Journal of Threatened Taxa |
www.threatenedtaxa.org | 26 January 2023 | 15(1): 22494–22500
ISSN 0974-7907 (Online) | ISSN 0974-7893
(Print)
https://doi.org/10.11609/jott.8163.15.1.22494-22500
#8163 | Received 28 August 2022 | Final
received 20 October 2022 | Finally accepted 09 January 2023
Flowering
and fruiting of Tape Seagrass Enhalus acoroides (L.f.) Royle
from
the Andaman Islands: observations from inflorescence buds to dehiscent fruits
Swapnali Gole
1, Sivakumar Kuppusamy 2, Himansu Das 3 &
Jeyaraj Antony Johnson 4
1 CAMPA_Dugong Project, Wildlife Institute of India, Chandrabani, Dehradun, Uttarakhand 248001, India.
2 Department of Ecology
and Environmental Sciences, School of Life Sciences, Pondicherry University,
R.V. Nagar, Kalapet, Puducherry 605014, India.
3 Unit Head - Marine
Threatened Species and Habitats, Terrestrial and Marine Biodiversity,
Environment Agency Abu Dhabi, UAE.
4 Department of Habitat
Ecology Wildlife Institute of India, Chandrabani,
Dehradun, Uttarakhand 248001, India.
1 gole.swapnali@gmail.com,
2 ksivakumarwii@gmail.com, 3 hsdas@ead.gov.ae, 4 jaj@wii.gov.in
(corresponding author)
Editor: Anonymity requested. Date of publication: 26 January 2023
(online & print)
Citation: Gole,
S., S. Kuppusamy, H. Das & J.A. Johnson (2023). Flowering and
fruiting of Tape Seagrass Enhalus acoroides (L.f.) Royle from the Andaman Islands: observations from
inflorescence buds to dehiscent fruits. Journal of Threatened
Taxa 15(1): 22494–22500. https://doi.org/10.11609/jott.8163.15.1.22494-22500
Copyright: © Gole
et al. 2023. Creative Commons Attribution 4.0 International License. JoTT allows
unrestricted use, reproduction, and distribution of this article in any medium
by providing adequate credit to the author(s) and the source of publication.
Funding: National CAMPA Advisory Council (NCAC), Ministry
of Environment, Forest
and Climate
Change, Government of India (Grant/Award Number: 13-28(01)/2015-CAMPA).
Competing interests: The authors
declare no competing interests.
Author details: Swapnali Gole, a PhD scholar and a researcher with the CAMPA_Dugong
Project, Wildlife Institute of India has been studying dugongs and seagrasses in the Andaman and
Nicobar Islands (ANI),
India. Her doctoral research entails
seagrass vegetation ecology, plant-animal interactions and habitat use of seagrass meadows by local
communities from the ANI. K. Sivakumar
has been working on conservation
and management of aquatic biodiversity especially marine biodiversity of India as well as of Antarctica. His
research involves understanding
species distribution pattern
and behavioural ecology. He has significantly contributed in the MoEFCC-CAMPA funded project on the recovery of Dugong and its habitats
in India. Himansu Das is working
as the Unit Head of Marine Threatened
species and habitat programs at the
Environment Agency Abu Dhabi, United Arab Emirates. His study involves research, monitoring and
rehabilitation/ restoration
of marine megafauna and critical
marine habitats in the Arabian Gulf / Persian Gulf. He
has been involved with local, regional and international research and conservation programs related to coastal
and marine ecosystem. J.A. Johnson has been working on conservation and management of aquatic
resources. His research includes species distribution patterns, community structure, understanding resource
(food and space) partitioning among co-existing fish species and conservation of rare and threatened aquatic species Currently he is coordinating the MoEFCC-CAMPA funded project on the recovery of Dugong and its habitats
in India.
Author contributions: SG—Study design, field
data curation, data processing and
analysis, conceptualization, and
drafting of the manuscript;
SK—Funding acquisition, Study design, supervision,
review, and editing of the
manuscript; HD—Methodology design, supervision of work, validation of phenophases,
review, and editing of the
manuscript; JAJ—Funding acquisition,
Supervision, review, and
editing of the manuscript.
