Diel patterns in three-dimensional use of space by sea snakes

Twenty-five individuals from two species of sea snake, spine-bellied sea snake (Hydrophis curtus previously Lapemis curtus: n = 19) and elegant sea snake (Hydrophis elegans: n = 6), were tagged within the study site and monitored between January and November
2013. The majority of monitored individuals were juvenile, which were difficult to
sex using external morphological feature. Only two adult female H. curtus and a single adult male H. elegans were monitored; therefore, to avoid inaccurate conclusions related to the small sample
size of reproductively mature individuals in this study, sex was excluded as a covariate
in further analyses. Data from depth sensors showed that individuals from both species
displayed a strong diel pattern in use of the water column (Fig. 2). Snakes were found at significantly greater depths during the day (06:00–18:00 h)
and were active on the surface at night (t test, H. curtus: t = 26.37, p  0.05, H. elegans: t = 9.51, p  0.05). Hydrophis curtus displayed a more varied dive profile, diving to an average depth of 3.2 m (SE: ±0.03 m;
max depth = 7.5 m) during the day and 2.1 m (SE: ±0.03 m; max depth = 7.4 m) at night.
While, H. elegans generally used comparatively shallower water and dived to an average of 2.5 m (SE:
±0.05 m; max depth = 5.7 m) during the day and 1.8 m (SE: ±0.04 m; max depth = 6.2 m)
at night. Individuals of H. curtus were generally present in deep water on the eastern side of Cleveland Bay (Fig. 3a, Additional file 2, https://dl.dropboxusercontent.com/u/31456301/3DSS/SM2/index.html), whereas H. elegans were restricted to shallow water directly adjacent to the southern shore (Fig. 3c, Additional file 3, https://dl.dropboxusercontent.com/u/31456301/3DSS/SM4/index.html).

Fig. 2. Patterns in diel use of different depths by tagged aHydrophis curtus (n = 19) and bHydrophis elegans (n = 6) over the monitoring period. Mean depths recorded by day (red) and night (blue) are represented as ticks on the y-axis

Fig. 3. Three-dimensional space use by a representative Hydrophis curtus (a) within the study site and b in closer detail, and by a representative Hydrophis elegans (c, d). Screenshots showing day-time core (50%-3DKUD; dark red) and extent (95%-3DKUD; light red) 3DKUD as well as night-time core (50%-3DKUD; dark blue) and extent (95%-3DKUD; light blue) 3DKUD. Surrounding bathymetry and sea surface are also rendered to provide context.
Black points in a, c represent the locations of acoustic receivers within the study site. Interactive,
three-dimensional versions of these 3DKUD models are available in the additional files
(Additional files 2, 3, 4 and 5)

The integration of depth data into analysis of space use showed a difference in the
three-dimensional kernel utilisation distributions (3DKUD) for individuals of both
species during day and night (Fig. 3b, d; Additional files 4, 5; https://dl.dropboxusercontent.com/u/31456301/3DSS/SM3/index.html, https://dl.dropboxusercontent.com/u/31456301/3DSS/SM5/index.html). Sufficient data from 12 of the 19 tagged H. curtus, and 5 of the 6 H. elegans were available to calculate reliable 3DKUDs to compare diurnal and nocturnal use
of space. Despite the difference in depths occupied by individuals, generalised linear
mixed models (GLMM) showed that volumes of core (50%-3DKUD) and extent (95%-3DKUD)
of space used by H. curtus were not significantly different between day and night (50%-3DKUD: F_1,11
 = 0.44, p = 0.52; 95%-3DKUD: F_1,11
 = 0.20, p = 0.66; Fig. 4a). Similarly, the volume of 50%-3DKUD used by H. elegans was not significantly different between day and night (F_1,4
 = 5.58, p = 0.07); however, H. elegans displayed significantly larger 95%-3DKUD volumes at night compared to the day (F_1,4
 = 18.79, p = 0.01; Fig. 4b).

Fig. 4. Diel differences in the use of volumetric space by tagged sea snakes. Mean diurnal
(50%-3DKUD: dark red; 95%-3DKUD: light red) and nocturnal (50%-3DKUD: dark blue; 95%-3DKUD: light blue) volumetric space used by aHydrophis curtus (n = 12) and bH. elegans (n = 5). Overlap between c 50%-3DKUD and d 95%-3DKUD diurnal and nocturnal use of space by H. curtus (white bars) and H. elegans (grey bars)

The overlap between areas used by individuals during the day and at night showed that
50%-3DKUDs of both species had a low proportion of overlap (Fig. 4c), suggesting that despite similar volumes, there was little overlap in the core
areas used. The mean proportion of overlap in 50%-3DKUDs between day and night was
significantly lower than that of a random pattern for both species (t test, H. curtus: t = ?2.47, p = 0.02; H. elegans: t = ?3.93, p = 0.02). In contrast, 95%-3DKUDs showed a high degree of overlap between day and
night (Fig. 4d). The mean proportion of overlap in 95%-3DKUD between day and night for both species
was not significantly different from random (t test, H. curtus: t = 1.52, p = 0.15; H. elegans: t = 0.07, p = 0.94).