American Horseshoe Crabs, Limulus polyphemus, in Mexico: Open Possibilities Jaime Zaldı´ var-Rae, Rene´ Elı´ as Sapie´ n-Silva, Martha Rosales-Raya, and H. Jane Brockmann Abstract Little is known about Mexican Limulus polyphemus, the southern- most population of the species. We present an overview of work on Mexican horseshoe crabs, their situation, and perceived threats and opportunities regarding the conservation of the species. Horseshoe crabs occur along the western, northern, and eastern coasts of the Yucata´ n peninsula, and are geneti- cally distinct from populations in the United States. Spawning aggregations and nests are found continuously throughout the year, commonly in protected lagoons where mangrove (Rhizophora mangle, Laguncularia racemosa, Avicen- nia germinans, and Conocarpus erectus) and sea grass (Thalassia testudinum) communities proliferate. Populations are thought to be dwindling since the 1960s and Limulus is listed as ‘‘in danger of extinction’’ in Mexican legislation since 1994. The most important localities are within protected areas. Direct exploitation is not an important threat, but coastline modification (especially of mangrove areas and coastal lagoons) for housing and tourism is a major concern. Additional potential threats are the oil industry and shrimp fishery in the southern Gulf of Mexico, but their effects on horseshoe crab populations have not been assessed. 1 Introduction Knowledge about the biology and ecology of Mexican Limulus polyphemus, the southernmost population of the species, is scant (Escalante et al. 1980; Gomez-´ Aguirre 1980; Bonilla-Gonza´ lez et al. 1986; Anderson and Shuster 2003). This chapter aims at providing an overview of the available information on the biology of horseshoe crabs in Mexico and our perspective on potential threats and opportunities for the study and conservation of these animals in Mexico. J. Zaldı´ var-Rae (*) Laboratorio de Conducta Animal, Instituto de Ecologı´ a, Universidad Nacional Autonoma´ de Me´ xico, AP 70-275, CP 04510, Coyoaca´ n, Me´ xico, DF, Me´ xico e-mail: [email protected] J.T. Tanacredi et al. (eds.), Biology and Conservation of Horseshoe Crabs, 97 DOI 10.1007/978-0-387-89959-6_6, Ó Springer ScienceþBusiness Media, LLC 2009 [email protected] 98 J. Zaldı´ var-Rae et al. 1.1 The Study of Limulus polyphemus in Mexico: A Historical Account The first description of horseshoe crabs in the Western world was provided by Friar Diego De Landa (1566; translated by Tozzer 1941) in his ‘‘Account of the Things of Yucata´ n.’’ Although he classified it as a fish, he clearly described the round shell, slender tail, and many legs of the horseshoe crab and used its Mayan name ‘‘Mex.’’ He also stated that the only edible part of the animal was its eggs, which were eaten by the Mayans. The next written accounts of Mexican horseshoe crabs date from the late 19th century. In them, Milne- Edwards (1879) and Ives (1891) mentioned that horseshoe crabs were abundant along the southern west coast of the Yucata´ n Peninsula in the Laguna de Te´ rminos, Campeche, and along the northern coast of Yucata´ n at Dzilam. Their presence along the coasts of the Yucata´ n Peninsula was later confirmed by Zarur-Menez (1961), Ramı´ rez et al. (1963), Gomez-Aguirre´ (1979), and Bonilla-Gonza´ lez et al. (1986). Occasional sightings in Veracruz, on the western portion of the Gulf of Mexico, were reported by Cha´ vez and Mun˜ oz-Padilla (1975). Gonza´ lez-Guzma´ n et al. (1967), at the National Autonomous University of Mexico (UNAM), described the cytology of hemolymph collected from horseshoe crabs of Laguna de Te´ rminos, Campeche, and correctly inferred a role for amebocyte granules in the coagulation of hemolymph. Gonza´ lez-Guzma´ n et al. (1967) got their specimens from Dr. Samuel Gomez-Aguirre´ of UNAM’s Institute of Biology, who had begun his pioneer- ing work with populations in the southern Gulf of Mexico in 1964. Dr. Gomez’s´ interest in L. polyphemus spanned 40 years until his passing in 2006, and his work included the first calls for attention to a decline in Mexican horseshoe crab populations (Gomez-Aguirre´ 1979, 1980, 1983, 1985, 1993, 2002). Dr. Gomez´ also led several groups of Mexican biology students in the first morphometric studies of the species and quantified exuviae, dead, and live individuals along the coasts of the Yucata´ n Peninsula (Bonilla-Gonza´ lez et al., 1986; Barba- Macı´ as et al., 1988; A´ lvarez-An˜ orve et al., 1989; Gomez-Aguirre´ 1993; Gomez-Aguirre´ and Ya´ n˜ ez-Martı´ nez 1995). He also provided the first descrip- tions on particular aspects of the biology of Mexican horseshoe crabs (e.g. Gomez-Aguirre´ and Flores-Mora´ n 1987; Gomez-Aguirre´ et al., 1992). 