Monday, May 23, 2016

Marine Toad, Rhinella marina (Family Bufonidae)

The marine toad, locally known as crapaud, can be found throughout Trinidad and Tobago as it occurs in a variety of terrestrial habitats from urban areas, agricultural lands to forests. It may be considered the largest toad in the world with a maximum size of 238 mm SVL, with females larger than males. This species is known to feed on anything it can swallow. It can be distinguished from the Beebe's toad also found on Trinidad & Tobago by the position of its nostrils and its heavily webbed hind feet. It has toxic chemicals in its skin that can result in fatality if consumed.

Marine Toad, Cane Toad
Rhinella marina (Linnaeus, 1758)

Rana marina
Linnaeus, 1758, Syst. Nat., Ed. 10, 1: 211. Type(s): By indication including specimen illustrated in Seba, 1734, Locuplet. Rer. Nat. Thesaur. Descript. Icon. Exp. Univ. Phys. Hist., Type locality: "America"; restricted by Müller and Hellmich, 1936, Wissenschaft. Ergebn. Deutschen Gran Chaco Exped., Amph. Rept.: 14, to Surinam.

Bufo marinus — Schneider, 1799, Hist. Amph. Nat.: 219.

Chaunus marinus — Frost, et al. 2006, Bull. Am. Mus. Nat. Hist., 297: 364.

Rhinella marina — Chaparro, Pramuk, and Gluesenkamp, 2007, Herpetologica, 63: 211.

Distribution. Marine toads (Rhinella marina) occur naturally from southern Texas and Sonora and Tamaulipas Mexico southward to the Orinoco and Amazon River Basins of South America. They have been introduced over much of the tropical world to control agriculture pests to most tropical regions as a control for agricultural pests. They occur naturally in Trinidad and Tobago.

Marine toads, or crapauds as they are known in the islands, use shallow bodies of clear water with a high pH and minimal vegetation to reproduce. The breeding sites may be in shallow brackish waters in coastal areas or they may use pools in streams running through agricultural areas or forests. Man-made bodies of water such as concrete ponds, old foundations that collect water, abandoned swimming pools, or natural ponds in forests or forest edge situations.

The marine toad is so prevalent in Trinidad and Tobago that it has taken on a cultural role, albeit a superficial one. While Amerindians may have lived on the islands for thousands of years, Trinidad and Tobago were not discovered by the European world until Christopher Columbus' third voyage in 1498. And, early European colonization was slow. Trinidad's human population probably numbered in the hundreds until 1785, and Tobago's population was similarly small. Thus these islands have only been inhabited with substantial populations of Africans and Europeans for less than 300 years.
The toad's local name, "crapaud," is French for "toad," and in Speyside, Tobago there is a street named after this largest native amphibian. The marine toad has also become part of local culture and colloquialisms. Tobagonians wishing to silence a hostile witness in court will attempt to intimidate the witness by sewing a crapaud's mouth shut and placing it in the witness' house the night before the trial. Also, in predicting misfortune or offering a warning the phrase, "Crapaud smoke your pipe!" may be used (Niddrie, 1980). As we will see the crapaud has taken on a more complex role in other cultures.

The marine toad was one of the first (actually the eighth) amphibians to be described by science. Carl Linneaus, the Swedish founder of the current binomial nomenclatural system used by biologists, named and described the giant toad, "Rana marina" on page 211 of his first volume of Systema naturae... in 1758. Sixteen years later John Julius Walbaum proposed that Linnaeus' name for this toad was inappropriate. Walbaum (1784) (in Smith et al. 1977) wrote,

I seriously doubt that it usually lives in the ocean, because the structure of the feet is not such that it would be able to swim with them through the rough waves of the ocean. therefore I assume it to be a land frog which lives at the banks of rivers or shores of lakes. If this assumption is confirmed in the future, the Latin specific name applied by Linnaeus would have to be changed because it does not correctly characterize the species.

