Engraulidae: D II A2

Thryssa vitrirostris (Gilchrist & Thompson, 1908)

Orangemouth glassnose

Egg diameter in µm	Number of oil globules	Diameter of oil globule in µm	Yolk texture	Perivitelline space	Position of oil globule at hatch	Gut length   at eye- pigment stage	Myomeres
985-1060	0	NA	Segmented	narrow	NA	74% of NL	44

Egg: Size, fine segmentation of the yolk and lack of an oil globule (A ), set this egg apart from all others found in the area, except for DIIA1 and HIIA4 (see notes below). The embryo does not develop any significant pigmentation in the egg. In my collections the egg hatched in about 30 hours (25°C), which is a reasonable estimate of incubation since they were collected freshly laid in the evening at the mouth of Durban Harbour.

Larva: Although newly hatched larvae have unpigmented eyes and no mouth, the 2-day larva has pigmented eyes, a developing mouth, and the faint black pigment spots on the notochord have moved ventrally, to form a continuous line along the notochord (B1). The pigment pattern of the older larvae can be seen by enlarging Plates C, D & E.  B: 2 days, C: 10 days, D: 18 days, E: 33 days.

Initially I was uncertain whether this was Pellona or Thryssa, since normally clupeids and engraulid larvae can be separated by whether the dorsal and anal fins overlap. Thryssa is an exception, having the dorsal fin entirely anterior to the anus and anal fin (Leis & Trnski, 1989). In addition, Pellona is the only clupeid in our waters, with an anal ray count above 30, matching Thryssa. Pellona however, has an oil globule in the egg (Delsman 1928), and Thryssa does not (Delsman, 1930). DNA barcoding of 5 larvae has subsequently confirmed this identification, matching the sequence of 5 adult T. vitrirostris collected locally (BOLD).

When seen in a sample without other familiar species as a comparative size reference, it is easy to misjudge size, and pass these up as E. teres (DIIA1). In size they are identical to the clear eggs of Saurida, HIIA4, but the latter always appear more “glassy” due to a clear yolk. It is wise to always hatch these out, to comfirm if they are Thryssa or Saurida.

This egg was the thirteenth most abundant egg in DHM samples (Introductory notes: Section 7, Table 2). It is a spring and early summer spawner (green graph). The egg was seen in the Park Rynie samples on 7 occasions, from October to February, which matches the DHM data. It has also been reported from Lake St Lucia, where it was incorrectly assumed to be the egg of Gilchristella (Connell 1996). The egg described by Connell (1996), from Lake St Lucia, as an unidentified clupeid, is probably Gilchristella (see GIIIA1).

Only 19 eggs have been recorded to date off Park Rynie, of which 68% were in offshore samples. This suggests spawning a little further offshore than I would have expected, given their abundance in Durban Harbour mouth, but numbers are small.