Scutus unguis (Linnaeus, 1758) 43mm
ANIMAL OF THE WEEK
Scutus unguis (Linnaeus, 1758) 20mm shell from the above 43mm animal
[shell not shown in the same proportion as the animal]
Opisthobranch-like Animal of the Week Data
Frequency on Okinawa:
- Class: Gastropoda
- Subclass: Prosobranchia
- Order: Mesogastropoda (Vetigastropoda)
- Family: Fissurellidae
- Individual Collection No: RFB #3937
- Location: Seragaki, Okinawa (26o 30.4'N, 127o 52.6'E)
- Date: 4 August, 2003 [many individuals have been seen but neither photographed nor collected]
- Depth: 15ft (5m)
- Water temperature at collection depth: 86oF (30oC)
- Size: TL 43mm
- Specimen deposited: Photo only, without specimen
- Photo Data: Digital film w/ Nikon F-100 & Nikon 105mm Micro-Nikkor lens
Once again, as with several earlier presentations, I've decided to depart from the usual opisthobranch presentation with the above featured animal. Although it superficially resembles an opisthobranch, it in fact is classified into an entirely different group of molluscs, the Prosobranchia. The conventional schema of mollusc classification is to divide the Class Gastropoda into the following three Subclasses:
- The opisthobranchs include the sea slugs and their relatives the sea hares, sea butterflies, and others. The bodies of most member of this group show evidence of detorsion. Shells may or may not be present; if present they may be reduced and/or internally located. Opisthobranchs usually lack a mantle cavity and operculum. Many species lack gills and respiration takes place through the skin, which may bear numerous projections and folds that increase the area for gas exchange. The head bears 1-2 pairs of tentacles. This diverse group of organisms is divided into around nine Orders. There is some disagreement about whether these form a monophyletic group.
- Prosobranchs are the largest subclass of gastropods. Most species are marine, but many freshwater species and a few terrestrial forms are also known. Prosobranchs have an operculum (lacking in pulmonates), and most have a spirally coiled shell. The head includes eyes that are located on tentacles. The mantle cavity is anteriorly directed and near the head. They are divided into three Orders;
- Pulmonates are the land snails and slugs (a few species are marine). A coiled shell is usually present, but it is lost in some groups. Some detorsion has occurred in many species. The Subclass derives its name from the fact that the mantle cavity forms lungs; these are filled with air as a result of contractions of the mantle floor. One or two pairs of tentacles are found on the head, depending on whether the snail or slug is terrestrial (two pairs) or aquatic (one). The nervous system is highly concentrated. Pulmonates are dioecious and hermaphroditic as are prosobranchs, but pulmonates develop directly (there is no larval form).
Scutus unguis is considered to be very common on Okinawa; numerous individuals have been seen here, primarily in the upper 30ft of water. They are commonly seen on the underside of coral rubble slabs as the rubble is overturned. Also, lone Scutus shells are a common sight in the intertidal zone, suggesting a fairly high mortality rate. Scutus species are not slugs but a marine snail with a white shield-like shell on its back. The shell is frequently covered by flaps of the mantle, but sometimes the mantle is withdrawn and exposes a portion of the shell. It belongs to the family Fissurellidae which includes the Slit-Limpets and Keyhole Limpets, named from the slit or "keyhole" opening in their limpet-shaped shell; these openings allow for the discharge of water from the mantle cavity. Scutus species have only a notched shell, as seen in the above image. The relatively large number of questions posted by home aquarists to Bill Rudman
on the Sea Slug Forum (Rudman, 2000) supports how slug-like (opisthobranch-like) these animals are.
In addition to the above prosobranch, I've also have links to the following animals which are similar in appearance to opisthobranch molluscs:
- Rudman, W.B., 2000 (August 15). Scutus sp. Elephant Slug, Shield Slug, Ducksbill Limpet. [In] Sea Slug Forum. http://www.seaslugforum.net/scutus.htm.
Page Date: 25 Aug '03
Page Modification Date: 01 Feb '04
Digitally manipulated photo
Copyright © 2004 Robert F. Bolland