Smooth Toadfish

With a total adult length of anywhere from 3 to 16 cm (1+1⁄8–6+1⁄4 in), the smooth toadfish has an elongate body with a rounded back and flattened belly. The body narrows posteriorly to the slender tail, and its fins are all elongate and rounded. The dorsal fin has 9 to 11 rays. The pectoral fin has 15 to 18 rays, the first of which is very short. It arises well below the level of the eye. The anal fin has 7–9 rays and caudal fin has 11. The smooth toadfish has a small mouth with thin lips at its apex and a tiny chin. The round eyes are adnate (unable to rotate), their upper border level with the profile of the back and their lower border well above the mouth. In a slightly depressed area just in front of the eyes are two small nipple-shaped structures (papillae) that are the nasal organs. The openings face to the rear of the fish and are closed by flaps attached to the walls closest to the fish's midline. The first pharyngobranchial gill arch is elongated and narrow with many tiny teeth. The smooth toadfish has tiny spines that are entirely within the skin layer; these run along its back from the nasal organs almost to the dorsal fin, and along its sides from the eye to the pectoral fine, and along its underparts from behind its mouth to its vent. The skin is smooth even when the fish is fully inflated. …

The smooth toadfish is found along Australia's eastern and southeast coast, from Moreton Bay in southeastern Queensland to Port Lincoln in South Australia as well as Kangaroo Island and Tasmania. It is one of the most abundant fishes in the muddy areas of Port Philip Bay. It generally lives in shallow water less than 3 m (10 ft) deep, often over mudflats in estuaries. In areas of seagrass beds, smooth toadfish are more commonly found in sand areas bordering on the seagrass patches. They are more commonly found in seagrass patches in water less than 1.5 m (5 ft) deep rather than deeper water of 3.5–6 m (11–20 ft). A South Australian field study on wrack and associated fauna found that the smooth toadfish was associated with larger volumes and aggregations containing green algae. Although its movements are poorly known, tagging patterns indicate that the smooth toadfish spends most of its life cycle and reproduces in estuaries. It can venture well into freshwater past brackish areas. In 1964 a school of toadfish were found in the Lang Lang River at the South Gippsland Highway—34 km (21 mi) from Western Port Bay and well beyond tidal areas.

Conservation. Its large range, abundance and stable population mean the smooth toadfish is classified as least concern on the IUCN Red List. …

The smooth toadfish has strong jaws that readily crush shellfish and crustaceans. It feeds predominantly on benthic (bottom-dwelling) organisms in the substrate of the bodies of water in which it forages. Its diet includes molluscs such as black mussels, pipis, white sunset shells (Soletellina alba) and oysters (Crassostrea), crustaceans such as semaphore crabs and shrimp, and brown algae. The proportions of crustaceans to molluscs can vary widely depending on the abundance of food items; hence in a 1999 field study, the soldier crab (Mictyris longicarpus) predominated in Cowan Creek while the black mussel did so in nearby Berowra Creek. Field experiments showed it was a consumer of oysters and the gastropod Bembicium auratum, and had a major impact on their numbers. Because it is a common estuarine fish, it has been used in studies of heavy metal contamination in coastal waters. Fish tested around Sydney showed uptake was highest in the gonads, then muscle, gills and liver. It is unclear why metal concentrations were lower in toadfish livers (compared with studies of contamination in other fish) but their liver cells may be more effective at removing these elements. Lead, cadmium and nickel levels corresponded with those in the sediment from which the fish were taken, suggesting dietary intake. …