Journal of
Threatened Taxa | www.threatenedtaxa.org | 26 October 2018 | 10(11):
12443–12450
The terrestrial life of sea kraits:
insights from a long-term study on two Laticauda
species (Reptilia: Squamata:
Elapidae) in the Andaman Islands, India
Zoya Tyabji 1,
Nitya Prakash Mohanty 2, Erina Young
3 & Tasneem Khan
4
1,2 Andaman Nicobar Environment Team, North Wandoor, South Andaman, Andaman & Nicobar Island
744103, India
2 Present address: Centre for Invasion
Biology (CIB), Department of Botany & Zoology, Stellenbosch University,
Stellenbosch 7602, South Africa
3 School of Vet and Life Sciences, College
of Veterinary Medicine, Murdoch University, Murdoch 6150, Australia
4 Founding partner, EARTH CoLab science, design and learning LLP, Indiranagar,
Bangalore, 560038, India
1 zoya.tyabji@gmail.com (corresponding
author), 2 nitya.mohanty@gmail.com, 3 erinayoung@gmail.com,
4 tasneemkhan85@gmail.com
Abstract: Sea kraits forage in water and return to
land to digest their prey, mate, slough, and lay their eggs. The temporal terrestrial patterns in
encounter rate and behaviour of two species of sea kraits Laticauda
colubrina and L. laticaudata
were studied over four years at the New Wandoor
beach in the southern Andaman Islands.
The encounter rate of L. colubrina was
found to be 20 times higher than L. laticaudata,
and sea kraits were observed to prefer the natural refuge that the microhabitat of uprooted trees provide. Additionally, nesting observations are
presented that emphasize the need to promote the conservation of these crucial
terrestrial habitats.
Keywords: Andaman Islands, encounter rate, habitat
use, India, Laticauda colubrina,
Laticauda laticaudata,
nesting behaviour, sea snake, terrestrial pattern.
doi: https://doi.org/10.11609/jott.4311.10.11.12443-12450
| ZooBank: urn:lsid:zoobank.org:pub:3940D739-DA0F-4230-9614-24021E46AD04
Editor: Harold
K. Voris, Field Museum of Natural History, Illinois,
USA. Date
of publication: 26 October 2018 (online & print)
Manuscript details: Ms
# 4311 | Received 02 June 2018 | Final received 21 September 2018 | Finally
accepted 10 October 2018
Citation: Tyabji, Z., N.P. Mohanty,
E. Young & T. Khan (2018). The terrestrial life of sea kraits:
insights from a long-term study on two Laticauda
species (Reptilia: Squamata:
Elapidae) in the Andaman Islands, India. Journal
of Threatened Taxa 10(11): 12443–12450;
https://doi.org/10.11609/jott.4311.10.11.12443-12450
Copyright: © Tyabji et al. 2018.
Creative Commons Attribution 4.0 International License.
JoTT allows unrestricted use of this article in any
medium, reproduction and distribution by providing adequate credit to the
authors and the source of publication.
Funding: Andaman Nicobar
Environment Team.
Competing interests: The authors declare no competing interests.
Author
Details: Zoya Tyabji is a researcher working on elasmobranchs and
coral reef and associated fauna in the Andaman Islands, India. She is currently
affiliated with Andaman Nicobar Environment Team and Wildlife Conservation
Society. Nitya Prakash Mohanty
is a researcher working on biological invasions, and behavioural
ecology of herpetofauna. He is currently pursuing a
PhD from the Center of Excellence for Invasion Biology (CIB), Stellenbosch
University, South Africa. Nitya is a Ravi Sankaran Fellow and recipient of the M. Krishnan Natural
History Writing Award. Erina Young is currently enrolled in a PhD
program where she is studying sea turtle health in Western Australia at Murdoch
University in collaboration with the State Government Conservation agency
(Department of Biodiversity Conservation and Attractions). She has a background
in veterinary science specializing in wildlife and is a locum veterinarian at
Perth Zoo. Tasneem Khan is a biologist, educator, photographer and sailor with
a fascination for the natural world. Her formal training in marine zoology;
experience with field research management and practice in the development of
experiential learning pedagogies has resulted in her co-founded initiative
‘EARTH CoLab’.