Acknowledgements: This study was
sponsored by National CAMPA Advisory Council (NCAC), Ministry of Environment,
Forest and Climate Change, Government of India (Grant/Award Number:
13-28(01)/2015-CAMPA). We acknowledge the Department of Environment and
Forests, Andaman and Nicobar Islands, for granting necessary work permits,
logistical support, and assistance. We acknowledge Ajay Kumar (local field
support) for his generous help in the fieldwork and sample processing. We thank
Dr. Len McKenzie (James Cook University) and Dr. Richard Unsworth (Swansea University) for validating
the phenophases observed during the study. We thank Snehal Gole for preparing the
maps and Dr. Nehru Prabakaran
(Wildlife Institute of India) for his valuable comments on the manuscript.
Abstract: Seagrass phenophases are crucial in understanding their reproductive
biology but are seldom documented. We studied flowering and fruiting phenophases of Enhalus acoroides from a mixed-species intertidal seagrass
meadow in Ritchie’s archipelago, Andaman Islands, India. The estimated mean
densities of pistillate and staminate flowers were 16.0 ± 12.0/ m2
and 12.7 ± 7.3/ m2, respectively. We observed the bloom of
free-floating male flowers (961.7 ± 360.4/ m2) during the spring low
tides (at mean sea surface temperature ~30°C). Seagrass cover, shoot density,
and canopy height of E. acoroides, along with
flowering densities, showed a zonal variation within the sampled meadow. We
report the first-time observations of several phenophases
of E. acoroides, such as female inflorescence
buds, male inflorescence, a bloom of released male flowers, pollination, and
fertilized flowers from the Indian waters. We also report the prevailing
threats to seagrass meadows, such as meadow scarring done by boat anchorage in
the Andaman Islands.
Keywords: Mass bloom, meadow
scarring, mixed-seagrass meadow, Swaraj Dweep.
Introduction
Characterizing
the demography of local seagrass populations is essential to understanding the
phenology and ecological processes of seagrass species (Inglis
1999). Such information is critical to improving the knowledge and management
of high ecological value species like Enhalus
acoroides (L.f.) Royle that regulates the food web, primary production,
& sediment dynamics and supports a diversity of benthic organisms &
fish communities (Estacion & Fortes 1988; Komatsu
et al. 2004; Yu et al. 2018). E. acoroides has
a wide distribution range in the Indo-Pacific region, extending from the
eastern coast of Africa to northern Australia (Waycott
et al. 2004; Short & Waycott 2010). The species
is dioecious and reproduces asexually (through clonal growth) and sexually
(pollination). Pollination in E. acoroides is
epi-hydrophilous, and fruiting and flowering occur
throughout the year (Hartog 1970; Brouns & Heijs 1986; Ackerman 2006; Rattanachot
2008). The positively buoyant seeds (Hartog 1970) and released fruits have a
higher potential for long-distance dispersal, thus facilitating the wider
species distribution and ensuring succession (Lacap
et al. 2002; Kendrick et al. 2012).
In
the Indian waters, E. acoroides is known to
occur on the southeastern coast, Andaman & Nicobar Islands (ANI), and
Lakshadweep Islands (Jagtap 1991, 1992; Das 1996). In
ANI, E. acoroides distribution is reported
from the North Andaman (Paschim Sagar and North
Reef), South Andaman (Tarmugli, Chidiyatapu,
Wandoor, Dugong Creek, and Vivekandapur),
Ritchie’s archipelago (Kalapatthar, Vijay Nagar, Inglis, and Henry Lawrence), and Nicobar archipelago (Pilomilow, Camorta, Trinket, Nancowry, Katchal, and Great
Nicobar) (Jagtap 1992; Das 1996; Thangaradjou
et al. 2010; D’Souza et al. 2015; Ragavan et al.
