Balanoglossus gigas is recorded as growing to 1.5 metres (4.9 ft) in length, and is thus considered the largest acorn worm currently known.[3][4][5] One specimen from the Brazilian coast was recorded as measuring 1.8 metres in length.[6]
When handled, B. gigas releases thick, sticky mucus with a strong smell of iodoform, which may serve as a deterrent to predators.[7] The odour is caused by 2,6-dibromophenol, a phenol[8] also reported from crustaceans, where it is the cause of a similar odour.[9][10]
Balanoglossus gigas also has accumulation of iodine throughout its body, something reported from no other acorn worm or protochordate. The iodine concentration is highest in the hepatic (liver) region, and lowest in the proboscis.[11][12] Other compounds were also found to be present in accumulations in the body of B. gigas: high concentrations of calcium were found in the skeleton, while phosphate was found in the proboscis, copper in the collar and the skeleton, chloride in the collar and branchial area, iron in the hepatic region, nitrogen in the genital and hepatic regions, and a mix of sodium, calcium, potassium and magnesium found at both the front and back of the body.[12]
A multipart study in the 1960s assessed the bioluminescence of Balanoglossus biminiensis, since synonymized with B. gigas. The reaction causing bioluminescence incorporates hydrogen peroxide and a luciferin common to balanglossids.[13]
