Morphometric Study of Swimming Crab (Portunus pelagicus) Using Truss Network Analysis for Determination of Stock Structure in FMA 711

The crab of Portunus pelagicus is a fishery resource with a wide distribution area and quite diverse environmental conditions in Indonesia, giving rise to morphometric diversity. The study aims to analyze differences in morphometric diversity of crab populations (P. pelagicus) in the waters of Indonesian Fisheries Management Area (FMA) 711, landed in Bintan, Bangka, and Ketapang, which can describe the condition of the crab stock units. A total of 300 individuals were collected from all locations using random sampling. The crab morphometric characters were analyzed using Truss Network body shape method (TNA). The Kruskal Wallis results analysis showed distinguishing in some morphometric characters for both carapace and abdomen. Cluster analysis shows that there were two population groups: The Bintan group, and the Bangka-Ketapang groups. The discriminant analysis on the carapace characters resulting that the crabs from three locations have the potential for more than one stock unit, so different management is needed.


Introduction
The crab of Portunus pelagicus with the common name as the blue swimming crab (BSC) is a fishery resource that has one of export commodities in Indonesian fisheries.Important economic values with high prices make demand quite high in the market [1,2].The export commodity of crab (P.pelagicus) is crab meat, with the destination countries being the United States, Europe, and China [2].The distribution of crabs is widespread in the waters of southeast and east Asia with sandy or muddy substrates [3].Due to their distribution in wide area can affect its morphological form as consequence of adaption and related to variation in degree of reproduction, growth, and mortality [4].
Studies on morphometric are related to the differences and variation in the body form or morphological characters of organism's dimension or objects size [5].Morphological shape disparities inter populations in distinctive geographical area are affected by divers of genetic structures and habitat characteristics.A population with similar morphological feature and body shape in certain area can be stated as a stock unit [4].The truss network analysis (TNA) can apply to assess the disparities of morphometric characters.This approach can recognize achievable distinguishes in the morphology and/or body shape of organisms, which is a close relationship for both the same and different population [6].The basic principle in TNA analysis is the difference in growth patterns between male and female of fishery resource that influence morphological and body form in the scope of truss network [6].
The Indonesian Fisheries Management Area (FMA) 711 is a large geographical area, from the South 2 China Sea to Karimata Strait (including the Natuna Sea) with a variety of environmental conditions [7].However, this area is a distribution area with an abundance of large crab populations with high utilization of crab fisheries resources.Therefore, it is necessary to analyze the morphometric character of the crab to observe and test the hypothesis whether it consists of the same stock unit or more in several waters locations within the FMA 711.This study aims to analyze a differences in the morphometric variation of the crab population (P.pelagicus) in the representative waters of FMA 711, namely Bintan, Bangka, and Ketapang, to describe the condition of the crab stock structure in the region using the Analysis with Truss Network (TNA) method.The results of this analysis can be used as a basis for managing crab fishery resources in various FMA with different stock units

Research time and location
This study was performed from June 2019 to December 2019.The BSC samples were collected at three locations within the FMA 711, that is Bintan, Bangka, and Ketapang (Figure 1).Sample identification and morphometric analysis were conducted at the Aquatic Molecular Biology Laboratory, Aquatic Resources Management, FFMS, IPB University.

Data collection
The crab (P.pelagicus) sample was taken using the Simple Random Sampling method.Samples of BSC were taken from collectors at three catch landing locations and representing different fishing ground within FMA 711, namely Bintan, Bangka, and Ketapang.The number taken samples were 100 crab in each landing site.In order to assess with truss network method, sample is place on scale paper and put landmark point around the crab carapace and abdomen.Subsequently, taken pictures in the form of photos using a Canon EOS 1300D digital camera and setting with the similar length to the object, as well as the same magnification, and light effect.In this way, we do not sacrifice the BSC samples.The sample is an edible organism, is not protected, and consumed.Thus, ethical approval is not required for this study.Using the utility program TPSUtil, those taken images were converted from JPEG (*.jpeg) format to TPS (*.tps) format.Eighteen morphological characters from the carapace (Figure 2 and Table 1) and ten from the abdomen were measured (Figure 3 and Table 2).Using TPSDig2, the characters are then placed in their appropriate locations to cover the entire body.Statistics for Paleontological Analysis (PAST) was then applied to extract the morphometric or body form data and do a Truss Network Analysis (TNA).All characters are formulated into the form used by Bhosale [8].Table 1.Landmark spacing characters on the carapace crab (P.pelagicus) using Truss Network Analysis (TNA) method (modified from Bhosale [8]) No Character Code Description    [9].The aims of conversion data were to eliminate the effect of different size and growth variations from body shape diversity, and not from the relative size [9].Data conversion was using the following formula: M -convers = log M -β (Log CW -Log CW mean) While: M -convers: determination truss conversion, M: original determination (mm), CW: carapace width (mm), CW mean: average carapace width (mm), β: Group regression slope from Log M to Log CW.