Author
Contribution: TK and EY
conceived the idea; TK and ZT collected the data; NPM performed the analysis;
ZT and NPM led the writing, all authors contributed to the writing.
Acknowledgements: We would like to thank the Andaman Nicobar Environment Team for
providing us with the opportunity and support to carry out this project and to
be part of it. We thank Dr. Rauf Ali, Dr.
Romulus Whitaker, and Dr. Suhel
Quader for advice and inputs during the initiation of
the project, Smita Madhusoodhan,
Aditi Rajagopal, and Pooja Gupta for the design of the handbook for data
collection, and Saw John for the content.
We thank and acknowledge the efforts of the volunteers and field
assistants, especially that of Anand James Tirkey, Saw Stanlee, Naveen Ekka, and Saw Agu, in carrying
out this project.
Introduction
Sea snakes of the genus Laticauda,
most commonly known as banded sea snakes or sea kraits, are amphibious in
nature (Shine 2003; Heatwole et al. 2005). They rely entirely on aquatic prey and
display adaptations to marine life such as a flattened paddle-like tail,
salt-excreting glands, and enlarged lungs (Heatwole
1999; Shine & Shetty 2001; Brischoux
& Shine 2011). Unlike the other
lineages of marine snakes, however, sea kraits retain an oviparous mode of
reproduction and hence must return to land to lay their eggs. They also mate, digest their prey, and slough
their skins on land (Heatwole 1999; Greer 1997). They possess large ventral scales that help them
move efficiently on land (Bonnet et al. 2005; Shine & Shetty
2001). Acquiring fresh water is crucial
for sea kraits (Lillywhite et al. 2008; Kidera et al. 2013) and a combination of availability of
fresh water on land and low salinity at sea may determine the sea kraits’
geographic distributions and indicate their environmental tolerances (Brischoux et al. 2012, 2013). Sea snakes may also act as indicators of
climate change effects (Lillywhite et al. 2008,
2014), thus heightening the interest in their ecology, behaviour, and
conservation (Brischoux et al. 2009a; Bonnet 2012; Elfes et al. 2013).
Sea kraits are widely distributed in the tropical and
subtropical coastal waters of the eastern Indian Ocean, southeastern
Asia, and archipelagoes of the western Pacific Ocean
(Heatwole et al. 2005). Of the eight Laticauda
spp., Yellow-lipped Sea Krait L. colubrina
and Blue-lipped Sea Krait L. laticaudata have
the largest range, extending throughout the southeastern
Asian islands and seas. Comparatively,
the distribution of L. laticaudata is
fragmented with a smaller extent of occurrence than L. colubrina (Gherghel
et al. 2016). Laticauda
colubrina grow to an avergae
total length of 100cm with females growing larger than males, whereas L. laticaudata grow to an average length of 80cm (Shetty 2000). While L.
colubrina are considered to be more terrestrial, L.
laticaudata are considered to be intermediate, using
both terrestrial and aquatic phases equally (Heatwole
1999; Greer 1997; Bonnet 2012).
Juveniles of both species rarely venture far from water but stay close
to its edge, whereas adults move inland (Shetty &
Shine 2002a,b). Mate-searching males
move about much more actively on land than do females (Shine & Shetty 2001).
In the Andaman Islands, L. colubrina
and L. laticaudata are known to occur
throughout the archipelago (Bhaskar 1996; Shetty & Sivasundar 1997,
1998). On South Reef Island off North Andaman,
the relative abundance of L. colubrina with
respect to L. laticaudata was found to be
200:1 (Bhaskar 1996).
Preliminary studies on the behaviour of sea kraits, conducted in the
late 1990s at South Reef Island (Bhaskar 1996; Shetty & Prasad 1996), show that sea kraits restrict
their terrestrial activities between 1800 and 0400 hr. During the day, they take shelter in the cool
microclimatic conditions provided by the crevices of live and dead trees (Shetty & Prasad 1996).
