Diagnosis: Shell turbinate; smooth with 16-22 minutely granulose threads at and above periphery and rugose suture, or granulose with 2-5 rows of granules at and above periphery; colour grey, cream, orange, brown or black. Mature shell height 5.4-13.9 mm. Shape turbinate to conical (H/B = 1.20-1.53; SH = 1.47-2.11); spire whorls lightly rounded to almost flat, suture distinct; spire profile slightly convex, concave at apex; periphery of last whorl weakly angled. Columella short, concave, slightly hollowed at base; no eroded parietal area; sometimes a pseudumbilicus in large shells. Sculpture granulose, nodulose, rugose or smooth; last whorl of strongly sculptured shells (Fig. 8A-F, H): 2-5 rows of granules or nodules, usually most prominent at periphery and shoulder, with granulose threads between; smooth shells with up to 16-22 fine minutely or indistinctly granulose threads above periphery; suture and adjacent posterior part of last whorl often coarsely rugose; spiral microstriae cover entire surface, but surface often eroded; base with 5-12 threads or weakly granulose ribs. Protoconch 0.26-0.29 mm diameter, 2.6-2.8 whorls. Colour: pale grey, cream, orange, brown or black, sometimes with broad dark band in middle of whorl and another on base; pattern usually absent, but sometimes a faint spiral pattern of brown marks between granules; aperture brown with pale band at base; columella white.

There are no obvious anatomical differences between E. arabica and E. marisrubri; details of the pallial oviduct, the shape of the penis, the curved rods of the paraspermatozoa, and the 5-6 cusps of the outer marginal teeth, are all similar. However, shells of the two are almost always distinct. Typically, those of E. ara¬bica are turbinate with rounded whorls, usually smooth with 16-22 minutely granular threads at and above the periphery, sometimes rugose or occasionally with 2-5 rows of small nodules. In contrast, shells of E. marisrubri are more conical with flatter whorls, bearing three rows of nodules and a beaded rib between each row, and brown to black shells are not known. Rare weakly sculptured shells of E. marisrubri have only eight beaded ribs at and above the periphery . Difficulty may be experienced with some of the most nodulose forms of E. arabica that are similar to moderately nodulose E. marisrubri ; in the latter there remain fewer ribs (8-13 at and above periphery), and it is the peripheral row of nodules that is largest, whereas in E. arabica the shoulder nodules are largest. Although nuclear genetic data are limited, these shell differences support the recognition of two distinct species. Furthermore, there may be an ecological difference, because the range of E. marisrubri extends into the oligotrophic northern Red Sea. In the Persian Gulf and Gulf of Oman E. arabica is broadly sympatric with E. millegrana. The latter species predominates in more exposed microhabitats and localities, and its shells can resemble those of granulose forms of E. arabica. The shell of E. millegrana is almost always white, the rounded whorls lack peripheral and shoulder angulation, and the surface is more uniformly granulose.

Persian Gulf and Gulf of Oman.
Habitat and ecology: Echinolittorina arabica occurs on a wide range of hard substrates in the uppermost eulittoral zone and low littoral fringe, but always in sheltered situations. It has been recorded on limestone, beachrock and concrete sea walls. It can be found on the trunks and pneumatophores of the mangrove Avicennia and among saltmarsh plants, and may crawl over firm sand and mud surfaces at low tide (G.R. Feulner, pers. comm.). It is abundant on the extensive black cyanobacterial mats ('sabkas') of the Gulf coast of the United Arab Emirates (Biggs 1973). It tolerates salinity of up to 50 ppt and water temperatures of up to 34°C (Biggs 1973).

The shell of this species is among the most variable in the genus, ranging from coarsely granulose to smooth. Although available samples differ in their degree of sculpture, it is unclear whether there is any correlation with habitat type. Extremes of sculpture can be found in the same microhabitat. The great majority of shells are of the smooth type. Despite the sheltered environment, the shells from the cyanobacterial mats on tidal flats of the southern Persian Gulf are of considerable thickness and curiously eroded. In addition, this species is extremely variable in colour and some samples are polymorphic, varying from black to orange or cream. Most samples from the black cyanobacterial mats have black to brown shells, as do those from mangroves, whereas samples from rock substrates are paler brown to orange (BMNH; G.R. Feulner and R. Hornby pers. comm.); this might imply an ecophenotypic effect through diet, or visual selection, and deserves further study. G.R. Feulner (pers. comm.) has suggested that crab predation plays an important role in determining the abundance of this species in the eastern Persian Gulf and Oman. Evidence of damage by crabs can sometimes be seen on the shells, and the shells can be remarkably solid .