Sciaenidae: L III A8
Argyrosomus japonicus (Temminck & Schlegel, 1843)
Dusky kob
|
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 |
|
865-985 |
1 |
240-265 |
clear |
narrow |
stern |
44% of NL |
25 |
Egg: This is another colourful egg that stands out in a collection of eggs of like size, due to the yellow pigment on the embryo and oil globule (A). The patchiness of orange/yellow pigment in the embryo (A), and absence of pigment on the yolk are key features. Black pigment follows the same pattern as yellow, but is masked unless viewed on a white background. Incubation is about 30 hours, but in mid-summer can be 24 hours.
Larva: The NH larva has no pigment in the finfold (C), but quickly forms a patch above the trunk (D & E), which seems to be common in sciaenids (see LIIA6, LIIB6 & LIIIA4). At this stage, the midtail patch in particular, can be distinctly orange. By day 4, the only yellow pigment retained is mid-tail in some specimens (F). At 11 days, flexion was just beginning (G, a preserved specimen), and was completed by 17 days (H). Note that by including Plates H and I in the series, does not imply that such a change will occur in 3 days. This illustrates both the big difference that can occur between larvae reared in the same tank, and why"larvo's" prefer to refer to changes occurring at a particular size, rather than a particular age. C: NH, D: 1 day, E: 2 days, F: 4 days, G: 11 days, H: 17 days, I: 20 days. J: 35 days (23-24°C).
Kob proved easy to rear, with 10-20 juveniles being reared from some egg batches. Three hatched larvae have been sequenced, and match the sequences of 6 adult A. japonicus collected locally (BOLD).
This was a common egg off Park Rynie, with as many as 350 being collected in a single sample. Spawning usually begins in July, and continues through to February (blue graph). It was the 15th most common egg off Park Rynie (Table 3: Introductory Notes), and was the most common big predatory fish egg. It was seen in the DHM samples on two occasions, in July and September (green graph). Abundance in samples off Park Rynie, has diminishing in the
| Linked samples | Offshore | Inshore |
| Eggs | 3153 | 1158 |
| Hits | 115 | 100 |
past 10 years, compared to the previous10 years (white graph). Spawning aggregations of this fish gather on reefs and wrecks in the area, at 30-50m water depth, where they are targeted by commercial fishermen, at night (see notes on LIIA6 as well). The recent declaration of the Aliwal Shoal as an MPA, encompassing the large steel wreck of the Produce, will serve to protect the spawning aggregation around the wreck-site. The Park Rynie linked samples had more of these eggs offshore (73%), which has been used as the second benchmark (with geelbek) to assess the offshore depth contours where other species are spawning relative to these two species. See Section 7.3 and Table 1 of the Introductory Notes, for more information on the linked samples..