Wallbaum's proposal is not acceptable today because Article 18(a) of the International Code of Zoological Nomenclature makes it clear that a genus, or a species-group name, cannot be changed once it is established, even if it is inappropriate. But, a species may be moved from one genus to another. Linnaeus recognized only 10 genera of amphibians and reptiles in his 1758 work. Joseph Nicolaus Laurenti was the next person to review amphibian and reptile classification, and he established the genus Bufo in 1768. Today, Article 34(b) of the International Code requires that changes in species names must made so that they agree with the gender of the genus. Thus, Rana marina is feminine, while Bufo is masculine. When marinus was moved to Bufo, it therefore became Bufo marinus. When the species was re-assigned to the genus Rhinella, it again became feminine and today it is known as Rhinella marina.

There are competing species for the honor of the largest toad. However, the largest toad species appears to be South American, and the contest for the largest species is between Blomberg's toad, Rhaebo blombergi, and the marine toad. Blair (1972) and Duellman and Trueb (1986) considered Blomberg's toad of Colombia the largest toad species, the latter authors give its length as 250 mm. Reed and Borowsky (1970) reported five female marinus ranging from 208-230 mm, all are museum specimens, and all are from Suriname and Guyana. A newspaper article (Anon., 1988) reported the death of a marine toad at the Blank Park Zoo, in Des Moines, Iowa. This animal, named "Totally Awesome," or Toad A, weighed 5 pounds 1.5 ounces (2.3 kg), and was 9.5 inches (241.3 mm) in total length. It was accepted into the Guinness Book of World Records (McFarland, 1991) as the largest toad. The zoo obtained the toad from a Miami animal dealer in 1983, thus the toad's exact age was unknown. A similarly sized specimen was reported by Tyler (1994), the specimen is currently in the Queensland Museum, and it measured 24.1 cm, and weighed (presumably when it was alive) 1.36 kg. Other toad species in the marinus Group also attain respectable sizes, Cei (1980) reported the maximum size of the kurúrú guazú (also spelled cururú), Bufo paracnemis as 210 mm in length. North American's largest toad is the Colorado River toad, Incilius alvarius, Stebbins (1985) described I. alvarius reaching 190 mm in body length. Asia also has toads that reach impressive lengths. As Boring and Lui (1934) pointed out, most continents have at least one toad species that reaches a large size.

The habitat used by Rhinella marina may have an impact on the size they reach. The largest specimens of Rhinella marina on Trinidad appear to come from the Northern Range. While the breeding population in Nariva is much smaller. This may be the result of completion for breeding sites. Shallow bodies of water are few in the Northern Range while breeding sites in Nariva Swamp are exceptionally common.

Breeding activity in Trinidad and Tobago coincides with the wet season which may start in May and continue until December or January. However, much of the activity occurs in the early and middle wet season in June and July. Males will call well after breeding has ceased.

Eggs are deposited in both permanent and ephemeral situations, the crapaud tends to prefer bodies of water with abundant aquatic plants and a neutral pH. One a female has selected a mate, they may remain in amplexus for a long period (up to two weeks). Clutch sizes of 4,000 to 36,000 have been reported, with larger females laying more eggs. Experiments with desiccating eggs fond the larvae could survive 10 hours without water as long as the surface was moist. Tadpoles emerge 48–72 hours after deposition. Tadpoles usually metamorphose 27–46 (however a range of 10–180 days has been reported). This may result from temperature differences of the water.

Marine Toad tadpoles feed on algae and other aquatic detritus (Hinckley, 1962) and its availability affects the time to metamorphosis (Tyler, 1975).

Tadpoles prefer warmer water and are adapted to high water temperatures, and are often exposed with little or no vegetative cover beside or in the water body. Tadpoles remain active throughout their development and swim in large aggregations mid water column (Straughan 1966; Krakauer 1970). Tadpoles may be cannibalistic and feed on individuals that are younger and smaller than they are, and they apparently tend to feed just on other Marine Toad tads, and not other species of anurans (Crossland, 1998a).

New metamorphosed and post-metamorphic toads remain near water because they can easily desiccate (Straughan, 1966; Alford et al., 1995a). As toads get older and larger they move farther from water and they shift their daily activity from diurnal to nocturnal behavior. Juveniles in the 30–70 mm emerge at dusk and are active at night—they are frequently found under lights, feeding on the insects attracted to the light (Krakauer, 1968).