2 Distribution and Habitat Mexican populations of L. polyphemus are restricted to the coasts of the Yucata´ n Peninsula, between about 188N and 218N (Cha´ vez and Mun˜ oz-Padilla 1975; Gomez-Aguirre´ 1979; Va´ zquez-Garcı´ a and Villalobos-Figueroa 1980; Bonilla-Gonza´ lez et al., 1986), with rare sightings in Veracruz (Cha´ vez and Mun˜ oz-Padilla 1975; Fig. 1). In 1985, Dr. Gomez-Aguirre´ and his students found a population on the northern shore of Bahı´ a de la Ascension´ in Quintana [email protected] American Horseshoe Crabs, Limulus polyphemus, in Mexico 99 Fig. 1 Distribution of American horseshoe crabs, Limulus polyphemus, in Mexico. Black and gray circles represent localities where mating aggregations have been recorded and localities where live or dead individuals or exuviae have been observed, respectively. Localities: LT=Laguna de Te´ rminos; 1= Isla Aguada, Cabrera and Isla Pa´ jaros; 2=Icahao and Champoton;´ 3=City of Campeche; 4=Isla Arena and Celestu´ n; 5=Progreso and Yucalpete´ n; 6=Dzilam de Bravo; 7=San Felipe and Rı´ o Lagartos; 8=Holbox; 9=Cancu´ n; 10=Bahı´ade la Ascension´ Roo, on the Caribbean coast of the Peninsula (Bonilla-Gonza´ lez et al., 1986), confirming an anecdotal account by Shuster (1979). This observation contra- dicts the speculation that the strong northward flowing Yucata´ n Current and the very narrow continental shelf on the Caribbean coast might prevent south- ward migrations from northern localities (Anderson and Shuster 2003). Most Mexican localities for horseshoe crabs are within coastal lagoons. Although rivers are scarce in the Yucata´ n Peninsula (Fig. 1), large volumes of underground fresh water flow into these coastal lagoons and create estuarine systems. Red, black, and white mangrove and buttonwood (Rhizophora mangle, Laguncularia racemosa, Avicennia germinans, and Conocarpus erectus, respec- tively) and vast seagrass beds (predominantly Thalassia testudinum) are com- mon in these coastal lagoons and estuaries, and L. polyphemus are commonly associated with these communities (Gomez-Aguirre´ 1979; Bonilla-Gonza´ lez et al., 1986; Barba-Macı´ as et al., 1988). Indeed, the presence of horseshoe crabs on the Caribbean coast of the Peninsula may be explained by coastal lagoons providing suitable habitats without exposure to strong currents and with the sheltering effect of the Mesoamerican Barrier Reef. These shallow water areas also provide valuable nursery grounds for juveniles, which require many years to mature. [email protected] 100 J. Zaldı´ var-Rae et al. 3 Morphometry Following the pattern for the species, in Mexico male horseshoe crabs are significantly smaller than females, with inter-sexual size ratios of 0.73–0.80 (based on intergenal distances; RE Sapie´ n-Silva, unpublished data), similar to those observed in the United States (0.75–0.79 based on prosoma widths; Shuster 1979). There are significant differences in size between populations along the coasts of the peninsula: animals from Champoton´ are significantly larger than those from Laguna de Te´ rminos, while the latter are significantly larger than those from the area near Progreso, Rı´ o Lagartos, and Holbox (see Table 1). Differences between populations do not follow a latitudinal pattern (RE Sapie´ n-Silva, unpublished data) as has been described for U.S. populations (Shuster 1979; Riska 1981). Horseshoe crabs from Champoton´ are similar in size to those from Chesapeake Bay measured from Shuster (1979), while those from Rı´ o Lagartos are smaller than those from the Gulf coast of Florida (RE Sapie´ n-Silva, unpublished data). A similar pattern was detected by one of us (HJ Brockmann, unpublished data) upon comparing horseshoe crabs from San Felipe-Rı´ o Lagartos, western Florida, and Delaware Bay (Table 1). If size has a genetic basis, the size differences between Mexican populations may indicate limited gene flow among them. 4 Population Genetics There seems to be no gene flow between the Yucata´ n populations and those in Florida (Anderson and Shuster 2003). This is supported by the findings from King et al. (2005) that the population at San Felipe-Rı´ o Lagartos (Fig. 1) on the northeastern coast of Yucata´ n is highly genetically differentiated from popula- tions in the United States. The San Felipe-Rı´ o Lagartos population also showed low genetic diversity and heterozygosity, suggesting that geographic isolation, small population size, and inbreeding may have spurred stochastic processes leading to a loss of diversity. King et al. (2005) suggested that the marked genetic differences between the Mexican and United States populations may warrant a taxonomic revision of the genus. An assessment of the genetic structure of Mexican populations has not yet been conducted. 5 Reproduction In Mexico horseshoe crabs are known to breed from