Slender Hardyhead

Atherinomorus lacunosus

The slender hardyhead is a robust, broad-headed silverside that usually measures 10-12 cm and tops out near 14 cm. Alive in the water it looks translucent with a bright blue stripe along the flank; freshly caught, it is silvery with a greenish back and dusky fins. Its wide mid-lateral band, low flat jaw bone and count of 40 to 44 scales along the side separate it from the several near-identical hardyheads it shares reefs with. It ranges across the Indo-Pacific from East Africa east to Tonga, north to southern Japan and south to northern Australia, apparently absent from the Andaman Sea and replaced in Hawaii by Atherinomorus insularum. It was also the second Red Sea fish ever recorded in the Mediterranean after the Suez Canal opened, reported there in 1902 and since spread as far as Greece. Divers find it along sandy shorelines and reef margins in schools running from a few hundred fish to aggregations more than 100 m long, a moving silver wall that opens and closes around a diver. It is largely nocturnal: the schools break up after dark and the fish forage singly on plankton and small bottom invertebrates. Breeding timing follows local water temperature, from August to December in New Caledonia, in two seasons in the Seychelles and year-round in the Marshall Islands, and the eggs carry fine threads that glue them to weed and other surfaces. Most fish die after their first spawning season. They are a keystone forage species, hunted by sharks, tuna, needlefish and amberjack from below and by terns, boobies and gulls from above.