2016; Savurirajan et al. 2018; Figure 1). Although
the seagrass distribution, status, and associated fauna of E. acoroides are well documented, the reproductive
phenology of this species was rarely observed from the Indian coastal waters
including from ANI (Patankar et al. 2019).
Seagrasses
in the ANI are vulnerable to human-induced (coastal modification and pollution)
and natural stressors (tsunami and recurrent cyclones). These threats may vary
in intensity and subsequently have caused habitat alteration or in worst-case
scenario, a complete wipe-out of the local populations. For example, the 2004
tsunami in the Indian Ocean critically impacted several seagrass meadows and
changed the species composition, with the local extinction of a few species (Thangaradjou et al. 2010). For recovering from such major
disturbances through recolonization, sexual reproduction (seeds) has proven to
be more effective than clonal expansion (ramets) in
the seagrass restoration initiatives (Darnell & Dunton 2016). Thus, for directing
local efforts for seagrass conservation and effective management of large-scale
loss, documenting the sexual phases of seagrass species is a prerequisite
(Short & Wyllie-Echeverria 1996).
Despite
sexual reproductive strategies of species like E. acoroides
contribute to the resilience of seagrass populations and genetic diversity
(Duarte et al. 1997; Yu et al. 2018), these observations are scarcely reported
from the Indian waters (Patankar et al. 2019). In
this context, the present study aims to fill the existing research gaps in
seagrass phenology of E. acoroides from the
Indian waters and reports rare phenological phases from a mixed-species
intertidal seagrass meadow of the Andaman Islands. Our study presents a
detailed natural history observation on 10 different flowering and fruiting phenophases of E. acoroides,
which provide a baseline for future research. Although opportunistic in nature,
we believe our findings establishes improved knowledge of seasonality in
phenology of the species, especially in the wake of E. acoroides
gaining attention as a target species in global seagrass restoration
initiatives (Lawrence et al. 2007).
Materials and Methods
As
a part of pan-archipelago seagrass exploratory surveys, we sampled a mixed
species intertidal seagrass meadow in Vijay Nagar, Swaraj Dweep
Island of Ritchie’s Archipelago (South Andaman; Figure 1), in January 2021 at
the afternoon spring low tides. We mapped the seagrass meadow by walking around
its fringes with a GPS and calculated the sampled area on Google Earth Pro
version 7.3. Quadrats (0.5 X 0.5 m size; n = 18) were placed randomly in the
selected seagrass meadow to document the species composition and seagrass cover
(Duarte & Kirkman 2001). The shoot density (shoots/ m2) of E.
acoroides was calculated by counting all the
shoots within the quadrat. Further, we randomly selected 20 shoots of E. acoroides from each sampling point and recorded canopy
height using a measuring scale (cm). In addition, environmental variables such
as sea surface temperature (SST), pH, and salinity were recorded at each
sampling point using handheld multi-parameter testers (Eutech
Oaklon- PCS Testr 35;
refractometer- LABART).
We
conducted field surveys for 12 consecutive days and studied different phenophases of flowering and fruiting of E. acoroides. We estimated the densities of flowers and
fruits within the quadrats across the sampling points. We measured peduncle
length, sepal, and spathal leaf lengths of flowers
using a measuring scale (cm). To study various stages of fruiting and seed
development, we collected fruits of all phenophases
(n = 5/ phase) except dehiscent fruits. Fruits were contained in seawater and
immediately transported to the laboratory for further analysis. We dissected
each fruit with a surgical blade and measured their diameter and length using a
measuring scale (cm). Lastly, we recorded each fruit’s seed development
(immature/ mature seeds), the number of seeds, and morphometric measurements
(seed length, seed base length).
We
validated different flowering and fruiting stages by referring to published
literature on the species (Bujang et al. 2006; Patankar et al. 2019) and through personal correspondences
with seagrass experts.
