-10~15m, Dived, Bohol Island, Central Visayas, Philippines, 43.8mm., 2012.
The « Robin Redbreast Triton » is a common but lovely ranellid with a very wide distribution throughout the Indo-West Pacific region. A carnivorous gastropod like other Septa species, it lives beneath or among corals of the reefs in shallow to moderate depths of around -5~30m. When alive the brilliant intense red colour is hidden under a layer of bristly periostracum, rather thick and chestnut yellow in colour. There is usually a diffused white band around the fifth spiral cord. Typical shell length around 40mm., very large specimens may exceed 55mm. It is similar to a number of other Septa species such as S. bibbeyi (Beu, 1987) and S. hepatica (Roding, 1798), and is sometimes confused with them. In Japan it is reknowned as one of the « Three Beautious Snails », along with S. flaveola (Roding, 1798) and S. hepatica.

Cymatium rubeculum rubeculum is the second smallest (rarely over 45 mm high) and much the most common taxon in the genus; approximately three times as many lots have been examined as for all other C. (Septa) taxa combined. A high proportion of specimens are uniform deep crimson, except for two consistent white splashes on each varix and a single pale basal spiral cord on the last whorl. However, a significant proportion of specimens are orange-red, orange, or even yellow; it is not at all clear whether this paler coloration is natural colour variation, or simply caused by fading of beach shells. Some apparently fresh specimens are a deep orange, so this is probably a natural (if uncommon) colour variant in living shells; probably most paler specimens are faded. A careful analysis of colour variation in the USNM collection (51 locality lots) and NZGS collection (20 locality lots) showed no correlation between colour and geographic range. As noted under C. hepaticum, a few specimens of C. rubeculum rubeculum from the Philippine Islands have narrow black spiral interspaces on early spire whorls, resembling the wider and more prominent dark interspaces of C. hepaticum. Almost all specimens have eight spiral cords on the sides of the last whorl, as in most other C. (Septa) taxa, but a few large ones have only seven (as in C. rubeculum mare¬rubrum and C. peasei).
Over almost all of its geographic range, specimens of C. rubeculum rubeculum closely resemble in shape, sculpture, and apertural characters specimens of C. hepaticum and C. closeli, having prominent but rounded spiral cords and 19 to 25 low, rounded collabral costae, gemmating the spiral cords, in each intervariceal space. However, specimens from the northwestern Indian Ocean (Magadiscio, Somalia, USNM673819,3; Male Atoll, Maldive Islands, WM13344, NZGS, 3) approach the following subspecies in having the hollow behind each varix weakly buttressed by raised spiral cords. The northwestern Indian Ocean shells appear to be intermediate in this feature between C. rubeculum rubeculum and C. rubeculum marerubrum, and for this reason it appears likely that the highly distinctive Red Sea form is a geographic subspecies of C. rubeculum.
Type data. Lectotype of Murex rubecula Linne, 1758 (designated below) in Linne's collection, Lin-nean Society of London, Burlington House; type(s?) of Septa scarlatina Perry, 1810 lost.
Dimensions. 29.8 X 17.6 mm (lectotype of Murex rubecula Linne); 31.4 X 19.1 mm (paralectotype of Murex rubecula Linne); 45.7 X 22.6 mm (WM11635, One Tree I., Capricorn Group, Queensland, NZGS); 42.4 X 20.2 mm; 38.7 X 20.3 mm (both in WM13157, Visayan Sea, Philippines, NZGS).