Abnormal record of coastal horseshoe crab (Tachypleus gigas Müller, 1785) in Subang, West Java, Indonesia

The xiphosurids are a typical chelicerae group with massive morphological, anatomical, biochemical, and ecological documentation. Despite this study, information about extant horseshoe crab abnormalities is underexplored, especially in the Indonesian region. Thus, this study aims to investigate the abnormal horseshoe crab in Subang, West Java. This research was conducted from January until February 2020 in Legon Wetan, Subang, West Java. Here, we only documented abnormalities of extant xiphosurids namely Tachypleus gigas. During the study, we greatly found extant abnormal xiphosurids by identifying 45 specimens with a wide range of abnormalities on the cephalothorax (prosoma), thoracetron (opisthosoma), telson, and appendages. The abnormal coastal horseshoe crab found also included adults (male and female) and juveniles ranging from 11.6 cm to 21 cm of carapace width. We note that the most common abnormal body parts are the cephalothorax and the thoracetron. Although the original cause may be unknown, the three main causes of the abnormality-inducing which are injuries such as self-injuries or fishing net injuries, physiologic developmental abnormalities or teratologies, and pathologies.

Previous research for horseshoe crab abnormalities explained that most of them were located in the telson area which has been caused by teralogies [24,25].In addition, some other abnormalities also located on the opisthosomal part and prosomal section have been recorded by injuries to both male and female crabs that might be incurred by molting, mating, digging, or other activities [24,26].The term abnormality illustrates a misshapen or an unusual growth [2,27].In brief, studying horseshoe crab abnormalities has been underexplored despite massive observations into the group.
Abnormalities cases in Indonesian horseshoe crab has been reported yet.Following Bicknell and Pates [27] described the malformed of all extant horseshoe crabs including Asian and American crabs using specimens from the museum.In doing so, we present several examples of abnormalities (including a rare abnormal appendage) and the first documented examples of abnormalities in T. gigas in Indonesia.Thus, this study aims to describe the abnormalities in T. gigas in Subang, West Java, Indonesia.

Methods
A total of 151 coastal horseshoe crabs entangled in fishing nets were obtained.All the samples were part of the by-catch from the blue swimming crabs fisher.These were obtained during the fieldwork observation in January and February 2020 in Subang district, West Java, Indonesia (Figure 1).A total of 45 individuals were malformed and they separated into separate groups for further observation.Moreover, all types of horseshoe crabs which have abnormalities sections were photographed with a camera.