Adults are tolerant and even thrive in human modified environments. They occur in gardens, around houses, and in water tanks (Wright and Wright, 1949). Krakauer (1968, 1970) notes that marine toads are frequently found in disturbed areas and rarely encountered in undisturbed habitats. Marine toads are nocturnal and attracted to house and patio lights that also attract the insects on which toads feed (Wright and Wright, 1949). Toads are only active 1 out of every three to five nights (Brattstrom, 1962a; Zug and Zug, 1979; Floyd and Benbow, 1984), and their activity tends to be correlated with rain (Floyd and Benbow, 1984). During the day they hide under debris on the ground and in burrows (Wright and Wright, 1949), and under long grass clumps out of direct sunlight (Cohen and Williams, 1992).

As their name suggests, marine toads are generally found along rivers and coasts in association with fresh and/or brackish water, including mangrove swamps. In a study by Krakauer (1970), adult toads were found to survive in 10‰ sea water, but quickly died in 15‰ sea water. We have observed this toad emerging from the surf along turtle nesting beaches in Trinidad, probably individuals that got washed down stream during heavy rains and followed the coast for a short distance until they could return to land.

Home range size is variable, dependent to an extent on the size of their water bodies and feeding sites (Brattstrom, 1962a; Carpenter and Gillingham, 1987). Displaced animals will return to their capture site using local landscape and visual cues providing the key inputs for orientation (Brattstrom, 1962a). Mark and recapture studies In Queensland, Australia, and the average minimum home range was calculated at 340 m2 (Pearse, 1979). A similar study by Zug and Zug (1979) found that at least some toads were familiar with an area of 2,812 m2.

Marine toads do not defend territories during the reproductive season at the breeding site or in their foraging area (Sabath, 1980). Adult toads do display some fidelity to shelter sites and prefer shelters with high soil moisture, and often increase soil moisture by urinating on the soil (Alford et al., 1995a).

Toads are able to reproduce at body lengths of 66–220 mm, with males averaging about 13 mm shorter than females (Wright and Wright, 1949; Easteal, 1986).

Marine toads feed on a wide variety of terrestrial arthropods. However, they will consume native frogs and toads, dog food, and feces (Alexander, 1964; Tyler, 1975; Rossi, 1983; Bartlett and Bartlett, 1999a). Other prey include snakes (Rabor, 1952), birds (Krakauer, 1968), and mammals (Oliver, 1949).

Noxious and toxic chemicals in the skin and tissues at all points in the toad’s life cycle keep the possible predators few in number. At the tadpole stage, dragonfly naiads will readily consume marine toad tadpoles and eggs, as will dytiscid beetles, water scorpions (Lethocerus sp.), notonectids (Anisops sp.), leeches, tortoises, Macrobrachium spp., and crayfish (Cherax quadricarinatus; Crossland, 1992, 1993; Alford et al., 1995a). Native fishes have been found to ignore or taste and reject toad tadpoles (Alford et al., 1995a; Lawler and Hero, 1997). The most frequent predators of toad eggs and tadpoles, however, are older cohorts of marine toad tadpoles (Alford et al., 1995b).

Toads are most vulnerable to predation immediately following metamorphosis, while the development of terrestrial skin glands is occurring (Cohen and Alford, 1993). Although there are no studies on predators of newly metamorphosed toads, several animals have been observed to eat them, including adult marine toads, ants, centipedes, wolf spiders, small mammals, and some birds (Cohen and Alford, 1993).

Marine toads are highly poisonous and secrete a whitish, viscous compound from their parotoid glands (in Wright and Wright, 1949; Allen and Neill, 1956; Licht, 1967b; Easteal, 1986). The parotoid glands produce and store a mixture of bufotenine and epinephrine—steroid-like substances that are toxic to most animals (Chen and Osuch, 1969; Freeland, 1986). When confronted by a predator the toad goes into a head-down defensive position marine toads assume to present their parotoid glands to potential predators. These toads are known to approach potential predators and attempt to force contact with their parotoid glands.

The oldest fossil of marinus is known from a floodplain deposit in the late Miocene (5.3-11.2 MYA) of Colombia (Estes and Wasserzug, 1963). Estes and Wasserzug considered marinus the most primitive member of the species group, and suggested it gave rise to populations adapted for life in wet and dry environments on mainland South America. Bertini and Cei (1962) suggested Rhinella marina had an Amazonian origin, later Cei (1972) hypothesized the Guiana shield was marinus' point of origin.


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