How to identify an asellotan isopod

George D.F. (Buz) Wilson

Asellotan species generally can be identified without dissection. Although the mouthparts, particularly the mandible, are helpful for identifying some genera, obtaining them will almost certainly destroy the head of the specimen. As can be seen in the database, many characters are associated with the shape of the head and structures on it.

To begin an identification of an asellotan, note the following things. Use the glossary if you are having trouble with a term below.

1. Is the specimen a male, female or a manca? The developmental state will affect how you answer some questions, because males and females have very different pleopods. Mancas resemble small females. You should not have to dissect the anterior pleopods to find out; look at the underside of the pleotelson and, if necessary, carefully rotate the anterior pleopods forward. Female Asellota lack the first pleopods and their second pleopods are opercular (both sides fused together); the female second pleopods will lack an appendix masculina. Adult females also have oostegites (thin ventromedial plates on the coxae of pereopods I-IV and forming a marsupium or brood pouch to hold eggs). Preparatory females (adult but not brooding) will have buds on the medial sides of the anterior coxae. Mancas (juveniles of instars 1-3) lack oostegal features and will have only 6 pairs of fully developed pereopods. Some deepwater Asellota (Brusca & Wilson, 1991; Poore, 2001a) may also lack the last pereopod, so careful inspection of the pleopods will be necessary. Adult males will have a well developed pleopod I with setae on distal margin; juvenile males lack setae on the distal end of the pleopods. Males will not have oostegites (except in some hermaphroditic taxa). The male first pleopod is distinctive, and is generally reduced to a single lobe that may be fused medially (janiroideans). The presence of a geniculate (bent) appendix masculina arising from the medial margin of the basal article (protopod) on the second pleopod is diagnostic of a male. Males also can be determined by the presence of paired penes: submedian processes arising from the vas deferens and found on the sternum (ventral surface) of pereonite 7 or pleonite 1; penes may be reduced or subcuticular (e.g., Paramunnidae). Adult males may have other secondary sexual characters (e.g., enlarged pereopod I, additional antennular or antennal aesthetascs, larger size), but these features will vary according to the group. Your inspection of the anterior pleopods will also determine the superfamilial groupings of the specimens. If the first 2 pleopods are not opercular, then you may have a stenetrioid or other non-janiroidean.
2. Pereopod I. The first leg will immediately identify the family in some groups, so rotate it out so you can see the distal podomeres. Again, avoid dissection if possible. Look at the distal finger carefully. Is it a single piece or does it have a short segment near the end? That will indicate the segments; you need to know which part is the dactylus, propodus and carpus (or Terminal, T-1 and T-2).
3. Shape of the pleotelson. Where is the anus? This character will get you quickly to some family groups. Are the uropods on the pleotelson missing or present? Large uropods are easily broken off most asellotans, but some taxa may have tiny uropods, so figuring out which is useful. Uropods are always present, so they need to be found. Look for sockets if the uropods were large but are broken off. Does the pleotelson have denticles on the margin?
4. Shape of the head. Where do the antennulae and antennae attach? What does the frontal margin of the head have on it? What is the shape of the head? Where are the eyes, if any? Are the eyes on short lobes? The head is diagnostic in many groups, so careful observation is necessary.
5. Shape of the body. Is it flat or vaulted? Can you see coxal spines in dorsal view? The pattern of the dorsal and lateral spines is also useful for identifying species. Where do the legs attach - on the side or underneath? Does the body have broad tergal plates, so that the pereopods attach ventrally and medially from the lateral margin?
6. Pereopods II-VII. Legs are often broken off, but if they are present, look for thin walking legs or paddles or powerful setose limbs. Are they elongate (longer than the body) or short? Observe the lengths of the bases. The comparative pattern of lengths can identify the genus or family.