Flowering of the seagrass Halodule wrightii in North Carolina, USA

Flowering of the seagrass Halodule wrightii in North Carolina, USA

Randolph L. Ferguson*, Brian T. Pawlak, Lisa L. Wood

NOAA, National Marine Fisheries Service, Southeast Fisheries Science Center. Beaufort Laboratory, 101 Pivers Island Road, Beaufort, NC 28516. USA

(Accepted 9 March  1993)

Abstract

Halodule wrightii Aschers. is a tropical euryhaline dioecious seagrass which is broadly distributed in shallow marine waters on the mainland side of the Outer Banks of North Carolina. Previous reports of H. wrightii in North Carolina have been limited to observations of sterile plants. During June through August 1990, and in May 1991, we observed and photographed male and female flowers of H, wrightii collected between Cape Lookout and Oregon Inlet, North Carolina. Although we did not find seeds of H. wrightii in the sediment of that region, seeds were found south of Cape Lookout in Bogue Sound in 1991. Our observations confirm that Dare County, North Carolina, is the northern limit for occurrence o f / / . wrightii along the Atlantic coast of North America.

Introduction

The genus Halodule is widely distributed along the coasts of tropical seas of the Atlantic and Indo-Pacific regions including the Pacific coast of Mexico, the Sea of Cortez, the Gulf of Mexico and the Caribbean Sea (Den Hartog, 1970; Rosas and Ruelas, 1985; Phillips and Menez, 1988 ). Along the Atlantic coast of North America, Halodule wrightii Aschers. occurs as far north as North Carolina, USA (Kenworthy, 1981; Thayer et al., 1984). Halodule wrightii is locally abundant along the Outer Banks between Bogue Inlet and Oregon Inlet in Onslow, Carteret, and Dare counties, North Carolina (Rad-ford et al., 1968; Beal, 1977). Here, H. wrightii often is associated with Zos-tera marina L. and Ruppia maritima L. One way to differentiate these species is examination of the leaf tip (Thayer et al., 1984). Leaf tips of H. wrightii are concave with a median point. Those of the other two species are lanceo-late (R. maritima) or convex with a median point (Z. marina).

This study documents flowering of H. wrightii in North Carolina; previous studies from this region have not reported flowers or fruits. Detection of H. wrightii flowers and seeds usually requires directed effort. Halodule wrightii flowers are small and except for anthers and stigmas are sheathed below the sediment; fruits are approx. 2.0-2.7 mm in diameter and mature at rhizome level (McMillan, 1981, 1985). Our initial discovery of H. wrightii flowers was serendipitous, while sampling plants and verifying signatures of seagrass in aerial photographs (Ferguson et al., 1989a,b, 1991; Ferguson and Wood, 1991; Thomas and Ferguson, 1991 ).

Methods

The study area was the shallow waters along the landward side of the Outer Banks, from west of Cape Lookout to north of Oregon Inlet, North Carolina (Fig. 1 ). Sixteen sites having beds ofH. wrightii in Bogue Sound, Core Sound, eastern Pamlico Sound and southern Roanoke Sound, were visited at least once between 26 June and 28 August 1990. Four sites near Ocracoke Island were sampled weekly during July 1990, on 28 August 1990, and in May 1991.

Dependent on water depth, plants were observed by wading, snorkeling, or scuba diving. Samples of plants with flowers and associated sediment were collected with a shovel, stored in plastic bags, and processed within 24 h after collection. The color, relative size, and development of the flowers were noted. Flowers and fruits were photographe d with a zoo m macroscope and darkfield illuminator (Wild), and automati c camera system (Leitz) or a 35 m m cam-era with a macro lens (reference to tradenam e does not imply endorsemen t by the Federal Government, USA) . Environmental data included water depth, temperature, and salinity.

Surficial sedimen t from sites of H. wrightii flowering was collected to de-termine the possible presence o f a seed reserve (McMillan, 1985). Samples of near surface sediments obtained with a shovel from six sites in late July 1990, and 20 cores (6 cm in diameter and 10-15 cm deep ) from the four sites near Ocracoke Island in May 199 l, were passed through sieves to isolate seeds.

Results and discussion

H.  wrightii flowered in the m o n t h s of May through August throughou t the study area and over a wide range of salinity. It was flowering near Ocracoke Island in May 1991. Male and female flowers were found from 26 June through 28 August 1990 at 14 o f the 16 sample sites from Cape Lookout to Oregon Inlet (Fig. 1 ). Flowers occurred at water depths from exposed to 1.5 m deep at low tide. The ambien t water temperatur e and salinity for flowering ranged from 28.5 to 30°C and from 12 to 34 ppt, respectively. Nort h of Oregon Inlet flowering plants occurred at a salinity as low as 12 ppt and sterile plants oc-

curred at a salinity as low as 8 ppt. The phenology and salinity of flowering of H. wrightii in Nort h Carolina is consistent with the data reported for H. wrightii in Redfish Bay, Texas (McMillan, 1976). The temperature range is higher than the 22 - 26 °C range reported for Halodule flowering by McMillan (1982) .

