Smooth toadfish

Appearance

With a total adult length of anywhere from 3 to 16 cm, 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, their upper border is level with the profile of the back and the lower border is well above the mouth. In a slightly depressed area just in front of the eyes are two small nipple-shaped structures 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. It swallows water or air via a flap in its throat to swell itself up.

The base colour of the upperparts is pale tan to yellow-green, heavily marked with irregular brown spots in a reticulated pattern, and several broad dark brown bands, including ones between the eyes, between the pectoral fins and at the level of the dorsal fin. Reminiscent of a leopard's spots, the reticulated pattern continues on the upper lateral side along the body of the fish, becoming silver-white on the lower lateral parts. The chin and belly are white. The fins have a faint yellow-orange tinge, more noticeably in the tail fin. Fieldwork in Sydney waters found females to be larger and heavier than males. Smooth toadfish grow steadily larger as they grow older, with one 16 cm long individual calculated to be 13 years old from examination of its otoliths. Their gonads develop when they reach a total length of about 7–8 cm. The smooth toadfish can be distinguished from the otherwise similar common toadfish by its lack of spines and its larger- and bolder-patterned markings on its upperparts.

Naming

French naturalist Christophe-Paulin de La Poix de Fréminville described the species in 1813, though early records confused it with its close relative, the common toadfish. The two are the only members of the genus "Tetractenos" after going through several taxonomic changes since discovery.

Distribution

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 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 deep rather than deeper water of 3.5–6 m. 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 from Western Port Bay and well beyond tidal areas.

Status

Its large range, abundance and stable population mean the smooth toadfish is classified as "Least Concern" on the IUCN Red List. Although no decline in numbers has been recorded, the effects of disappearance of its habitat—mangroves and seagrass beds—is unknown.

Habitat

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 from Western Port Bay and well beyond tidal areas.

Reproduction

The breeding habits of estuary-dwelling pufferfish have been little researched in general. Fieldwork in the Hawkesbury River and tributaries north of Sydney found that the smooth toadfish breeds between April and July, building up fat stores in its liver from February to April beforehand.

Food

The smooth toadfish has strong jaws that readily crush shellfish and crustaceans. It feeds predominantly on benthic 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 and oysters, 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 predominated in Cowan Creek while the black mussel did so in nearby Berowra Creek. Field experiments showed it was a consumer of and had a major impact on numbers of oysters and the gastropod "Bembicium auratum".

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 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. The gonads of male fish had twenty times as much arsenic than those of females, while the gills of female fish contained thirty times as much lead as those of males. Raised levels of arsenic, cobalt, cadmium and lead in gills suggested the fish absorbed these from the surrounding water. An experiment exposing smooth toadfish to radioactive cadmium and selenium in either food or water found that cadmium in food was taken up in and excreted by the liver, while cadmium in water was taken up in the gut lining and excreted in liver, gills and kidney, indicating the fish were consuming a lot of water. Selenium was taken up in the gills, kidneys and liver regardless of whether it was in food or water....hieroglyph snipped... Fieldwork in Sydney waterways showed that higher arsenic, lead, cadmium and cobalt corresponded with decreased lipid levels in liver and gonadal tissue, and raised cobalt and nickel correspond to increased protein levels in muscle, liver and gonadal tissue. Raised lead levels were consistent with smaller egg size. A study of asymmetry of fish bones in smooth toadfish in various parts of Sydney and Hawkesbury River estuaries showed a relationship between exposure to organochlorine pesticides but not heavy metals, indicating the finding may correlate to stress from organic toxicity.

Defense

Notorious for taking bait from fish hooks, the smooth toadfish is an unwanted catch for anglers as its flesh is highly poisonous and unfit for human consumption. However, its lack of spines makes it easier to handle than other toadfish when it inflates itself after being caught. Its toxicity had been reported by local aborigines in Sydney to William Dawes in the late 18th century. A man named John Buff was fatally poisoned after catching and eating toadfish in Duck River in 1821 near Parramatta, his case and subsequent coroner's inquest published in the "Sydney Gazette". The smooth toadfish was responsible for the deaths of the wife and two children of Captain Bell of New Town near Hobart in a widely publicised case in March 1831. Colonial surgeon James Scott wrote,

"The melancholy and dreadful effect produced by eating it was lately instanced in the neighbourhood of Hobart town ... The poison is of a powerful sedative nature, producing stupor, loss of speech, deglutition, vision and the power of the voluntary muscles, and ultimately an entire deprivation of nervous power and death."

An inquest into the deaths took place on 29 March 1831. The family's three servants, one of whom appeared to have been poisoned as well and was ill, were placed in custody separately to stop them communicating with each other while the investigation proceeded. The jury replicated the effects by feeding the fish to two cats. The jury learnt that the servant, Speed, had caught the 20 or so fish and taken them home to eat. A neighbour called out to him not to eat the fish as they were "no good", but he took it as a joke. The inquest concluded with a finding of accidental death, with some deliberation over whether Speed should have been charged with manslaughter or even murder after hearing the warning. Warnings about toadfish were subsequently issued.

Its toxicity is due to tetrodotoxin, which is concentrated particularly in the liver, ovaries, intestines and skin. Many species of pufferfish bear this toxin, obtaining it from tetrodotoxin-containing bacteria in their diet. Eating the fish can have fatal consequences. The symptoms of poisoning, which are predominantly neurological, include ataxia, in addition to numbness and/or paraesthesia around the mouth, lips, and limb extremities. Cases of pets being poisoned have occurred when the fish have been left where they can eat it.