Vol. 40: 369–373, 2019 ENDANGERED SPECIES RESEARCH Published December 19 https://doi.org/10.3354/esr01003 Endang Species Res OPENPEN ACCESSCCESS NOTE Conserving the understudied invertebrates: a call for a systematic monitoring protocol for Asian horseshoe crabs in nursery habitats Chun-Chieh Wang1,2,3, Shiang-Lin Huang2, Xueping Wang4, Peng Xu4, Xing Huang4, Yongyan Liao3,4, Xiaoyong Xie5, Kit Yue Kwan3,4,* 1Marine Biology Institute, College of Science, Shantou University, Shantou, Guangdong, 515063, PR China 2Guangxi Beibu Gulf Marine Research Center, Guangxi Academy of Sciences, Nanning, Guangxi, 530000, PR China 3Guangxi Key Laboratory of Beibu Gulf Biodiversity Conservation, Beibu Gulf University, Qinzhou, Guangxi, 535011, PR China 4College of Marine Sciences, Beibu Gulf University, Qinzhou, Guangxi, 535011, PR China 5Key Laboratory of South China Sea Fishery Resources Exploitation & Utilization, Ministry of Agriculture of China, South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences; Guangdong Provincial Key Laboratory of Fishery Ecology and Environment, Guangzhou, Guangdong, 510300, PR China ABSTRACT: Baseline information on population size, trends and spatial distribution are important for population status assessments of threatened taxa. The 3 species of Asian horseshoe crabs (Tachy pleus tridentatus, T. gigas, Carcinoscorpius rotundicauda) are declining throughout their geo graphic ranges, and identifying their essential nursery habitats and monitoring juvenile popu- lation changes is necessary for conservation management. We reviewed juvenile population stud- ies published during 2001−2019 to evaluate which sampling methods were commonly adopted and how baseline data were presented. The applied methods included random or systematic quadrat (35%), random visual search (41%) and belt transect (47%). Juvenile population sampling and data reporting were generally insufficient for assessing population status and clarifying nursery habitat configuration. We propose a conceptual framework and minimum standards for collecting and reporting juvenile population and nursery habitat baseline information. We also outline the impor- tance of showing population density, rather than abundance, and spatial distribution in relation to habitat seascape or landscape satellite images. The framework and recommendations will improve the feasibility of robust, long-term monitoring of juvenile Asian horseshoe crab population status and nursery habitat, thus serving regional conservation management purposes. KEY WORDS: Baseline · IUCN Red List · Assessment · Carcinoscorpius rotundicauda · Tachypleus gigas · Tachypleus tridentatus 1. INTRODUCTION rig et al. 2019). Baseline information, including an estimate of population size, growth and extent of A robust monitoring program on distribution and occupancy, is necessary for population status assess- population size of taxa is essential not only to ecolog- ment following IUCN Red List criteria (Gärdenfors et ical studies but also to the design and evaluation of al. 2001). To collect relevant baseline data for evalu- effective conservation practices (Conway 2011, Per- ating population status, however, can be challenging © The authors 2019. Open Access under Creative Commons by *Corresponding author: [email protected] Attribution Licence. Use, distribution and reproduction are un - restricted. Authors and original publication must be credited. Publisher: Inter-Research · www.int-res.com 370 Endang Species Res 40: 369–373, 2019 for widely distributed marine species, due to the Supplement at www.int-res.com/articles/suppl/ n040 information gap across the geographic range, and p369_supp.pdf). The retrieved data were screened the incompatability of sampling methods. The lack of manually to remove duplicate references. Publica- baseline data can impede the progress of effective tions in which horseshoe crabs were sampled, but conservation and management planning. which were not specifically designed as juvenile The 3 Asian horseshoe crab species, i.e. tri-spine population studies, were excluded. We applied 20 Tachypleus tridentatus, mangrove Carcinoscorpius detailed criteria in 3 categories (Table 1; Tables S2− rotundicauda and coastal horseshoe crab T. gigas, S4), including population baselines, nursery habitat are distributed widely across the subtropical and baselines and sampling conditions, to assess whether tropical waters from East Asia, including the Seto the population studies contained all important infor- Inland Sea of Japan and the southeastern coast of mation required for population status assessment China, through Southeast Asia to the Bay of Bengal and nursery habitat configuration. (John et al. 2018, Liao et al. 2019a). Natural popula- tion overexploitation