The Sydney seahorse, holding to sponge, soft coral and shark nets in harbours and estuaries. Its population has fallen far enough for it to be listed as endangered.
Between about 15 and 17 cm. Pale to dark brown or nearly black, sometimes entirely yellow, with fine pale spots or streaks, saddle-shaped marks on some of the body and tail rings, and a snout finely barred with dusky lines. The crown on the head is tall with five points. It has no spines on the upper shoulder ring near the gill opening, which separates it from the two seahorses most like it.
It is active by day and extremely faithful to one spot. It lives in estuaries, bays and harbours on seagrass, large seaweed, soft corals, sea fans and sponges, and increasingly on jetties, piles and shark nets as the natural habitat disappears. Juveniles favour sea fans and adults sponges and soft corals. Pairs are monogamous, greet each other daily, and court for up to nine hours; the male carries the eggs for about three weeks. It lives between about 1 and 25 m and has been recorded to 46.
It is endemic to eastern Australia, from Mackay in Queensland south to Sussex Inlet in New South Wales, with its strongholds at Port Stephens and in Sydney Harbour. Older ranges extending it to the Solomon Islands rest on misidentifications.
It is endangered, and the cause is habitat loss rather than fishing. Port Stephens lost 95 to 98 per cent of its seahorses between 2006 and 2015 and Port Jackson about 40 per cent, through dredging, anchor damage, sedimentation and coastal development destroying the soft coral, sponge and seagrass it depends on. Its extreme site fidelity makes the losses worse. It was the first Australian seahorse listed as threatened, and artificial habitat structures — seahorse hotels — are being placed in New South Wales estuaries for it.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.