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.
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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
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p h o t o g r a p h s .
M . D u r a k o
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a n d
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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 .
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wrightii flowers,
a n d
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m a d e
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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 ,
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S. W y l l i e - E c h e v e r i a , P.
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M c R o y , a n d
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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
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early d
r a f t o f the
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m a n -
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uscript
. T h i s
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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 .
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