The Family Strombidae

by Capt. Alex Roth, Jr. (1988)

Although the class Gastropoda is known from the pre-Cambrian period, about six hundred million years ago, the Super-family Strombacea only dates from the early Triassic, or about one hundred ninety million years ago, with the genus Strombus first appearing in the early Miocene. This makes this family's members among the oldest of extant mollusks.

The shells we have under discussion are classified taxonomically as follows:

PHYLUM: Mollusca; there are about 250,000 living species of mollusks
CLASS: Gastropoda; single shelled mollusks mostly
SUBCLASS: Prosobranchia; there are about 90,000 in this large group
ORDER: Mesogastropoda
SUPERFAMILY: Strombacea
FAMILY: Strombidae
GENERA: There are five generally recognized genera in the Strombidae, containing about seventy-five species. The Rimella are often placed in Tibia, as a sub-genus, and renamed Varicospira.

Terebellum Röding, 1798: according to Peter Jung and Tucker Abbott, there is only one living species of Terebellum: Terebellum terebellum Linné, 1758, and it is only found in the Indo-Pacific area. It is relatively common. Normally it lives buried in coral sand and is found from the low tide mark down to a depth of 165 ft (50.2 m). It can move quite rapidly through the water. The shell is elongate and smooth, with a high polish on it; its length to breadth ratio is approximately 1:4. This genus, like Tibia, does not have the stromboid notch. It has a sickle shaped corneous operculum and no periostracum. Its range is from Samoa and Tonga in the east through northern Australia to Madagascar and northeast Africa, the Red Sea and southern Arabia, the southern tip of India through Malaya, and as far north as southern Honshu, Japan; thence back to Samoa, including the Marianas and the whole of the former Trust Territory.

There are a number of variant forms of T. terebellum Linné, 1758, such as T. terebellum lineatum Röding, 1798; T. terebellum punctulorum Röding, 1798; T. terebellum delicatum Kuroda & Kawamoto, 1961; T. terebellum terebra Bosc, 1801; T. terebellum variegatum Link, 1807; T. terebellum nebulosum Röding, 1798; and T. terebellum sibum Link, 1807.

Strombus Linné, 1758: the genus Strombus is found world-wide, almost always in waters suitable for coral growth. 76% of the known full species live in the Indo-Pacific area and we will concern ourselves with them. The majority of them live from the low tide line to a depth of about 20 feet (6 m); 25% of the species occurring from the 15 to 30 feet depth (4.5 to 9 m) and two species may be found as deep as 400 feet (122 m). Of the sixty living full and sub-species, thirty-eight are found in the Pacific Ocean and twenty-two in the Indian Ocean areas.

J.G. Walls lists eleven sub-genera of the Strombus:

R. T. Abbott adds one additional sub-genus: Dilatilabrum Cossman, 1904.

Strombus are agile animals; they have a long, narrow foot to move on, with a narrow corneous, sickle-shaped operculum, whose edge is frequently serrated, which assists in their movement. The shells are characterized by a large flaring lip, which thickens in the adult, and the presence of a U-shaped stromboid notch near the anterior end. This notch is used by the shorter right eye peduncle to peep out of. Incidentally, the eyes at the ends of these peduncles or stalks are large, and have circular rings, either red, yellow or orange on them. About one-third of the distance from the eye on the peduncle is a short tentacle whose use is not fully known. The genus Strombus have a periostracum that is thin and a semi-translucent brown. In size they vary from the largest of 13 inches (33 cm), Strombus goliath Schroter, 1805, from Brazil, to the tiny Strombus maculatus Sowerby, 1842 of 5/8 inch (1/6 cm) found throughout the Central and Western Pacific.

The genus Strombus are dioecious, with most of the species exhibiting some sexual dimorphism, with the females being larger, or heavier.

As with most of the marine mollusca, Strombus fall into two groups as far as habitat is concerned; one group, generally limited to shores of continents with highly nitrogenous waters, or well vegetated volcanic ("high tropic") islands; the second group lives mainly in clear oceanic waters surrounding smaller coral islands and atolls, submerged banks or similar conditions around large islands or continental shelves. The genus members are herbivores or detritus feeders, preferring filamentous algae. They are generally found in colonies.

In our local (Guam) area the following species are found:

Lambis Röding, 1798: this genus is only found in the Indo-Pacific area, and consists of nine species of the so-called spider conchs. They are characterized by the long fingers or spines on the aperture side of the shell, these spines varying in number and length from species to species. Males are normally smaller than females, with two smaller (lower) knobs on the shoulders of the last whorl, whereas the female has a longer, single high knob. Also, the spines of the male are shorter and flatter, growing more toward the horizontal, as compared to the spines of the female which are longer and have a definite curl upwards. All of the Lambis have the stromboid notch.

The Lambis are shallow water gastropods and herbivores. The genus has three sub-genera; Lambis, Millipes, and Harpago. The species lambis, truncata, and crocata are member so the first group; millepeda, digitata, scorpius, robusta and violacea of the second; and chiragra the only member of the last named sub-genera.

In the Guam area are found the following:

Tibia Röding, 1798: the genus Tibia is found from Japan down through the Philippines, around India, the Persian Gulf, Red Sea, and northeast Africa. None are found in our area. They are characterized by a very high spire, an extra long siphonal canal, and finger-like projections or teeth on the outer lip. The various species dwell in moderately deep water, and are herbivorous. They may reach a length of 18 inches (45.7 cm).

Rimella Agassiz, 1841: this genus is frequently considered a sub-genus of the Tibia, and is then name Varicospira. They are small shells, with high spires. The posterior canal extends up to the spire, and the anterior canal is fairly short. In length they range up to two or three inches (5.1 to 7.6 cm).

The various species of the Strombidae often are listed under synonyms by dealers. It is therefore advisable to carefully check on the names - correct ones, that is! - of any shells prior to purchasing them. Identical species from different areas may show marked differences in colors and sizes; because of these, dealers frequently assign new or other names to these shells; again, check carefully before buying. Caveat emptor...let the buyer beware!