Edible frog

Amphibia → Anura → Ranidae → Pelophylax kl. esculentus

Rana vërde, Gritta

The edible frog ( Pelophylax kl. esculentus ) is a unique case in the European fauna, as it is a fertile hybrid between the pool frog ( Pelophylax lessonae ) and the marsh frog (Pelophylax ridibundus). It is medium to large in size with extremely variable coloration: from bright green to brown-olive tones, often with marked dark spots on the back.

Average adult sizes are:

• Males: 2.4–3.1 in (6–8 cm)
• Females: 2.8–3.5 in (7–9 cm)

Sexual dimorphism is expressed in several distinctive features:

• Presence of dark grey vocal sacs in males, especially evident during the breeding season
• Generally larger size in females
• Dark nuptial pads on the thumbs of males in reproductive period
• More robust forelimbs in males

Tadpoles, at hatching, average 0.28–0.31 in (7–8 mm) and show a brownish-green coloration with tiny golden spots, an adaptation useful for aquatic life in early development stages.

In Western Liguria, Pelophylax kl. esculentus is the most common and widespread green frog species. It is regularly found from sea level up to about 2,600 ft (800 m) above sea level, successfully colonizing almost all suitable wetlands, both in the valleys and in coastal and sub-coastal areas. Its constant presence is a key factor for the local wetland biodiversity.

This species prefers a wide variety of aquatic environments, showing remarkable adaptability. The most frequently occupied habitats include:

• Natural or artificial ponds and small lakes
• Irrigation canals and storage basins
• Slow-flowing streams
• Coastal wetlands and marsh areas
• Flourishing seasonal pools, fountains in urban and periurban areas, provided with aquatic vegetation

The ability to exploit both rural and periurban areas makes Pelophylax kl. esculentus particularly resilient compared to other amphibians.

The activity of the edible frog ( Pelophylax kl. esculentus ) is both diurnal and nocturnal, with a distinct preference for periods of greater sunlight, necessary for thermoregulation. The winter dormancy period generally lasts from November to March, but can vary according to altitude and local climate conditions.

The breeding cycle takes place from April to July: males emit powerful and repeated calls, especially in the evening and at night. Females lay from 1,000 to 4,000 eggs, which are aggregated in gelatinous masses anchored to aquatic vegetation, providing protection and nourishment for the tadpoles. Metamorphosis from tadpole to adult is completed in about 3–4 months, a time span which can vary according to temperature and food availability.

The diet of the edible frog ( Pelophylax kl. esculentus ) is very varied and reflects its opportunistic nature:

• Adults:
• Aquatic and terrestrial insects (beetles, diving beetles, chironomids, etc.)
• Crustaceans such as freshwater shrimps (Atyaephyra desmarestii)
• Mollusks (Planorbarius corneus and others)
• Small fish such as mosquito fish (Gambusia holbrooki)
• Occasionally other small amphibians, including common toad tadpoles ( Bufo bufo ) and smooth newt ( Lissotriton vulgaris )
• Tadpoles:
• Algae
• Plant detritus
• Aquatic microorganisms

This composite diet allows the species to adapt to different ecological contexts and reduces food competition with other sympatric frogs.

The main threats to Pelophylax kl. esculentus in the Ligurian area are multiple and often human-made:

• Alteration and destruction of wetlands and aquatic habitats, due to land reclamation, urbanization, or invasive hydraulic works
• Water pollution caused by domestic discharge and pesticides used in agriculture
• Introduction of non-native fish species such as pumpkinseed (Lepomis gibbosus) and common carp (Cyprinus carpio), which prey on eggs and tadpoles
• Spread of emerging amphibian diseases such as chytridiomycosis (caused by Batrachochytrium dendrobatidis)
• Illegal harvesting for food or commercial purposes
• Alteration of the natural water regime due to drawing and canalization

The maintenance of wetland ecosystems remains crucial for the survival of the species.

Pelophylax kl. esculentus stands out for a few unique features among European anurans:

• It is a naturally occurring, fertile hybrid, with a complex genetic reproduction system ('hybridogenetic' system)
• It shows a remarkable ability to colonize new habitats and disturbed environments
• It demonstrates greater resilience to environmental change compared to other green frog species
• It has historically been exploited by humans for food (hence the name 'esculenta')

In Western Liguria, this species is under constant monitoring to assess population status and the impact of environmental changes. Its presence serves as a biological indicator of the quality and connectivity of aquatic habitats. Ensuring the conservation of the edible frog means protecting a tightly interconnected network of wetlands—for the benefit of amphibians and all local aquatic biodiversity.