The general shape of the otolith is ovally pointed. The sulcus is straight and runs over the whole length of the otolith. The sulcus is open at both sides, but this is not always clear at the posterior end. The sulcus is not well developed and ostium and cauda can not be distinguished. The surface of the sulcus is smooth though somewhat irregular at the edges. The surface of the inside and outside is smooth, whereas the margins are a bit irregular. The anterior margin is slightly indented at the ostium. The posterior end is rounded. The rostrum and antirostrum of the otolith are well developed and rounded. There is another protruding, strongly pointed, element between the rostrum and antirostrum visible. The otolith is thin and flat. Twaite shad is the largest of the clupeiformes but otoliths can only reach a length of 5.5 mm.
Twaite shad otoliths are difficult to distinguish from other clupeiformes. Herring Clupea harengus otoliths have a sulcus that is better developed The rostrum of herring otoliths is better developed, but there is only one antirostrum. The posterior end of herring otoliths is indented and at 90° with the ventral side, whereas the twaite shad otolith is rounded and at a wider angle with the ventral side. Otolith length-width ratio is smaller in twaite shad. The otoliths of sprat Sprattus sprattus are much rounder and have a smaller rostrum, one antirostrum and a posterior end that is indented and at less than 90° with the ventral side. Pilchard Sardina pilchardus has a rostrum that is more pointed and only one antirostrum. Anchovy Engraulis encrasicolus has a closed sulcus which ends at the centre of the otolith. The rostrum is more pointed but less developed and smaller compared to twaite shad otoliths.
When eroded the sulcus becomes less distinct and the margins, rostrum and antirostrum are smoothed. There is often only one antirostrum visible when eroded. Often the rostrum is missing after the otolith has gone through the digestive tract of an animal.
Otoliths of juvenile fish are already ovally pointed. The sulcus is not well developed and the margins are much smoother.
Fish length and distribution
Twaite shad can grow up to 60 cm. Twaite shad is a pelagic schooling roundfish (ALOSFALL.TIF) that is mostly found in inshore waters and connecting fresh waters. It spawns in spring in fresh water and juveniles are often found in brackish waters. (Wheeler, 1978, Nijssen and De Groot, 1987, De Nie, 1996, Muus et al., 1999).
It is found in the coastal waters and estuaries of the NE Atlantic, North Sea and Baltic.