Differentiate of morphological characters.
T h e Kruskal-Wallis test was performed to determine which morphological characteristic is significantly different among crab populations from all sites.The Kruskal-Wallis test is a technique to differentiate of population characteristics from several samples of the same population from different habitat and/or environmental condition.

Groupings and classify populations according to various geographic locations.
Cluster analysis was used to compare population groupings between locations using morphometric characteristics.Moreover, discriminant analysis was used to group populations based on different locations.The purpose of discriminant analysis is to estimate the relationship between a single categorical non-metric dependent variable and a set of morphometric independent variables in general form [10].The equal value of morphometric characters indicate that the existence of measurable population is mixing.The final result of the analysis shows whether or not the centroids of the population morphometric characters are separated between the observed locations.

Differentiate of morphological characters
Morphological characters comparison of the BSC at three locations can see based on the significance value (p-value) obtained from the Kruskal-Wallis test.There are 29 characters on male and 28 characters on female carapace crabs shown differences (p-value <0.05) among 34 characters analyzed (Table 3).All 28 character on the female carapace are also differentiator in male carapace crabs.Meanwhile, there are 14 of 19 characters on male and 16 of 19 characters on female abdomen crabs shown differences (pvalue <0.05).However, there are 11 characters on male abdomen also differentiator in female abdomen crabs.

Groupings and classify populations according to various geographic locations
The level of similarity between locations to see the grouping of crab populations based on the suspected crab morphometric characters using cluster analysis.The cluster analysis results can explain the relationship between crabs between locations in the dendrogram form.The outcome of the inquiry obtained from the computation of the carapace and abdomen crabs for both male and female using the TNA method appearance that the populations of Bangka and Ketapang crabs had the high similarity morphometric characteristics, so they formed one group (Figure 4).Meanwhile, the crab populations in the Bintan location have different morphometric characteristics and form their own or separate groups from the Bangka and Ketapang crab populations.The TNA method on the carapace of female crabs has also shown the same result (Figure 4c & 4d).The classification of morphological characters of each location and the formation of character groupings decided by variation in the habitat or location of the centroid were estimated by using the analysis of discriminant.The discriminant analysis on the morphological characters from carapace determinations for both male and female crabs using the Truss Network approach showed that there were three centroids or spreading centers and were completely separated, especially the crab population of Bintan (Figure 5a and 5b).Meanwhile, abdominal measurements showed that there were three centroids or spreading centers, but not completely separated.Abdominal morphometric characters of crabs in Bintan, Bangka, and Ketapang locations were overlap each other (Figure 5c and 5d).This shows the high similarity on majority of abdomen characters among locations.