Terrestrial activities such as digesting of prey and sloughing have been
encountered with one record of copulation (Bhaskar
1996; Shetty & Prasad 1996). The reproductive behaviour
of these two species in the archipelago is not well known, with no nesting
observations apart from one record of egg laying in captivity (Bhaskar 1996; Shetty & Prasad
1996). In addition, sea kraits
are known to exhibit positive phototaxis on land at
night, making them extremely vulnerable to anthropogenic activities (Bhaskar 1996).
Therefore, there is a necessity for a contemporary study to observe the
terrestrial behaviour of sea kraits, specifically of reproduction and habitat
use.
In this context, a long-term study was carried out to
investigate the temporal patterns in abundance and behaviour of two species of
sea kraits L. colubrina and L. laticaudata in the Andaman Islands. The relative abundance of sea kraits was
primarily investigated with respect to ambient climatic and tidal conditions,
along with the sea kraits’ behaviour and substrate use. In addition, incidental natural history
observations were documented.
Methods
The study was carried out at the New Wandoor beach, located between 11.594°N & 92.607°E and
11.600°N & 92.608°E, in the southwestern corner
of the South Andaman Island (Fig. 1a,b).
The study area lies adjacent to the Mahatma Gandhi Marine National Park
and Lohabarrack Crocodile Sanctuary and is lined with
fringing reefs (Raghuraman & Raghunathan
2013). The islands are significantly
influenced by the southwestern and northeastern monsoons from May to December, receiving up to
3,500mm of rainfall annually (Andrews & Vasumati
2002). The site experiences high
anthropogenic activity as it is within the boundaries of Wandoor
Village that is used for tourism and a part of the location lies in front of
the Sea Princess Hotel, a tourist resort.
The study was conducted as a volunteer program to
introduce groups of laypersons to conservation-related experiential learning and
all groups were led and monitored by the Andaman Nicobar Environment Team
(ANET) staff. As part of the ANET
volunteering program, 27 surveyors participated in the overall monitoring and
walked one transect of 680m parallel to the sea 181 times between 2012 and
2016. The transect lies within the
intertidal region, consisting of a sandy shoreline with rocky outcrops at the
start and a small estuarine inlet at the end.
Ipomea pes-caprae,
a pantropical salt tolerant creeper (Devall 1992), lines the supratidal
zone. During the 2004 tsunami, many
littoral trees (e.g., Sea Mohwa Manilkara
littoralis) were uprooted in the region; in the
study area, there were 19 such uprooted trees that were sparsely distributed on
the beach.
The transect was walked between 1830 and 2130 hr as sea kraits are known to restrict their terrestrial
activities between 1800 and 0400 hr with a peak in
activity between 1800 and 2300 hr (Bhaskar 1996). An
average of two people wearing headlamps actively searched for sea kraits on the
intertidal area, scanning from the sea up to the sandy vegetation (Ipomea sp.) lining the supra-tidal zone. At the start of each transect, the time, moon
phase, weather conditions (clear, cloudy, or rainy), and ambient climatic
variables (sand temperature & humidity, atmospheric temperature &
humidity) were recorded using a pocket weather meter (Kestrel 3000).
Upon encountering a sea krait, the following data were
recorded: i) species – L. colubrina
or L. laticaudata, ii) activity when
sighted — moving, resting, mating, or dead, iii) behaviour/condition —
sloughing, gravid, and cluster size, and iv) substrate — Ipomea
sp., sand, or wood. The
microclimatic factors of the sea kraits were not recorded to avoid disturbing
them and all participants were instructed not to handle any specimens as part
of the study. In addition to these
parameters, natural history observations were recorded ad libitum.
Analyses
Since the same transect of a fixed
distance was walked by the surveyors on all occasions, the number of
encounters of individuals reported are as such and not as a time-constrained
encounter rate. Further analyses of data
on L. laticaudata were not performed
as the number of encounters was low. The
influence of microclimatic variables and moon phase on the number of encounters
of L. colubrina was tested using univariate linear regressions. A one way-ANOVA was performed to evaluate the
differences between tide categories based on time to the nearest high tide (0–2
h, 2–4 h, >4h) coupled with the direction of the tide (Shetty
& Shine 2002a) on the number of encounters in the case of L. colubrina. Additionally, effect sizes-η2 in
percentage for one-way ANOVA (Zar 2010) are reported.