Results and Discussion
A total of 151 coastal horseshoe crabs (Tachypleus gigas) confirmed as by-catch from blue swimming crab fisher were assessed.Approximately 30% of samples have abnormalities featured both adult and juvenile T. gigas were documented (Table 1).The mean of entangled female carapace width by fishers and experiencing abnormalities is 17.76±3.28cm, for males is 15.88±0.71cm, while for the juvenile is 13.33±1.62cm.The number of individuals with abnormalities for T. gigas was more in males than females and juveniles an amount of 21 individuals.The mean weight of abnormalities entangled T. gigas in this study reached 559.00±298.75 g for females, 288.05±56.46g for males, and 202.21±90.46g for juveniles.Coastal horseshoe crab in Subang has the most diverse range of deformities in the sample collection.Previous research [25,27] observed that telson abnormalities were the most frequently documented horseshoe crab abnormalities.However, in this study, most abnormalities were spotted in cephalothoracic and thoracetronic with small abnormalities while extreme abnormalities have been also located in several individuals.For example, type of malformation featured an abnormal right posterior cephalothoracic with an outside V-shaped producing a sharply buckled border (Fig. 2A).The second abnormal example shows the left thoracentronic section forming a V-shapes with a bulge that develops on the slightly elongated dorsal edge and the right movable spine does not fully develop (Fig. 2B).The third example has the impression of forming an inside V-shaped letter on the left anterior cephalothorax which changes the shape of the surrounding exoskeleton (Fig. 2C, F).The fourth example shows Y-shape on the right cephalothorax creating a sharply forked edge (Fig. 2D).The cephalothoracic border has a small U-shaped embayment on the left anterior (Fig. 2D) and has a pointed end to the shape (Fig. 2D).The fifth example has damaged appendages spotted on the first and second walking leg in pretarsus, tarsus and tibia sections are the most documented appendage abnormalities (Fig. 2E).Damaged and amputated walking legs have also been noted (Fig. 2E).Other pieces of evidence also had extremely abnormalities shape (Fig. 2G), anterior and posterior right cephalothoracic has been eroded and also on the right thoracetronic including movable spines.Moreover, there was also a recorded of the hole in the right section anterior cephalothoracic.Another hole was also documented in the left section anterior cephalothoracic in the dorsal margin (Fig 2H ) for other individuals.Additionally, two other individuals have been noticed with U-shapes that were spotted in the posterior right section (Fig. 2I) and anterior right section (Fig. 2J) of cephalothoracic.
Abnormalities in horseshoe crabs could be caused by several things including injuries, teritologies and pathologies (Bicknell et al. 2018).In this study, most of the abnormalities that occurred were assumed to be due to injuries originating from mechanical damage [27], especially injuries with V (Fig. 2A, B, C, F), and  Y-shapes (Fig 2D).Mechanical damage that occurred was mostly caused by predation, burrowing, and also the mating process.However, the injuries that occur with U-shaped final results are due to the molting process that occurs at each individual stage or instar.Injuries with a V and Y shape may indicate injuries resulting from tearing of the softer exoskeleton after molting, or injuries that have undergone substantial recovery, or both [5,27].Here only one individual sample showed evidence of recovery (Fig. 2E), under these conditions it was suspected that there was damage to the appendages.The visible abnormality revealed the possibility of a regeneration or selection process in that part with several molting times [27].There are  The existence of horseshoe crabs globally faces a serious population threat.Habitat changes, reclamation, anthropogenic, consumption, and excessive utilization of blood lead to drastic population decline [28,29,30].In this context, in Subang, most of the coastal horseshoe crab threats consist of anthropogenic activities, including sand mining and by-catch.Refers to our previous research (unpublished data), sand mining in Subang was carried out around the spawning area of coastal horseshoe crabs.On either side, domestic tourists exploit the spawning area as a tourism area.On the other hand, coastal horseshoe crabs in Subang mostly become by-catch that is unwanted by fishers because it tends to damage their fishing net.Most of the fishers forcibly removed the horseshoe crabs from the net, so some parts of the crab's bodies were amputated.Supporting the horseshoe crab conservation efforts in Subang, documentation of carapace conditions and abnormalities of the crabs may become one of the significant things for researchers to identify possible spawning areas that require more conservation attention.Thus, it can be seen which areas with extreme severity of abnormalities or the frequency of occurrence of abnormalities are most commonly found.Identification of abnormalities is not only carried out in adult individuals but also abnormalities in eggs (the presence of bacteria in the vicinity), embryos, and juveniles.This effort is made to maintain horseshoe crabs as an animal that has been able to survive the phenomenon of mass extinction.

Conclusion
The most common abnormal body part of Tachypleus gigas in Subang parts are the cephalothoracic and the thoracentronic areas.Although the original cause may be unknown, the most likely cause of the abnormalities particularly in V, Y, and U shapes are several types of injuries such as self-injuries, fishing net injuries, or matting process injuries.
Field experiments were permitted by the Animal Ethics Commission from IPB University No. 205-2021IPB and The Directorate General of Nature Resources and Ecosystem Conservation, Ministry of Environment and Forestry No. SK.115/KSDAE/SET.3/KSA.2/6/2021.

Figure 1 .
Figure 1.Sampling location in Subang, West Java, Indonesia Subang, most of the fishers could be forcibly removed the horseshoe crab from the fishing net resulting in various of injuries to the crabs.

Figure 2 .
Figure 2. Several types of abnormalities on Tacypleus gigas in Subang, West Java, Indonesia;