Frogs stranded from sea

Frogs and other amphibians are absent from islands and prior to introductions by Man had a lower diversity in England than mainland Europe. Two dead frogs found stranded at Southend-on-Sea recently (Sept. 21 2013, Hut 249 Thorpe Bay; November 18 2013 Lynton Road Bastion 855 m to West) had respective stranded lengths of c.160 and c.130 mm consistent with them being adult Common Frogs Rana temporaria L. But it is possible that they were small individuals of the Marsh Frog R. ridibunda introduced into Kent marshes in 1935. Their dorsal skin was uniformly dark grey and both it and the grey and white mottled ventral skin displayed thickenings into sub-mm diameter spots seen on Marsh Frogs. It is also unclear if they had been flushed out of the habitat of those frogs, or the more general terrestrial and pond environments. The diagram shows the geometry of the bones and orientation in seawater of the clearly dried frog when refloated after another day in rising tides and about nine hours in a plastic bag on November 19th. When seen on the 18th, it had a ventral-up orientation parallel to the seaward side of the sandy berm of the latest morning tide, with the legs pointing west (or more exactly towards 280°, 110 mm South of the sand wet limit trending 269°). On the 19th it had much the same position except it was now dorsal-up and the strandline itself had moved 2.5 m up the beach. The skin was entirely intact but darker than on living frogs (presumably green) and thrown into longitudinal folds plus a few transverse doral ones behind the head. The length of the body (excluding limbs) was 60 mm. After being placed in a tank of seawater at 9°C (decreasing to 8°C overnight), the corpse developed a posterior-down orientation with the waterline level with the eyes. In this initial desiccated state the hind limbs were flexed and twisted so that they touched each other where the tibio-fibula joins the split astragalus and calcaneum bones. My measurements made via the glass tank wall in mm of the depth and horizontal distance after 3 and 13 hours are illustrated on graph paper with a cm grid. The 3 hour stage is on the left and the ventral side is facing the glass. The hind limbs returned to the more straight position seen on the beach and at least in the region around the femur had swollen into a more natural diameter. The hind limbs were found to have been more flexible than they ever were on the beach when the frog was finally taken out of the water after 16 hours (having then just sunk in the orientation seen on the 13 hour drawing). By contrast, the forelimbs remained in a relatively bent and stiff posture not fully explained by the ventral view. In an anterior view, the left humerus extended ventrally to a position 20 mm from the dorsal surface of the head, while the right humerus was raised above that level with the radio-ulna flexed back down parallel to it. On the left side, the radio-ulna was parallel to the median plane of the body and formed a right-angled joint with the carpus lying parallel to the body about 18 mm from it. On the right side, this region of the forelimb was directly pressed against the ventral surface with the metacarpals curving away from it. When vertical in the water they also curved downwards. Originally the water line had formed a chord that included the eyes, but after three hours this had been reduced to a dry elevation around 3.5 mm around the nose. A gas bubble was observed in the eye socket and suggested that air was slowly lost that way, while water entered via the skin rather than the closed mouth. After 13 hours, the displacement volume had clearly been enlarged by swelling of the torso and hind limbs, while dry emergent cords on the nose had a reduced elevation of around 1 mm. Therefore, although decomposition gases may well have been generated to permit the frog to strand and during refloation, the overall density must have increased by addition of water inside the skin until it sank at 1.026 g/m density. Initial drowning would involve water entering the lungs but there might also be osmotic processes operating via the skin that the kidneys of the non-marine and largely terrestrial adaptations of adult frog could not cope with. One would imagine that water would move out, or more likely salt would move into the living frog, and in this dead intact state any initial drying was clearly reversed, but evidently not far in seawater. The September 2012 find represented a more likely swimming orientation with both the forelimbs bent around the head and the hind limbs extending parallel to each other down the sand (pointing towards 224°, situated 180 mm from the wet sand limit trending 304°). It now seems likely that this corpse, which had a more realistically swollen region around the backbone, had arrived with the hind limbs hanging below a vertical axis like a dragging anchor.

Dead frog in seawater for 3 (left) and 13 hours