Results
Seagrass
meadow characteristics
We
observed six seagrass species from a continuous meadow spread across ~16.8
hectares in Vijay Nagar (Swaraj Dweep), viz: Enhalus acoroides, Thalassia hemprichii (Ehrenberg)
Ascherson, 1871, Halophila ovalis (R. Brown)
Hooker f., 1858, Cymodocea rotundata Asch. & Schweinf.,
Halodule uninervis
(Forssk.) Asch, and Syringodium
isoetifolium (Asch.) Dandy.
Enhalus
acoroides
was the dominant of all species with the highest mean cover, followed by T. hemprichii and C. rotundata
(Table 1). Seagrass species exhibited spatial variation in distribution
within the meadow. In the high tide zone, S. isoetifolium
and H. uninervis occurred in a mixed
substratum of very fine sand and silt (Table 1). The distribution of C. rotundata was patchy across the mid-tide edges, and the
species preferred fine sand. Halophila ovalis and T. hemprichii occupied coarse sand and rubble in the
meadow’s mid and low-tide edges (Table 1). Distribution of E. acoroides was spread across high and mid-tide zones,
where the species was found either as monospecific strands in fine sand mixed
with silt and clay or co-occurred with C. rotundata,
H. uninervis, S. isoetifolium,
and T. hemprichii.
Seagrass
cover, shoot density, and canopy height for E. acoroides
varied considerably within the high and mid-tide zones of the sampled
meadow. The total mean cover of E. acoroides
was estimated as 36 ± 39.3 % (Table 1), but we observed a reduced species
coverage from high (64.8 ± 33.5 %) to mid tide zone (19.3 ± 33 %). Similarly,
overall shoot density for E. acoroides was
289.9 ± 103.9 shoots/ m2 (Table 1); however, mean densities in the
high and mid tide zones varied as 144.9 ± 130.8 shoots/ m2 and 30.3
± 55.2 shoots/ m2, respectively. We observed longer shoots of E.
acoroides in the high tide patches (33. 9 ±
10.1 cm). Shoots in the mid-tide zone were comparatively shorter (19. 4 ± 7.2
cm), with signs of herbivory.
Flowering
phases and natural history
In
the present study, we recorded different stages of both pistillate and
staminate flowers of E. acoroides—female
inflorescence bud, pistillate flower at anthesis, male inflorescence, the bloom
of free-floating male flowers, empty male spathe (post-release of male
florets), pollination (released male florets attached to female inflorescence),
and fertilized flowers (Table 2). Like species characteristics, a significant
zonal variation was observed in flowering densities of E. acoroides within the sampled meadow. Densities
of pistillate flowers in high and mid tide zones were 22.8 ± 13.4/ m2 and
4 ± 1.4/ m2, respectively. Similarly, densities of staminate flowers
were much higher (17.7 ± 10.4/ m2) towards the high tide shore than
in the mid-tide region (4 ± 1.1/ m2).
We
observed solitary female inflorescence buds on the terminal shoots. Peduncles
of female buds were shorter than pistillate flowers at anthesis (Table 2; Image
1A). Female inflorescence appeared as solitary flowers on the terminal shoots,
with visible sepals, petals, and pistils/ styles (Image 1B). Petals (3) were
pink and had 2–3 longitudinal ridges with folded margins, enclosing 5–6 styles.
Long peduncles aided the pistillate flowers to sway in the tidal waters, with
petals wide open, floating at the surface. The male inflorescence had multiple
white male flowers on the spadix enclosed at the base of widely open spathal leaves (Table 2; Image 1C). All male inflorescences
we observed were submerged in the water column with shorter peduncles (5.2 ±
1.1 cm above the substratum) than the female inflorescence at anthesis (26.1 ±
8.0 cm; Table 2).
A
noteworthy observation in the present study was the mass bloom of released male
florets free-floating in the high tide zone. Male florets (white) were 0.2 ±
0.1 cm long, with 2–3 stamens and 5–6 tepals. We observed released male florets
in masses (961.7 ± 360.4/ m2; n=3 quadrats) along the sandy
coastline (~1.5 km), floating on the water surface and trapped in seagrass
blades (Image 1D). After the release of male flowers, a male spathe with two
valves and a barren spadix (light orange) was visible. Shorter peduncles
supported empty spathes filled with sand in the seagrass meadow (Image 1E).
