; Marmelstein et al., 1968). We have also noticed, by general observation and use of frequency transects, patchiness in the spatial pattern of reproductive short-shoots.

258

T h e i n f o r m a t i o n p r e s e n t e d c o n t r i b u t e s t o c o n t i n u e d and m o r e e x t e n s i v e studies on r e p r o d u c t i v e d y n a m i c s in this a n d o t h e r seagrass species. I m p r o v e m e n t o f s a m p l i n g p r o c e d u r e s m a y allow a b e t t e r u n d e r s t a n d i n g o f t h e relative i m p o r t a n c e o f e n v i r o n m e n t a l a n d genetic c o m p o n e n t s in r e s o u r c e allocation, v e g e t a t i v e a n d sexual r e p r o d u c t i o n , f o r p a r t i c u l a r p o p u l a t i o n s . Sexual r e p r o d u c t i o n in Thalassia p o p u l a t i o n s a p p e a r s t o p r o v i d e a c o m p a r atively l o w ( b u t i m p o r t a n t ) s o u r c e o f n e w p l a n t g r o w t h in t h e T a m p a Bay area. O u r d a t a d e m o n s t r a t e , h o w e v e r , t h a t q u a n t i t i e s o f seed s t o c k m a y be i n s u f f i c i e n t f o r r e s t o r a t i o n efforts. J

ACKNOWLEDGMENTS This p a p e r is C o n t r i b u t i o n No. 308, F l o r i d a D e p a r t m e n t o f N a t u r a l R e s o u r c e s Marine R e s e a r c h L a b o r a t o r y ( F D N R M R L ) . We t h a n k K a r e n A. Steidinger ( F D N R M R L ) f o r h e r s u p p o r t a n d c o n s t r u c t i v e advice during t h e c o u r s e o f this s t u d y , including her review o f t h e m a n u s c r i p t . We are also grateful t o D.K. C a m p ( F D N R M R L ) , w h o d e v o t e d his t i m e a n d e x p e r t i s e t o w a r d t h e editing o f t h e paper. We e x t e n d special a p p r e c i a t i o n to Dr. R.C. Phillips ( S e a t t l e Pacific College, Seattle, W a s h i n g t o n ) f o r his critical review and suggestions. A p p r e c i a t i o n is e x t e n d e d to L.P. L o r a a m m f o r p r o v i d i n g t h e Thalassia p h o t o g r a p h s .

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Orpurt, P.A. and Boral, L.L., 1964. The flowers, fruits, and seeds of Thalassia testudinum KSnig. Bull. Mar. Sci. Gulf Caribb., 14: 296--302. Phillips. R.C., 1960. Observations on the ecology and distribution of the Florida seagrasses Fla. Board Conserv. Mar. Lab. Prof. Pap. Ser., No. 2, 72 pp. Thayer, G.W., Wolfe, A. and Williams, R.B., 1975. The impact of man on seagrass systems. Am. Sci., 63: 288--296. Tomlinson, P.B., 1969. On the morphology and anatomy of turtle grass, Thalassia testudinum (Hydrocharitaceae). III. Floral morphology and anatomy. Bull. Mar. Sci., 19: 286--305.