Diagnosis: Shell angled at shoulder, with spiral threads and 1-3 raised or carinate ribs on last whorl, sometimes bearing granules or sharp nodules; surface dull because of fine spiral microstriae; brown with white dashes on prominent ribs, fading to blue-grey.

Shell: Mature shell height 5.5-14.7 mm. Shape high turbinate (H/B = 1.39-1.54, SH = 1.45-1.69); spire whorls slightly angled at shoulder and with 2 enlarged ribs visible at suture and periphery, suture distinct; spire profile straight to slightly concave towards apex; last whorl angled at shoulder and slightly at periphery. Columella long, straight, slightly hollowed at base; inner lip of aperture slightly rounded and continuous with eroded pseudumbilical and parietal area. Sculpture of last whorl: surface covered by spiral ribs and threads of unequal size, 15-20 at and above periphery and 7-11 below; usually 3 ribs are enlarged or sometimes carinate (at shoulder, mid-point and periphery; rarely only shoulder rib; or up to 6 sub-equal ribs above periphery); larger ribs are made coarsely or minutely granulose by intersection with growth lines, but granules are not conspicuously axially aligned; granules sometimes obsolete or eroded away on last whorl; rarely the three large ribs are represented by rows of as few as 13 sharp nodules; surface made dull by fine spiral microstriae. Protoconch 0.28 mm diameter, 2.5-2.6 whorls. Colour: apical 3 whorls brown to black, with white dashes on ribs; most shells fade to blue-grey on last 2 whorls, with whitish base, ribs with white dashes separated by brown spots; occasionally last whorl with brown axial flames; aperture dark brown, pale band at base and sometimes faint bands at periphery and shoulder; columella dark purple-brown, inner lip purplish.

On exposed and moderately exposed rocky shores with clear, oceanic water; recorded from basalt, granite, sandstone, limestone and concrete, often on open or vertical surfaces. This species extends lower on the shore than all sympatric littorinids, being found in the low littoral fringe and upper part of the barnacle zone.

DNA-sequence data suggest that the sister species of E. tricincta is E. australis, but support for this rela¬tionship is not strong (Williams & Reid 2004). The shell of the latter is unlikely to be confused with that of E. tricincta, being larger, thicker and often bearing strong nodules. Much more similar, and sometimes indistinguishable, are the shells of E. biangulata. Anatomically, all members of the E. leucosticta group are closely similar, but the distal part of the penial filament of E. tricincta is often more swollent than in the others. In Taiwan E. tricincta occurs on the same shores as E. malaccana, but at lower tidal levels; the two can be similar , but nodulose specimens of E. tricincta are distinguished by their three, not two, rows of nodules and their rounded, not sharp, inner lip. Some shells from Taiwan also approach the sympatric E. radiata and E. melanacme , but E. tricincta differs in the two prominent ribs visible on the spire whorls and in the rounded inner columellar lip. Also found in Taiwan is E. cinerea, but shells of that species have four prominent ribs on the last whorl and a sharp inner apertural lip. Anatomical features distinguish E. tricincta from E. malaccana, E. radiata, E. melanacme and E. cinerea.

Range: Ryukyu Is, Taiwan, Moluccas, Papua New Guinea, northeastern Australia, New Caledonia.

Despite its 6000-km range from the Ryukyu Islands to New Caledonia, few records are available for this species. It is abundant on the exposed eastern coast of Taiwan and on a short stretch of the northeastern coast of New Caledonia, but at the other recorded localities it appears to be scarce. It has not been found at Okinawa, and only a single shell is reported from the Tokara Islands (Kurozumi 1994). Only a single specimen has been found in Queensland, despite intensive collecting, so this is evidently an extra-limital record. Relatively little collecting of littorinids has been done on the exposed east coast of the Philippines

Echinolittorina tricincta is found in areas with low to moderate levels of oceanic primary productivity (Rutgers University Primary Productivity Study). It is only known to be abundant in two areas, one on the exposed eastern coast of Taiwan (at two localities, Tali and Chialoshui, listed above) and the other on a short stretch of the northeastern coast of New Caledonia where there are gaps in the barrier reef. This apparent preference for wave-exposed sites may explain the paucity of records of this widely distributed species; on the western fringes of the Pacific Ocean exposed rocky shores that are not sheltered by reefs are scarce and often inaccessible. This habitat contrasts with that of the more sheltered sites where E. philippinensis occurs, and may explain why in Sulawesi E. philippinensis was found on the sheltered north coast while E. tricincta occurred on the more exposed coast facing the Molucca Sea.