Results
Encounter rate
Eight-hundred-and-five individuals of L. colubrina and 39 individuals of L. laticaudata were encountered during the surveys. The number of encounters of L. colubrina per transect was 4.45 ± 0.21 and that of L.
laticaudata was 0.22 ± 0.04. The average encounter rate for L. colubrina ranged from 2.93 in October to 7.6 in August,
whereas for L. laticaudata it ranged from 0 in
June to 0.55 in May (Fig. 2). The number of encounters for L. colubrina had
a statistically significant correlation with atmospheric temperature (R2
= 0.072, β = 0.387, SE = 0.018, p < 0.001), sand temperature (R2
= 0.066, β = 0.368, SE = 0.108, p < 0.001), atmospheric humidity (R2
= 0.024, β = -0.099, SE = 0.046, p = 0.032), and sand humidity (R2 =
0.022, β = -0.096, SE = 0.046, p = 0.039).
These relationships, however, had poor model fit values. The phase of moon did not influence the
number of encounters in the case of L. colubrina (R2
= 0.01, β = 0.04, SE = 0.03, p = 0.185).
Tide categories based on time to high tide did not correlate with the
number of encounters for L. colubrina (F
= 1.68, p = 0.14, η2= 4.86).
Behaviour
Sloughing of L. colubrina
was encountered on six separate occasions, more frequently in August to
December; sloughing of L. laticaudata,
however, was not observed (Table 1).
Gravid females of L. colubrina were observed
throughout the year on 37 separate occasions, with the maximum number of
observations in April, whereas only one gravid female of L. laticaudata was observed, in February (Table 1). Clusters of two to four individuals were
observed on 19 separate occasions for L. colubrina
and once for L. laticaudata (Table 1).
The most utilized terrestrial substratum of the sea
kraits was found to be uprooted Manilkara littoralis while resting (Image 1), followed by sand
(Table 2). Eighty per cent of the sea kraits
encountered on sand were moving (Table 2).
Five dead sea kraits were recorded during the study, out of which one
was found on Ipomea sp., whereas
the other four were found on sand (Table 2).
Natural history observations
Some of the sea kraits living in the crevices and
hollows of the uprooted trees were seen to be sharing their space with hermit
crabs, unperturbed by them. Sea kraits
displayed signs of disturbance when tourists photographed them using a
flash. The snakes, however, were not
aggressive but only changed their direction of movement. During the sampling period, only two
aggressive snakes were encountered even though the surveyors were at a distance
of more than 5m away. Out of the two
snakes, one was injured and struggling on the dry section of the sandy beach.
On 02 December 2015, a gravid L. colubrina was recorded to be resting in the hollow of
an uprooted tree with an egg partially covered by the snake’s coiled form
(Image 2). The individual was photographed
and no further disturbance was caused.
The next day, the egg was observed to be fully exposed,
outside the snake’s coiled form (Image 3).
The egg was creamy-beige speckled with small dark spots, oviducal, approximately 7cm long, and appeared soft-shelled. The following day, the hollow was empty with
no signs of the sea krait or the egg.
The hollow was located 9.6m from the water’s edge at low tide at an
elevation of 0.8m from the ground. Due
to its low location, the hollow would have received an influx of water during
high tide.
On 04 December 2015, courtship behaviour was
observed. A female L. colubrina was observed to be moving from the waterline
towards the beach. Once on the trunk and
root of the uprooted tree, the sea krait disappeared into a crevice. A conspecific male was observed following the
path the female had taken. The male was
seen to be repeatedly tongue-flicking while following
the path. Once near the crevice, it
circled the area and entered another crevice, close to where the female had
entered.
