Genetic and morphological differentiation of the Indo-West Pacific intertidal barnacle Chthamalus malayensis

Abstract

Chthamalus malayensis is a common intertidal acorn barnacle widely distributed in the Indo-West Pacific. Analysis of sequences of mitochondrial cytochrome c oxidase subunit I reveals four genetically differentiated clades with almost allopatric distribution in this region. The four clades exhibit morphological differences in arthropodal characters, including the number of conical spines and number of setules of the basal guard setae on the cirri. These characters are, however, highly variable within each clade; such that the absolute range of the number of conical spines and setules overlaps between clades, and therefore, these are not diagnostic characters for taxonomic identification. The geographic distribution of the four clades displays a strong relationship between surface temperatures of the sea and ocean-current realms. The Indo-Malay (IM) clade is widespread in the tropical, equatorial region, including the Indian Ocean, Malay Peninsula, and North Borneo. The South China (SC) and Taiwan (TW) clades are found in tropical to subtropical regions, with the former distributed along the coasts of southern China, Vietnam, Thailand, and the western Philippines under the influence of the South China Warm Current. The TW clade is endemic to Taiwan, while the Christmas Island (CI) clade is confined to CI. There was weak or no population subdivision observed within these clades, suggesting high gene flow within the range of the clades. The clades demonstrate clear signatures of recent demographic expansion that predated the Last Glacial Maximum (LGM), but they have maintained a relatively stable effective population in the past 100,000 years. The persistence of intertidal fauna through the LGM may, therefore, be a common biogeographic pattern. The lack of genetic subdivision in the IM clade across the Indian and Pacific Oceans may be attributed to recent expansion of ranges and the fact that a mutation-drift equilibrium has not been reached, or the relaxed habitat requirements of C. malayensis that facilitates high concurrent gene flow. Further studies are needed to determine between these alternative hypotheses.

Fig. 1

(A) Chthamalus malayensis belongs to the malayensis subgroup of Chthamalus and is the major occupier of space on the IWP high shore. The key diagnostic characters of the species in the malayensis subgroup include conical spines on the dorsal surface of the anterior ramus of cirri I (B and C) and II and the presence of multicuspidate setae which have basal guards (bg) on cirrus II (D). (E) Length and angle parameters measured in tergum (left) and scutum (right) of C. malayensis for comparison of opercular geometry among different clades. a occludent margin, b length of scutum, c basal margin of scutum, d angle between occludent and basal margin, f angle 1 of the tergal margin, g angle 2 of the tergal margin, h tergum height, i tergum width, j basal margin of tergum, k angle between occludent margin and scutal margin, l basi-scutal angle, m spur angle. Scale bars in μm, except A, E which are in mm.

Fig. 2

(A) Sampling sites of C. malayensis in this study with pie charts showing the relative frequency of each clade. The area of the pie chart is proportional to the sample size and colors denoting different clades correspond to that in the NJ tree. (B) NJ tree of COI haplotypes. The percentage of bootstrap support for the NJ and ML analyses is shown for the major clades on the corresponding branch for all values ≥70. See Table 1 for abbreviations of different localities and Appendix 4 for identity of the out-groups.

Fig. 3

Arthropodal characters of C. malayensis measured for comparison among clades. (A) Number of anterior and posterior rami on cirri I and II, number of segments having conical spines in the anterior ramus of cirri I and II, and number of conical spines per segment in the anterior ramus of cirri I and II. (B) Number of segments having the basal guard setae on the posterior side of the anterior and posterior rami of cirrus II. (C) Number of major teeth on the mandible. (D) Number of small teeth on the pecten of the mandibles and number of pectinations at the inferior angle of the mandible. (E) Number of setules and number of basal guards measured for the morphological comparison of basal guard setae. (F–K) Variation of the morphology of basal guard setae (F–K, same scale bar as G). (F) Short basal guard setae with sharp setules in the SC clade. (G) Short setae with blunt setules in the TW clade. (H) Basal guard setae with numerous setules in the SC clade. (I) Basal guard setae without basal guards are occasionally found in the IM clade. (J) Basal guard setae with two pairs of basal guards in the IM clade. (K) Basal guard setae with three pairs of basal guards in the IM clade. Scale bars in μm.

Fig. 4

(A) nMDS plot of the ordination of opercular plate geometry of the IM, SC, TW, and CI clades. (B) Variation in the average (+1 SD) number of segments with conical spines (CS) among IM, SC, TW, and CI clades. (C) Variation in the average (+1 SD) number of conical spines (CS) in the first segment of the anterior ramus of cirrus I. (D) Variation in the average (+1 SD) number of conical spines in the first segment of the anterior ramus of cirrus II. (E) Variation in the average (+1 SD) number of setules of basal guard setae among clades. Asterisks indicates significant difference (ANOVA) from other clades. Note that the CI clade was not included in the ANOVA analysis due to its small sample size.

Fig. 5

Zonation pattern of different clades of C. malayensis. For clarity, the tidal levels of transects are abbreviated as H – upper limit, M – abundant zone, and L – lower limit of barnacles. The vertical intervals of height between H, M, and L were 25 cm. Grey bars indicate IM clade, white bars indicate SC clade, and black bars indicate TW clade. In Muine, although SC and IM clades were both recorded, the IM clade was in very low abundance. As it is impossible to identify the clades from external morphology, all barnacles in Muine were regarded as belonging to the SC clade in the zonation graph. In PL and NE in Taiwan, the SC clade is present at lower abundance than the TW clade. The SC and TW clades cannot be distinguished by morphology in the field, therefore all individuals in these two sites were regarded as TW clade.

Fig. 6

(A) Distribution pattern of the IM (yellow), SC (green), TW (purple), and CI (red, slightly out of range in the map, approximate location shown by white arrow) clades (relative abundance of clades is shown as pie charts for each site) in relation to SSTs from a satellite image of the IWP in January–Febraury 2009. (B) June–July 2009 (Analyses and visualizations used in A–B was produced with the Giovanni online data system, developed and maintained by the NASA GES DISC). (C) Distribution of IM, SC, and TW clades (CI clade is not shown as it is out of the geographical range) in the SC Sea, in relation to different oceanic currents. The map was retrieved from Yang et al. (2008) which simulated the model bathymetry and velocity from the standard run in the SC Sea, showing the SCSWC along the northern SC Sea flowing northwards and entering the Taiwan Strait. The model also illustrates the intrusion of the SCSKB from the Luzon Strait into the SC Sea basin, where it impinges upon the SE Vietnam coast and mixes with the Southeast Vietnam Offshore Current (SVOC; Cai et al. 2007). Oceanographic map reproduced from Figure 3 in Yang et al. (2008). Copyright (2008). American Geophysical Union, Reproduced/modified by permission of American Geophysical Union.

Fig. A2