and shoreline reclamation have been identified as the main threats driving apparent 3. RESULTS population declines (Hsieh & Chen 2015, John et al. 2018, Fu et al. 2019, Liao et al. 2019b). While C. We identified 17 published articles or book chap- rotundicauda and T. gigas are categorised as Data ters that documented the juvenile population size, Deficient on the IUCN Red List; T. tridentatus has natural growth and nursery habitat conditions. The been listed as Endangered (Laurie et al. 2019). Direct number of studies that used the 3 different sampling species protection and indirect habitat protection methods, i.e. random/systematic quadrat (35%), ran- measures at specific sites for Asian horseshoe crabs dom visual search (41%) and belt transect (47%), are limited to a few countries or regions. Long-term was similar (Table 1). Nearly half of the studies (47%) population monitoring and habitat baseline collec- sampled within <5% of the total suitable habitat area tion are thus essential to inform scientifically sound or did not justify whether the sampling area was suf- conservation management. ficient to represent the entire habitat (Table 1). Issues In this Note, we summarise the sampling methods regarding the comparability of studies using differ- and data presentation of juvenile Asian horseshoe ent methods (29%) and the presentation of data in crab population studies published during 2001−2019. terms of abundance instead of density (12%) were Due to the differences in habitats used by horseshoe also noted. Most studies included sampling condition crabs throughout their ontogeny, we did not attempt information such as survey period (month) and sea- to extrapolate the findings to feeding and spawning son, number of transects and quadrats, area of habitats of adults. We tabulate common pitfalls of quadrat, as well as length and distance between sampling and data reporting that may hinder scien- transects. However, information which informs sur- tific information transmission, and propose a concep- vey efforts, such as sampling hour, number of survey- tual framework and minimum standards to guide ors and sampled shore length, was seldom included future population monitoring and collecting of base- (Table 1). line habitat data on nursery grounds. 4. DISCUSSION 2. MATERIALS AND METHODS The number of studies on quantifying juvenile pop- Databases and search engines, mainly Web of Sci- ulation and habitat baselines to serve conservation ence, Scopus, China National Knowledge Infrastruc- management purposes has increased in recent years. ture, Scholarly and Academic Information Navigator, However, important baselines and survey informa- Japan, and Google Scholar, were used to identify tion are often not provided to a sufficient degree, articles relevant to population studies and nursery thus limiting the practicality of long-term juvenile habitats of Asian horseshoe crabs. Seven key words population monitoring at local nursery habitats. Con- were used in combination: horseshoe crab, Tachy- sidering the patchy distributions and considerable pleus, Carcinoscorpius, juvenile, nursery, population, seasonality of juvenile horseshoe crabs on nursery and habitat. Reference searching using local names grounds, direct comparisons between datasets col- of horseshoe crabs was also conducted using differ- lected from different locations and times may result ent languages to include more studies (TableS1inthe in spurious conclusions and flawed management Wang et al.: Systematic monitoring of juvenile Asian horseshoe crabs 371 Table 1. Survey methods for populations of juvenile Asian horseshoe crabs, and baseline data, based on the cur- the accompanying issues and deficiencies in reporting population and habitat rent review of existing literature baseline data published between 2001 and 2019. Total Percentage Random visual searching with - count in a predefined and representa- Issues in reporting population baselines 17 tive area of nursery habitat is Data expressed as abundance (no. of individuals)a 2 11.8 recommended for the collection Sampling area <5% of the entire habitat/cannot be justified 8 47.1 of juvenile horseshoe crab oc - Comparing studies using different sampling methods 5 29.4 Comparing studies with different life stages (juvenile vs. adult) 1 5.9 currence data across wider geo- Polling data from different sampling methods 1 5.9 graphic ranges (Weng et al. Deficiencies in reporting sampling methods 2012). To sample juvenile popu- Random visual search 7 lations in local nursery habitats, Nursery habitat baselines systematic surveys with tran- Sampling location/point coordinates 2 28.6 Total suitable habitat area 3 42.9 sects and quadrats are encour- Sampling period/month/season 0 0.0 aged (Hu et