Wheel animals

Code JO999999

Key terminates here

A phylum of exceedingly small metazoan animals the largest of which barely exceed 1mm. The name Rotifer (wheel animal) refers to the often paired, corona of cilia at the head end. In life, the beating cilia look like a rotating wheel.

Preserved specimens often contract to an indeterminate small blob but identification to class and order may still be possible if the trophus (mouthpart apparatus) can be seen.

Identification

Identification to Rotifera can be based on size, on general body form, and on the presence of cilia in a corona of bands or lobes around the mouth. Some rotifers have the body more or less elongated while others are round or barrel-shaped. Some have soft bodies while others are protected by a hard case (lorica) which may carry distinctive radiating spines or other protrusions. Some secrete a soft, gelatinous case instead of or in addition to the lorica. In one freshwater ploimid genus, the body is almost spherical with a band of cilia around the middle. Organisms with which rotifers can readily be confused include the equally small Gastrotricha and various unicellular protists, especially the more barrel-shaped of the Ciliata.

Identification below phylum level is difficult and is best done with live specimens. The classes are defined on gonad number, the orders on the structure of small cuticularised grinding, scraping or biting organs (trophi) in the anterior foregut (mastax). External morphology can be an unreliable guide although some families and genera can be readily recognised by shape alone. In most cases, identification requires careful microscopic examination. Shiel (1995) keys the described Australian fauna to family, to genus or in some cases to species level.

Two classes and four orders of rotifer occur in inland waters. The following may suffice to identify most specimens to ordinal level:

Class Digononta (Code JD999999): Order Bdelloidea

Digononta are distinguished by having paired gonads. All freshwater species are in the order Bdelloidea., in which all species are parthenogenetic and males do not occur. Most species attain a length of only 0.1-0.5 mm, but the largest may exceed 1mm.

Bdelloid rotifers have the corona almost or completely divided into two lobes, often retractable, each bearing a "wheel" of cilia. The head is more or less distinct. The body is subdivided into pseudosegments which can be retracted telescopically. There may or may not be a lorica (case) around the main body segments. A foot, if present, is toeless or with 2-4 toes. The paired vittelarial glands and/or the gonads usually are clearly visible. The trophi are always of ramate type.

The most likely mis-identification for bdelloids is as ploimoids.

Most bdelloid species and genera have previously been regarded as cosmopolitan. Shiel (1995) commented that the Australian fauna is poorly know and that there may be many unrecognised endemic species.

Class Monogononta (Code JM999999)

Monogononta have a single gonad. The class contains three orders and all three occur in Australian inland waters. Most species are microscopic but the largest may exceed 1mm. Macroscopic colonies or aggregations are not uncommon. Most individuals are female, the males rare or seasonal in many species and unknown in others. Males tend to be smaller than females, and often have a reduced digestive system.

Identification to ordinal level may be difficult. The trophi are indicative but require microscopic examination. The key in Shiel (1995) should be consulted for positive identification beyond class level. The following is an approximate guide:

Order Ploima

Mostly motile, solitary animals. Some look like bdelloid rotifers but can be distinguished by (i) having a single gonad, not two, and (ii) never having a ramate trophus (although a wide variety of other trophi occur).

Order Flosculariacea

Mostly sessile, some are colonial. The only definitive identificatory character is a trophus of malleoramate type. Some species have a short, round body with several 'arms' or spines radiating from the lorica. These are mostly solitary and may be motile. Other species have a long, tubular or tapered body. These include the majority of sessile and sessile-colonial forms.

Order Collothecacea

Mostly sessile and solitary, inhabiting submerged or fringing vegetation. The only definitive identificatory character for this order is a trophus of uncinate type. The coronal margin may be developed into lobes, stalked lobes or tentacles. When it is, the coronal cilia are distributed in tufts on the apices of these lobes.

Reference:

Shiel, R.J. (1995) A Guide to Identification of Rotifers, Cladocerans and Copepods from Australian Inland Waters . Co-operative Research Centre for Freshwater Ecology Identification Guide No. 3., Albury, NSW.