Further, we observed released male florets attached to the pistil of a
wide-open female inflorescence on the water surface (2–8 male flowers/ female
inflorescence; Image 1F). Lastly, a fertilized inflorescence observed
had shed its petals, and the ovary was swollen, indicating the beginning of
fruit formation (Image 1G).
Fruiting
phases, seed development, and natural history
Based
on the stages of seed development, we categorized the fruits observed as
immature, mature, and dehiscent fruits. Immature fruits were fleshy,
greenish-brown in color, with an uncoiled peduncle. Solitary fruits were erect
on terminal shoots of the peduncle and concealed 10–13 spherical white seeds
still developing (Images 2A & 2B). Mature fruits were large,
ovoid-shaped, and fleshy, with a pointed tip. The fruit cover was greenish,
with longitudinal rows of brown spikes, and the coiled peduncle positioned
above the substratum supported the fruits (Image 2C). We found 8–14 fully
developed, germinating seeds per mature fruit (Image 2D). A membranous white
seed coat concealed the seeds. Seeds were conical, yellow at the base and dark
green at the apex. We observed visible shoot buds with a length of 1.2 ± 0.3 cm
in each germinating seed (3 shoot buds/ seed; Image 2E). Dehiscent fruits were
observed at the base of plant shoots, right above the ground (Image 2F). Fleshy
fruit cover (mean diameter 12.6 ± 0.7 cm; Table 2) was broken into 6–7 halves
post-release of seeds.
Discussion
The
lack of information on the phenology of E. acoroides
from the Andaman Islands and Indian waters limits our understanding of the
species’ reproductive phases and seasonality. Densities of shoots, fruits, and
flowers, in the present study (post-monsoon) were higher than previous reports
in pre-monsoon (Patankar et al. 2019), possibly due
to different sampling seasons. Additionally, no male flowers (inflorescence or
released), pollination event, or fertilized flowers were recorded by Patankar et al. (2019). Since both the studies were
opportunistic in nature, our findings supplement and strengthen the previous
observations Patankar et al. (2019) made on E. acoroides phenology in the Andaman Islands.
In
the present study, no correlation can be established between zonal variation in
meadow characteristics and flowering densities of E. acoroides,
given limited data. However, this aspect credits detailed investigation as
studies have highlighted the role of meadow characteristics (seagrass cover,
shoot density, and canopy height) and herbivory on the reproductive success of E.
acoroides (Vermaat et
al. 2004; Rattanachot 2008). Novel observations on
the mass bloom of released male flowers (at SST ~30°C; mean) align with similar
notes reported for the species (Hartog 1970; Rollon
1998).
In
conclusion, based on higher densities of multiple phenophases
observed in the present study (from buds to dehiscent fruits) as compared to
previous reports (Patankar et al. 2019), we presume
that January could be a critical period for E. acoroides
phenology at a local scale, but this needs further validation through seasonal
studies. Furthermore, the fruit ripening period for E. acoroides
is long (2–3 months; Rollon 1998), after which the
seeds are released. Thus, we assume that pollination is somewhere in
October–November for the fruits observed in the present study. Thus, we
recommend long-term seasonal monitoring studies to understand the peak
flowering and fruiting season of E. acoroides
and assess local drivers influencing the species’ phenology in the Andaman
Islands.
Lastly,
our observations also report meadow scarring of the seagrass bed, as the study
site is a fishing transit lane used for boat anchorage by local fishers
(personal observations). Also, the entire inhabited coastline of Swaraj Dweep is known for gleaning activities and recreational and
commercial fishing using ‘khevla/ feka
jaal’ (cast net) and ‘taana
jaal’ (shore seine). Moreover, anecdotal reports
(from local fishers) and our field observations (direct encounters) suggest
that these seagrass beds are important to support threatened species like green
sea turtles and dugongs. Hence, detailed baseline information on the seagrass
meadow, including its biodiversity, needs to be established to emphasize its
management and conservation and to understand the species’ natural history.
