Rotifer trophi web page
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Rotifer trophi

     Most rotifers are filter feeders, or feed by browsing the bacterial film on substrata. Food particles are directed to the mouth by the metachronous beating of the coronal cilia. Many, however, are active predators or feed on large algae. They either completely engulf their prey or suck out its content. Food is transported to a muscular masticatory apparatus, the mastax, via a short, ciliated pharynx. This mastax contains hard, jaw-like structures, the trophi, and associated muscles. The trophi are of the most important features used in the classification and identification of monogononts. From there, particles are transported through a short oesophagus to a stomach, intestine and anus situated dorsally of the foot. In a number of taxa, the stomach is a blind sac, and food remnants are extruded by the trophi (Asplanchna).
     The trophi are a complex set of extracellular cuticular sclerites. Each trophi element is made up of tubular structures with a dense osmiophilic core embedded in a matrix. They are formed by a number of different epithelial cells, which can be appreciated in the structure of the rami and manubria containing compartments. The mastax and associated trophi are highly adaptive, and structurally and functionally complex in conjunction to food type and feeding habits. The primary function of the mastax is to grind food particles, however, common adaptations are for grasping or sucking in complete food particles or the contents of large prey.
      Basically, the trophi consists of five sets of elements. An unpair median fulcrum (yellow) connects two rami (green) at their joint, and serves as insertion for abductor muscles for the rami. The fulcrum is mostly rectangular in lateral view, but can be elongate, and/or provided with a basal plate. The rami are hollow, more or less triangular structures. Their inner margin is provided with numerous elongate elements, the rami scleropillii, which may be fused forming a reinforced ridge and/or a series of blunt or sharp tooth-like projections. Commonly, the rami are provided with antero-lateral projections, the alulae. Fulcrum and rami are joined in a functional unit, the incus (Latin, anvil). The unci (blue) are plates formed by connection of a variable number of teeth into one rigid structure. The teeth are mostly unequal, with the ventral ones the largest, and consist of a shaft and an arrow-shaped head. Minute subuncinal teeth are mostly present, and are situated under the unci plate. The manubria (red) are crescent-shaped and compartmented supports of the unci with (Ploima) or without (Flosculariaceae) an elongate shaft. Unci and manubria together form the malleus (Latin, hammer) of the trophi. Diverse accessory sclerites occur in monogononts.
     The trophi can be distinguished in almost any female monogonont, but are completely or strongly reduced in most males. The basic pattern is considerably modified in the different families or species according to their mode of life and their feeding habits. Seven trophi types are recognised in monogononts, one in bdelloids, and one in seisonids:

Malleate: Similar to the malleoramate type: fulcrum short, rami more or less triangular or lyriform, in a straight angle with the fulcrum. The unci plates have several teeth, but generally less than in the malleoramate type (4-12 teeth), and the connection between the teeth is more strongly developed. The manubria are provided with a shaft, which is typically fairly short (more elongate in the submalleate trophi of Lecanidae and some Proalidae).

Malleoramate: Fulcrum short, rami more or less triangular, flat; unci teeth numerous, occasionally resembling striations; manubria crescent-shaped, without shaft.

Virgate: Fulcrum elongate, rami bent towards dorsally, unci with few teeth or reduced, and manubria mostly with elongate shafts. The virgate trophi type is the most variable of all trophi types; occasionally it is asymmetrical (e.g., Trichocerca). Virgate trophi occasionally consist of thin, hence hardly discernable elements (e.g., many Synchaetidae).

Forcipate: Fulcrum mostly short, rami elongate, long pincers usually armed with anterior and median teeth. Unci strong, but with a single or few teeth only, dagger- or sword shaped. Manubria long, thin, often with intramallei. In Dicranophoridae and Ituridae.

Incudate: Fulcrum short, rami elongate, pincer-shaped. Manubria and unci strongly reduced. In Asplanchnidae.

Cardate: Fulcrum short, rami lyriform, manubria with a shaft and a characteristic, additional ventral projection. Accessory trophi elements are mostly present, and numerous. Restricted to Lindiidae.

Uncinate: Similar to the malleoramate type, but all trophi elements except the unci strongly reduced. Unci teeth few (2-5 teeth per uncus), elongate and curved supporting rods for the mastax. In Atrochidae and Collothecidae only.

Ramate: Fulcrum absent, rami semicircular, flat; unci teeth numerous, occasionally resembling striations; manubria as lateral bands. In bdelloids.

Fulcrate: Fulcrum long, manubria absent; the pumping action is performed by the hypopharynx muscle. In Seison.

A few monogonont taxa have trophi that can hardly be placed in any of these types (e.g., Tetrasiphon).

Rotifer trophi preparation
     Reliable identification of many, especially illoricate, monogonont rotifers requires the examination of trophi morphology. In the case of large specimens or for routine identification light microscopy can suffice, but for some small species or taxonomic studies scanning electron microscopy (S.E.M.) may be required. For light microscopy, animals are transferred to a slide containing only a minute amount of liquid. After examination of the complete animal, its trophi can be isolated by adding a drop of commercial sodium hypocloride (NaOCl, ‘bleach’), KOH or NaOH (4%) near the edge of the cover slide. Through capillarity the solution will spread under the cover slide and clear out the trophi, which can then be studied.
     For the preparation of trophi for S.E.M. examination, the following method can be used:

1) prepare a piece of clean (degreased in alcohol, rinsed in distilled water and dried) cover slide or circular cover slide, and place it on a microscope slide. A minute quantity of glycerol placed between cover slide and slide is used to prevent movement of the cover slide during the preparation process;

2) place a small number of clean (rinsed in distilled water) animals (3-10) on the cover slide;

3) add a small drop of sodium hypochlorite to the specimens. The dissolving process of the soft tissues can be monitored using a dissection or compound microscope and will last, depending on the species and the concentration of the reagent, from 5 to 20 minutes. It may be necessary to remove the trophi from the lorica, by gently squeezing the lorica with a fine needle (e.g., Tungsten needles sharpened by electrolysis in a KOH solution) after most soft tissues are dissolved;

4) as soon as the trophi are isolated, remove the remaining reagent using the capillary suction of an ultrafine pipette, and add a minute quantity of distilled water;

5) rinse the trophi by adding, and subsequently removing distilled water. This is done 3-6 times, until no crystal formation can be observed anymore when some of the removed rinsing water is dried on a different slide. When necessary, the orientation of trophi can be changed by moving a fine needle near the trophi.

6) when the trophi are sufficiently rinsed, the preparation is dried by evaporation and the trophi will automatically stick to the cover slide. The preparation can then be attached to a S.E.M. stub, sputter-coated and examined.

     This method can also be used to prepare trophi for inclusion in a permanent microscope slide, in which case the cover slide is inverted on some mounting medium. Critical point drying is not necessary for the preparation of trophi for S.E.M., it is, however, required when complete rotifers are prepared, even in loricate species.


(modified from Segers H., 2004. Rotifera Monogononta. In: Yule C.M. & Yong H.S. (eds.), Freshwater invertebrates of the Malaysia Region. Academy of Sciences, Malaysia, in press.)

     The nomenclature and the systematic hierarchy we are following in this web site is that of Segers, H., 2002. The nomenclature of the Rotifera: annotated checklist of valid family- and genus-group names. Journal of Natural History 36: 631-640.