Discussion
This study provides long-term data on the number of
encounters, habitat use, and behaviour of sea kraits. The encounters of L. colubrina was found to be 20 times higher than
that of L. laticaudata, similar to the
findings on South Reef Island (Bhaskar 1996). This rarity in abundance likely contributes
to the fragmented distribution of L. laticaudata. Both the rate of cutaneous evaporative
water loss and the extent of terrestrial dependence are known to differ among
the sea krait species with L. colubrina having
relatively higher dehydration rates in seawater (Brischoux
et al. 2012), lower rate of cutaneous evaporative water loss, and higher
dependence on the terrestrial habitat, and with L. laticaudata
showing intermediate levels of water loss and terrestrial dependence (Liu et
al. 2012). Laticauda
laticaudata are known to utilize shelters almost
exclusively under mobile beach rocks, which are both easily accessible from the
sea and regularly submerged at high tide (Bonnet et al. 2009). In contrast, L. colubrina
frequently travel long distances overland, with males moving significantly
further from the beach than females (Bonnet et al. 2005; Shine & Shetty 2001). The
availability of both shelter and fresh water are important to the habitat
selection of sea kraits; while L. laticaudata requires
a source of fresh water, L. colubrina is
characterized by a greater preference for the terrestrial habitat and possibly
greater opportunities for access to a freshwater source (Liu et al. 2012). The difference in the number of encounters
between the two sea kraits is likely due to the varying degree of dependence on
terrestrial habitats or site selection (Bonnet et al. 2005) and naturally low
densities of L. laticaudata (Bhaskar 1996).
Avoidance of L. colubrina by L. laticaudata and vulnerability of L. laticaudata to anthropogenic disturbance, however,
cannot be ruled out. Relatively high
numbers of sea kraits were encountered between April and September. Although the mean abundance of sea kraits has
been observed to be three-fold higher in the wet season compared to the dry
season due to the dependence on availability of fresh water (Bonnet et al.
2009; Lillywhite & Tu
2012), the results from this study could be skewed due to the low sampling size
during these monsoon months.
Ambient abiotic factors such as atmospheric
temperature & humidity and sand temperature & humidity were found to
significantly correlate with the number of encounters of L. colubrina; however, these relationships were not
biologically meaningful given the poor model fit values. Neither lunar nor tidal phases were found to
influence the number of encounters. Moreover,
these results do not match the findings of studies that show nocturnal
high tides to influence encounters with sea kraits (Girons
1964; Heatwole & Guinea 1993). Sea kraits of different age and sex classes,
however, use terrestrial habitats differently and thus vary in their temporal
and spatial distribution (Shetty & Shine 2002a). The results from this study could be
attributed to the influence of moon phase and tide on all the individuals of
the species collectively. As the
microclimatic conditions of the sea krait sites were not recorded in order to
avoid disturbing them, comments on the sea kraits’ microclimatic distribution
cannot be made.
The use of substrates by sea kraits was studied where
a majority of the individuals were encountered on wood where they were
predominantly resting, followed by sand where they were moving (Table 2). Sea kraits are known to use natural refuges
on beaches, which are both easily accessible from the sea and regularly
submerged at high tide (Bonnet et al. 2009).
Moreover, the spatial arrangement of terrestrial coastline
microhabitats, especially refugia, plays a key role
in the distribution of different sea krait species (Bonnet et al. 2009). In the current study area, sea kraits
utilized the gradual slopes of the sandy beach to move from the sea to refugia in the form of uprooted trees dispersed along the
intertidal zone. The protection of such refugia can lead to the conservation of these species (Webb
et al. 2000; Berryman et al. 2006; Bonnet et al. 2009). Clustering of sea snakes was observed to
increase from January to May; unconfirmed reports of mass aggregations (ca. 200
individuals) on Rutland Island were recorded and are pending validation. The sea kraits’ resting behaviour on wood and
clustering could possibly be explained by 1) thermoregulation (Shetty & Shine 2002a) and kleptothermy
(Brischoux et al. 2009b), 2) sharing of resources to
increase body temperature, or 3) mating behaviour (Parrish & Edelstein-Keshet 1999; Gregory 2004).