Therefore, although our observations have provided detailed documentation of
the meadow and natural history notes on different phenophases
for E. acoroides, this baseline needs to be
supplemented with future research and long-term studies.
Table 1. Meadow characteristics, species‘ substratum preference, and spatial distribution
of seagrasses at Vijay Nagar, Swaraj Dweep.
Seagrass species |
Mean seagrass cover
(%) |
Shoot density (shoots/ m2) |
Shoot length (cm; n=20) |
Substratum |
Species
distribution within the meadow |
Enhalus acoroides |
36 ± 39.3 |
289.9 ± 103.9 |
35.3 ± 12.1 |
Fine sand mixed
with silt and clay |
high and mid-tide
zones |
Thalassia hemprichii |
10.9 ± 4.8 |
70.3 ± 43.6 |
10.7 ± 4.8 |
Coarse sand and
rubble |
mid and low-tide
zones |
Cymodocea rotundata |
6.5 ± 23.1 |
30.3 ± 18.5 |
6.0 ± 3.6 |
Fine sand |
mid-tide zone |
Halophila ovalis |
5.9 ± 17.1 |
29.3 ± 17.9 |
0.4 ± 0.6 |
Sand and rubble |
mid and low-tide
zones |
Halodule uninervis |
1.9 ± 5.7 |
46.9 ± 41.6 |
8.5 ± 2.6 |
Very fine sand
mixed with silt |
high-tide zone |
Syringodium isoetifolium |
1.3 ± 4.6 |
53.3 ± 46.0 |
8.2 ± 3.0 |
Very fine sand
mixed with silt |
high-tide zone |
SST (°C)- 30.1 pH- 7.7 Salinity
(ppt)- 30.9 |
Table 2. Different phenophases of flowering and fruiting
of Enhalus acoroides reported from the sampled
seagrass meadow in Andaman Islands, India.
Stages of flowering |
Density/ m2 |
Peduncle length
(cm) |
Sepal/ Spathal leaf length (cm) |
||||
Female
inflorescence bud |
3.2 ± 1.8 |
23.2 ± 7.8 |
3.9 ± 0.7 |
||||
Pistillate flower
at anthesis |
16.0 ± 12.0 |
26.1 ± 8.0 |
3.9 ± 0.8 |
||||
Male inflorescence |
12.7 ± 7.3 |
5.2 ± 1.1 |
4.2 ± 0.3 |
||||
Male spathe
(Post-release of male flowers) |
3.3 ± 1.5 |
6.4 ± 1.6 |
4.1 ± 0.1 |
||||
Pollination (Male
flowers attached to female inflorescence) |
10 ± 1.3 |
26.4 ± 8.0 |
3.9 ± 0.7 |
||||
Fertilized flower |
2.2 ± 1.0 |
20.3 ± 4.9 |
4.0 ± 0.6 |
||||
|
|
|
|
||||
Stages of fruiting |
Density/ m2 |
Diameter (cm; n =
20) |
Fruit length (cm; n
= 20) |
No. of seeds/ Fruit
(cm) |
Seed length (cm) |
Seed base (cm) |
|
Immature fruits
(Seeds still developing) |
7.3 ± 2.0 |
4.6 ± 2.2 |
5.2 ± 0.8 |
11.3 ± 1.5 |
0.7 ± 0.1 |
0.8 ± 0.1 |
|
Mature fruits
(Developed seeds) |
8.0 ± 3.9 |
9.2 ± 2.8 |
6.7 ± 1.0 |
11.8 ± 3.3 |
0.9 ± 0.2 |
1.1 ± 0.1 |
|
Dehiscent fruits |
2.2 ± 0.9 |
12.6 ± 0.7 |
- |
- |
- |
- |
|
Values expressed as
mean ± standard deviation; (- not recorded).
For figure &
images - - click here full PDF
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