Our observations of H. wrightii flowers in Nort h Carolina are consistent with previous descriptions (De n Hartog, 1970; Phillips et al., 1974; Johnson and Williams, 1982 ). Halodule wrightii is dioecious. Staminate flowers have a pair o f anthers on a single stalk which are enclosed (Fig. 2a) or exserted from the sheath at the leaf base (Fig. 2b ). Because o f fibrous pollen, maturing anthers were cream colored and had a cottony exterior. Mature and dehisced anthers were about 3 m m in length and the anthers o f a pair were vertically displaced from each other by up to 1 m m (Fig. 2c). Following dehiscence, anthers were dark, at the end of a stalk that measured up to 28 m m in length, and visible approx. 2 cm above the surface of the sediment (Fig. 2d) . In our samples, dehisced anthers were not prone to excise from the stalk as reported by Johnson and Williams ( 1982 ).

Pistillate flowers o f H. wrightii have two free ovaries, each with one long style (De n Hartog, 1970). One o f the two ovaries may differentiate to a fruit. We did not observe both ovaries of a pair differentiated to fruit. In early July

Fig. 2. (a) Immature staminate flowers of Halodule wrightii encased in sheath; (b) pollen filled anthers, exserted from the sheath; (c) a dehisced pair of anthers with visible tannin cells - - note the displacement between superior and inferior anthers and the small pigmented protuberance just above the inferior anther; (d) male plant with dehisced anthers separated from sediment. Rule indicates approximate distance of anthers above surface of sediment.

Fig. 3. (a) Pistillate flower ofHalodule wrightii encased in sheath; (b) pistillate flower dissected free of sheath; (c) enlarged fruit dissected free of sheath-note (see arrow) the undifferentiated ovary of the pair; (d) mature fruit exserted from sheath.

1990, the two ovaries in each pistillate flower were equal in size (approx. 1.0 m m × 0.8 ram ) and were light green to yellow-green in color. Styles were 0.25 m m wide and up to 25 m m long. Ovaries did not appear to differentiate prior to the middle of July (Fig. 3a). From mi d July through August 1990, one of the two ovaries in many pistillate flowers was larger (1.3-2.5 m m in major dimension) than the other ovary and had a shorter, less robust style (Fig. 3b). It was assumed that the larger ovary had been fertilized and was differentiating into a fruit. Fruits varied from light colored, soft and only slightly larger than the undifferentiated ovary, to dark brown, hard and sev-eral times the diameter of the undifferentiated ovary (Fig. 3c). The undevel-oped ovary of a pair ultimately darkened but remained small. With the expan-sion of the fruit, the sheath of the female plant first became distended and then ruptured as the mature fruit, up to 3.0 m m in major dimension, exserted the sheath but still remained attached to the parent plant (Fig. 3d). In August 1990, both ovaries in some plants were dark but were not hardened or en-larged although the styles were reduced. This may have indicated failure of fertilization or aborted development.

Pistillate flowers were more widely distributed and where flowers of both sexes were present they were more numerous than staminate flowers. Vari-able with location, male and female plants were closely associated or segre-gated. Unique to our description are occasional pairs of staminate flowers associated with a single node.

Seed reserves for H. wrightii have been reported in sediments in Florida, Texas, and St. Croix, Virgin Islands (McMillan, 198 l, 1985 ). Seeds of this species were found neither in sediments from the six sites sampled in eastern Pamlico Sound and northern Core Sound in July 1990, nor in the sediment samples from Ocracoke Island in May 1991. Both flowers and seeds of H. wrightii were found in Bogue Sound in summer 1991 (J. Jewitt-Smith, per-sonal communication, 1991 ). Further study is required to determine the ex-tent and viability of a seed reserve in North Carolina.

The northern limit for occurrence ofH . wrightii along the Atlantic coast of the USA is confirmed as Dare County, North Carolina. We observed H. wrightii as far north as southern Roanoke Sound (Fig. 1 ) at an ambient sal-inity of 8 ppt. Salinity decreases rapidly north of Oregon Inlet. Water is brack-ish in northern Roanoke Sound and farther north in eastern Albemarle Sound and throughout Currituck Sound (Giese et al., 1979; Ferguson et al., unpub-lished data) . Sampling in North Carolina north of Roanoke Sound has re-vealed a variety of brackish water species but not the seagrass H. wrightii (Ferguson et al., unpublished data) . Halodule wrightii is reported to tolerate

a  salinity range of 9-52.5 ppt (McMahan, 1968). Thus northward limit ap-pears to be coincident with a salinity barrier.

 

Acknowledgments

C.	L e w i s  c o n s t r u c t e d  t h e  figure  a n d  p r i n t e d  t h e		p h o t o g r a p h s .  M .  D u r a k o
a n d	R .  Phillips  c o n f i r m e d o u r i d e n t i f i c a t i o n  o f H .		wrightii flowers, a n d		m a d e
s u g g e s t i o n s f o r t h e  study .  J.  K e n w o r t h y ,  M .  F o n s e c a ,				S. W y l l i e - E c h e v e r i a , P.
M c R o y ,  a n d		R .  Phillips p r o v i d e d  h e l p f u l r e v i e w o f a n		early d r a f t  o f the	m a n -
uscript .  T h i s		r e s e a r c h w a s  f u n d e d  b y N O A A ' s  C o a s t a l  O c e a n  P r o g r a m .