Previous observations on the mating behaviour of sea
kraits describe the male locating the female through vomerolfaction
(Bhaskar 1996; Shine 2003). This corresponds with the description stated
in this paper of an instance where a male followed a female by regularly
flicking its tongue at short intervals (we determined the sex of the sea kraits
by body sizes). A sophisticated vomeronasal system not only allows male snakes to locate
receptive females by following scent trails but also facilitates pheromonally mediated mate choice (Shine 2003). The presence of gravid L. colubrina peaked concurrently in April with the first
nesting observation in the Andaman Islands, India, recorded in this paper. The breeding cycle for L. colubrina varies geographically with some populations
showing aseasonal breeding whereas others showing
seasonal breeding patterns (Gorman et al. 1981; Guinea 1986; Brischoux & Bonnet 2009). In oviparous reptiles, incubation requires
well-buffered thermal and hydric conditions (Packard & Packard 1988;
Ackerman 1991; Shine et al. 1997; Delmas et al.
2008). Coastal sites catch greater
volumes of rain and, coupled with ambient temperature, are more stable compared
to remote islets (Bonnet et al. 2014).
Thus, the New Wandoor beach
with the crevices that uprooted trees provide may provide thermally buffered
and relatively humid microhabitats that are suitable for incubation. Sea kraits are known to lay their eggs in below-ground caves or in narrow crevices to exclude human
entry (Guinea 1986). With only two
recorded incidents of egg laying from the wild,
however, the nesting behaviour of L. colubrina
is unknown. Further studies are required
to investigate the breeding and nesting behaviour of sea kraits to determine
the factors influencing reproductive failure such as stressors leading to observed egg-laying or abandonment of the egg. In 2016, a crocodile
exclusion net was introduced by the government to provide a safe swimming area
for tourists. The net cordons off 200m
of the New Wandoor beach coinciding with the sites
where sea kraits were most commonly sighted.
In addition, the authorities removed the uprooted trees after the end of
our study to clear the beach for tourism purposes. These actions, collectively, could have
negative repercussions for the sea kraits that utilized the natural refuge that
these uprooted trees on the beach at New Wandoor once
provided. Overall, our findings
emphasize the need to promote the conservation of crucial terrestrial habitats
of sea kraits and justify their inclusion in marine protected areas, which in
turn will benefit a wide array of other organisms also dependent on beach
substrates.
Table 1. The encounter rate of sloughing
snakes, gravid females, and cluster size for Laticauda
colubrina (LC) and L. laticaudata
(LL) on New Wandoor Beach, South Andaman Island
|
Sloughing |
Gravid female |
Cluster size (≥2) |
|||
LC |
LL |
LC |
LL |
LC |
LL |
|
Jan |
0 |
0 |
1 |
0 |
4 |
0 |
Feb |
0 |
0 |
3 |
0 |
5 |
1 |
Mar |
1 |
0 |
0 |
0 |
0 |
0 |
Apr |
0 |
0 |
14 |
0 |
3 |
0 |
May |
0 |
0 |
2 |
1 |
5 |
0 |
Jun |
0 |
0 |
0 |
0 |
0 |
0 |
Jul |
0 |
0 |
4 |
0 |
0 |
0 |
Aug |
2 |
0 |
0 |
0 |
0 |
0 |
Sep |
0 |
0 |
5 |
0 |
0 |
0 |
Oct |
1 |
0 |
0 |
0 |
0 |
0 |
Nov |
1 |
0 |
4 |
0 |
2 |
0 |
Dec |
1 |
0 |
4 |
0 |
0 |
0 |
Table 2. Terrestrial substrate use and
associated behaviour of Laticauda colubrina and L. laticaudata
on New Wandoor Beach, South Andaman Island
Substratum |
Activity |
L. colubrina |
L. laticaudata |
Ipomea pes-caprae |
Dead |
1 |
0 |
Sand |
Dead |
3 |
1 |
|
Mating |
1 |
0 |
|
Moving |
132 |
8 |
|
Resting |
29 |
1 |
Manilkara littoralis |
Mating |
6 |
0 |
|
Moving |
272 |
14 |
|
Resting |
357 |
15 |
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