Morphology

Close-up

The European pond turtle is a medium-sized turtle, and its straight carapace length varies quite a bit across its geographic range, from 12 to 38 cm (4.7 to 15.0 in). The carapace is dark brown to blackish, with a hint of green. The head and legs are spotted with yellow. The plastron is yellowish.

An important factor that affects the development of E. orbicularis is temperature and thermal conditions. It has been reported that differential growth rates of the same species occur, including variation of body size and clutch size, because of varying temperatures in certain areas.^[12] Due to evident patterns of sexual dimorphism, E. orbicularis adult males are always found to be smaller than females. In males, smaller plastra offer them a wider mobility compared to females. In females, due to their differential diet and foraging habits, there may be a correlation to an adaptive effect on their skull and head morphology.^[12]

Diet

E. orbicularis eats a mixed diet of plants and animal matter that can increase the efficiency of its digestive process.^[11] It has been reported that an adult's diet starts from a carnivorous diet and progresses to a more herbivorous diet as it ages and grows in size. This is similar to other omnivore emydid turtles.^[11] As E. orbicularis grows in age and becomes an adult, the amount of plant material consumption increases during the post breeding period. E. orbicularis may prefer less energetic food after the breeding season, a period of time where most of its energy is spent to recover from reproduction.^[11]

Nesting

Nest with eggs

Most freshwater turtles lay their eggs on land, typically near a water source, but some species of Emys have been found to lay their eggs no less than 150 m (490 ft) from water.^[3] The search for nesting areas, by adult females, can last several hours to several days. Once an appropriate site is found, females take their time with the construction of the nest, painstakingly excavating a small pit out of the soft substrate purely by usage of her small forelimbs. Once satisfied with the depth of the nest, she will turn around (facing away from the nest) and proceed with egg-laying, gently dropping the eggs down and into a small pile. This process varies in duration; laying can take merely half an hour or upwards of several hours, depending on weather, interference by other animals, humans, etc. When laying is complete (and still facing away from the nest), the female turtle will use her back limbs this time, to cover and close the nest. This is another variable routine which can take up to another four hours.^[13]

Nest fidelity is a characteristic that is unique to female European pond turtles—selecting a nesting site based on its ecological characteristics—and then returning there for future laying, so long as the site has not changed.^[3] E. orbicularis females tend to look to a new nesting site if there are visible changes to the original nest’s surroundings, or because of dietary and metabolic changes. If an E. orbicularis female must change from nest to nest, she will typically select a site in relatively close proximity.^[3] In addition, females may also lay eggs in an abandoned nesting site if the conditions are an improvement, and deemed to be better suited for egg survival. If the environmental conditions of a nesting site change, this may influence the development of the eggs, the survival of the hatchlings and/or their sex ratio. Due to unforeseen ecological changes, such as thick vegetation growing over a season (and blocking sun to the nest), a nest site may become inadequate for incubating eggs. Females that do not exhibit nesting fidelity, and continue to lay in the same area for long periods of time—even with the ecological changes—may end up producing more male offspring, as the cooler and darker conditions promote more males developing.^[3] Since the sex of these turtles is temperature-dependent, a change in temperature may produce a larger number of males or females which may upset the sex ratio.^[3]

Mortality

Climate has an effect on the survival of E. orbicularis hatchlings. Hatchlings are only able to survive under favorable weather conditions, but due to regular annual clutch sizes and long lifespan, E. orbicularis adults, along with many freshwater turtles, balance out loss of hatchlings due to climate.^[13]

The species E. orbicularis has become rare in most countries even though it is widely distributed in Europe. The building of roads and driving of cars through natural habitats is a possible factor that threatens the populations of the European pond turtle. Road networks and traffic often carry complex ecological effects to animal populations such as fragmenting natural habitats and creating barriers for animal movement. Mortality on the road is most likely due to females selecting nests near roads which places a potential danger for the hatchlings as well. Hatchlings that wander too closely to roads are more likely to be killed and put the future population in danger. Although the possibility of roads being a major causation for the mortality of E. orbicularis is a rare phenomenon, long-term monitoring is necessary.^[13]

Introduced exotic species such as Trachemys scripta scripta and T. s. elegans, known commonly as Florida turtles, also put in danger the native Emys species in many parts of Spain (and possibly in other parts of southern Europe), since these exotic turtles are bigger and heavier than the native pond turtles.^[14][15] The usual life span of E. orbicularis is 40–60 years. It can live over 100 years, but such longevity is rare.

Parasites

E. orbicularis hosts several species of parasites, including Haemogregarina stepanovi, monogeneans of the genus Polystomoides, vascular trematodes of the genus Spirhapalum, and many nematode species.