Clibanarius symmetricus (Randall, 1840)

Melo, Ana Carolina, Rodriguesb, Melo & Martinelli-Lemos, Jussara Moretto, 2019, Spatiotemporal distribution and population structure of Clibanarius symmetricus (Randall, 1840) (Crustacea, Diogenidae) in an Amazon estuary, Turkish Journal of Zoology 43 (5), pp. 490-501 : 493-495

3.2. Spatiotemporal distribution of C. symmetricus

A total of 380 C. symmetricus specimens were collected during the study period. Hermit crab density varied significantly between seasons (dry and rainy; P <0.01), sector (lower and middle; P <0.01), and zone (upper and lower midlittoral; P <0.01). However, no significant variation was found between the estuary margins (A and B; P = 0.46) or in the interaction between the abiotic factors analyzed ( Table 1).

Themeantotaldensityrecordedduringthestudyperiod was 5.28 ± 11.69 ind./m 2. Factors associated with different densities were plotted to verify how they influenced the distribution of the species in the environment. This showed that the density of C. symmetricus was higher during the rainy season ( Figure 2a ) and in the lower estuary (sector 1), and highest on the eastern margin (B1) and lowest on the western margin at site A2 ( Figure 2b ), and highest in the lower midlittoral zone ( Figure 2c ).

The mean density of hermit crabs, as well as the temperature and salinity of the water, varied considerably during the study period ( Figure 3 ). The multiple regression analysis showed a significant negative correlation between hermit crab density and temperature (β = –0.37; P =

sampling points in the equatorial Marapanim estuary, Pará, Brazil.

(Se) seasons (dry and rainy), (Sc) sectors (lower [1] and middle [2]), (Ma) margins (A and

B), and (Zo) zones (lower [L] and upper [U]). DF = degrees of freedom, SS = sum of squares,

MS = mean squares. P = probability (α = 0.05). Significant values are highlighted in bold.

RODRIGUES and MARTINELLI-LEMOS / Turk J Zool

0.009), but little relationship with salinity (β = 0.18; P = 0.18). This analysis explained 39% of the variation in the data (R = 0.39).

3.3. Population structure of C. symmetricus in the study area

It was possible to examine 336 C. symmetricus specimens for sex determination, and the sex was identified in 287 cases, including 163 males (56.80%), 103 nonovigerous females (35.89%), 5 ovigerous females (1.74%), and 16 intersex individuals (5.57%). The sex of a further 49 individuals could not be identified; these specimens were designated as undetermined sex. Ovigerous females were present in September, November, July, and March, i.e. during both the dry and the rainy seasons ( Figure 4 ). Two ovigerous intersex specimens were collected in August.

The CSL ranged from 0.99 mm (in a specimen of undetermined sex) to 8.91 mm (male), while the smallest ovigerous females had a CSL of 4.51 mm ( Table 2). Size did not vary significantly between males and females overall (t = 1.86; P = 0.06), although the mean CSL of the males was significantly longer than that of nonovigerous females (t = 2.47; P = 0.01), but significantly shorter than that of ovigerous females (t = –2.08; P = 0.03). Nonovigerous females were also significantly smaller than ovigerous females (t = –3.48; P <0.001). Intersex hermit crabs were significantly larger, on average, than both males (t = –2.28; P = 0.02) and females (t = –3.75; P <0.001).

The smallest size (CSL) class contained only individuals of undetermined sex ( Figure 5 ). Both males and females were present in almost all CSL classes except for the largest one (8–9 mm), which was comprised exclusively of males. All the females in the largest classes (4–5 mm and above) were ovigerous. A unimodal size distribution was recorded in most months ( Figure 6 ), with continuous

RODRIGUES and MARTINELLI-LEMOS / Turk J Zool juvenile recruitment throughout the year, which was more intense from April through September. The sex ratio was significantly male biased (1.5:1; Χ 2 = 11.53; P <0.01) in all rainy season months except March ( Table 3).

The CSW vs. CSL ratio presented the breakpoint in 3.6 mm (CSW: –0.24 + 0.89*CSL, R 2 = 0.98), indicating a shift in the growth pattern that shows that this is the size at which C. symmetricus reaches morphological sexual maturity.

Figures

Figure 2. Median Clibanarius symmetricus density by season (a), sector (b), site (c), and midlittoral zone (d) in the Marapanim River estuary (PA), between August 2006 and July 2007, based on the results of the PERMANOVA.

Figure 3. Mean Clibanarius symmetricus density, and the temperature and salinity recorded in the Marapanimestuary, Pará,Brazil, between August 2006 and July 2007.

Figure 4. Frequency of occurrence of Clibanarius symmetricus specimens of nonovigerous females (F), ovigerous females (OF), males (M), and intersex individuals (INT) collected each month between August 2006 and July 2007, in the Marapanim estuary, Pará, Brazil.

Figure 5. Frequency of occurrence of Clibanarius symmetricus specimens of nonovigerous females (F), ovigerous females (OF), males (M), and intersex individuals (INT) by size class (cephalothoracic shield length, in mm), collected in the Marapanim estuary, Pará, Brazil.

Figure 6. Frequency distribution of cephalothoracic shield length (in mm) classes of the Clibanarius symmetricus specimens collected in the Marapanim estuary, Pará, Brazil.The vertical line represents the onset of sexual maturity (3.6 mm).