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Alacrán is an emergent platform type reef that forms part of a group known as the Campeche Bank Reefs, because they are located along the outer shelf of the Campeche Bank. It is the largest reef in the entire southern Gulf of Mexico, as well as the most northerly in location. It has five vegetated islands associated with it, Isla Desterrada, Desertora, Pérez, Chica and Pájaros, all of which have been the subject of several scientific studies that have focused on the dynamic conditions of vegetation changes along with changes in shape through time.
Since 1900, when the lighthouse was erected on Pérez, the island has been host to permanent residents, there are three buildings that host four families.
Alacrán faces threats from activities like fishing, tourism and contamination. The latter, is primarily caused by boat washing on open waters where toxin detergents are used and by oil exploitation.
Location on Continental Shelf: Outer-Shelf Bank
Coordinates: 22.49° N 89.70° W
Nearest Largest City: Merida, Yucatán, Mexico
Area of Coverage: 3,337 km2
Width: 13.0 km
Length: 25.0 km
Maximum Depth: 36 m
On Isla Pérez the main vegetation is composed of Suriana, which has taken the place of the species Sesuvium and Sporolobus that existed in 1865. Suriana covers the island With exception of a thin patch along the coast where Opuntia dillenii forms three circular patterns, Suriana covers the entire island. It is important to mention the presence of Coccoloba and Cordia, located around the permanent human dwellers. Other species include: Atriplex pentandra, Portulaca oleracea, Cakile edentula var alacranensis, Chamaesyce buxifolia and Tournefortia gnaphalodes.
Isla Desertora is covered by a low thicket of Chamaesyce buxifolia, which is accompanied by isolated examples of Cenchrus insularis, C. pauciflorus and scarce bushes of Tribulus alacranensis. Towards the East end, Desertora hosts a small patch of Sporolobus and Opuntia dillenii, which was introduced from Isla Pérez. Other species include: Cakile edentula var. alacranensis, Atriplex pentandra and Sesuvium portulacastrum.
The main vegetation of Isla Pájaros is composed of a meadow of Sporobolus that covers part of the island. The island also hosts a small forest of Avicennia nitida, which borders the salty lagoon located on the southern tip. Other species are Cenchrus insularis, Tribulus alacranensis, Portulaca oleracea, Atriplex pentandra, Conocarpus erectus and Cyperus planifolius.
A uniform low and semi-open thicket of Chamaesyce buxifolia covers Isla Chica, as well as small residual patches of Sesuvium. The marginal vegetation is represented by examples of Cakile and Portulaca, located along the beaches.
Isla Desterrada is formed by two sandy cays separated by a strait of 250 m. On both cays the main vegetation only occupies a central portion dominating Tournefortia gnaphalodes on open formations. Other species include:
Chamaesyce buxifolia, Portulaca oleracea, Cenchrus pauciflorus, C. insularis and Tribulus alacranensis.
Significant seabird nesting colonies exist on the islands of Alacrán Reef. Most are ground-nesters such as Masked Booby on Isla Pájaros and Desertora, Brown Booby on Isla Desterrada, Laughing Gull on Isla Chica, Desertora and Desterrada and Royal Tern on Isla Chica. Other ground-nesters such as Sandwich Tern are present on Isla Chica, and Sooty Tern on Isla Pérez and Desertora. Other species like Red-footed Booby on Isla Desertora, Magnificent Frigatebird on Isla Desterrada and Desertora and Brown Noddy on Isla Pérez prefer elevated vegetation. The Red-footed Booby and the Magnificent Frigatebird nest primarily in Tournefortia gnaphalodes. Brown Noddy apparently prefer Suriana maritima, but some nests are found in Casuarina equisetifolia. Brown Noddy are also known for utilizing molluscan shell fragments as part of their nests. On Isla Pérez, 90% of the shell fragments used were of the burrowing clam, Codakia orbicularis, a common inhabitant of the sea grass beds in the lagoon.
Trans-Gulf migrants primarily use the island for resting, but food and water, if available, can be important too. Midges are common on Isla Pérez, as well as house flies near human habitation and may offer food for the migrants. Other insects, noted on great numbers, apparently migrate at the same time as the birds, such as butterflies, dragonflies, lacewings, and grasshoppers. Freshwater is scarce on these semi-arid islands. Mirants are generally weak or tired an can often be easily approached without flying. The islands of Alacrán and other Campeche Bank islands are believed to be far more important for trans-gulf migrants than their counterparts in the Veracruz reef- islands, because of their greater distance from shore, 137 kms
Although little is known about the terrestrial fauna of the southern Gulf of Mexico reef islands, over 30 families of insects representing over 10 orders are known from the islands of Alacrán Reef, along with over 50 species of insects, and 12 families of spiders representing four orders.
Sea turtles occur around all of the southern Gulf of Mexico coral reefs, and nesting occurs on some islands like Desertora and Desterrada. Unfortunately, little information is available about nesting, but sea turtles are known to be exploited for their eggs and meat. The most likely species are the loggerhead and hawksbill, both of which are the most likely nesters, as well as the green turtle.
The shape of the islands of Alacrán have remained in essentially the same location and maintained a similar shape. A 1984 study showed however, that all but Isla Pérez had increased in size when compared to the previous studies, in an 82 year period.
Instituto Nacional de Ecologia (INE). 1996. Comisión Nacional para el Conocimiento y Uso de la Biodiversidad, 1996. URL: http://www.ine.gob.mx/ueajei/publicaciones/libros/2/alacranes.html?id_pub=2 (Accessed on Feb 24, 2004).
Tunnell, J.W., Jr. 2007. Reef Distribution (chapter 2). In Tunnell, J.W. Jr., Chavez, E.O. and Withers, K. (eds) Coral Reefs of the Southern Gulf of Mexico. Texas A&M University Press, College Station, TX, 216 p.
Instituto Nacional de Ecologia INE
Keywords: Reef, Emergent platform, Campeche bank reefs, Islands, Lighthouse, Permanent residents, Turtles, Birds, Insects
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