Discussion
The swimming crab at three different locations in FMA 711 is assumed to have dissimilar morphometric characters.Based on the result of analysis, at least 28 characters on the male carapace are also differentiator in female carapace crabs, whiles 11 characters on male abdomen also differentiator in female abdomen crabs (Table 3).The dissimilar morphometric characters of the crabs are phenotypic variations [11,12].Crab populations in certain area or habitat have specific distinctive with distinguish variation in degree of growth related to environmental quality and their adjustment to condition [13].Environmental qualities such as temperature and salinity have important influences in crab activity, movement, and distribution.Differences growth rates are also affected by water temperature, salinity, sex, food, and reproductive processes [14].Changes in morphological appearance into a form of body shape modification of each population to their habitat condition, while the distinction in morphology of an organism in population generally take place related to differences in the environment and geographical location [13,15].
The crab population at three locations within FMA 711 can be grouped based on morphological characteristics and can be observes from the degree of closeness between locations which estimated using cluster analysis in the dendrogram pattern.Dendrogram cuts are seen based on the furthest distance level [16].The Euclidean distance can also determine the degree of population sameness, where the smaller the Euclidean gap, the more similar the crab population is in the group [12].The male and female crab populations viewed from the carapace and abdomen using the TNA have similarities, so the crabs in the Bangka and Ketapang locations have a closer relationship than the crab populations in the Bintan location (Figure 4).This condition may be due to similar ecological impacts [17] or passive migration during larval stage due to current factor between connected sites [18], since Bangka and Ketapang are relative close ecological relation affected by west and east monsoon compare to Bintan.West monsoon flew from north west Bangka Island to the south east are, including to Karimata strait, Ketapang and Java Sea, while east monsoon is opposite direction [19].might bring larval stage of crab from and to both areas in the form of passive migration.Although Bangka and Bintan has close geographical area compare to Bangka and Ketapang, physical barrier between Bangka and Bintan (Lingga and Singkep islands) might obstruct crab larval passive migration from and to both areas.
Population groupings can be affected by similar aquatic environmental conditions between locations [20] and proximity of geographic locations [21] or the sameness of environmental quality between locations [22].Environmental quality is the main limiting factor cause of morphological characters' diversity at the intra-species level [23].The differences in population groups were suspected from differences in environmental conditions affected by their adaptability [15].
Classification of morphometric characters of several populations among observed location was determined by discriminant analysis.Result of analysis display that morphological character analyzed was create pattern and remarkable by the location of the centroid [24].The existence of contact between populations shows the proximity of groups among population [25].This is presumed to be the similarity of certain characteristics between locations [26].The distribution of crab morphometric characters using the TNA approach at each location display different centroid midpoint among observed locations in the waters of FMA 711 and the carapace characters were completely separated, while the abdomen characters was not completely separated and overlapped each other.Based on such condition, the crab of Bintan population tend to be a sub-population in FMA 711.Meanwhile, the overlap between the morphological characters of more than one populations shows a great morphological and body shape similarities [20].Even so, the differences in morphological characters obtained at this time as a form of phenotypic changes in different environments may have been followed by genetic changes or even have not yet occurred significantly.
Cluster analysis results are in line with the results of the discriminant analysis which shows that the BSC population from Bangka has great sameness to the population from Ketapang, while the population from Bintan appears to be separate from other populations.Differences in environmental conditions including exploitation pressures lead to different consequences of the crab adaptations.Based on this condition, blue swimming crab from Bintan waters seem to be separate sub-population or stock unit, so that it becomes a consideration in its management at FMA 711.

Conclusions
The BSC in three locations of FMA 711 showed differences in some morphometric characters for both carapace and abdomen crabs.The crabs in the waters of Bangka and Ketapang consist of the same group and separated from the waters of Bintan based on the differences in the crab's morphometric characteristics of the carapace and abdomen using TNA method.The crab in FMA 711 has the potential for more than one stock unit and require differed management option.

Figure 4 .
Figure 4. Dendrogram of cluster analysis results on the population of crab (P.pelagicus) established on morphometric characters using the TNA method in: (a) male's carapace, (b) female's carapace, (c) male's abdomen, and (d) female's abdomen

Figure 5 .
Figure 5. Morphometric distribution plot and grouping of the crab (P.pelagicus) population established on discriminant analysis using the TNA method in: (a) male's carapace, (b) female's carapace, (c) male's abdomen, and (d) female's abdomen

Table 2 .
Landmark All of obtained morphometric or body shape characters data were converted using adjustment formula by Ihsan et al. 1981 in Marini [8]cing characters on the abdomen crab (P.pelagicus) using Truss Network Analysis (TNA) method (modified from Bhosale[8])

Table 3 .
Carapace and abdomen morphometric characters of crab (P.pelagicus) were different by TNA method