INTRODUCTION TO THE SYNDERMATA

The Syndermata (Figure 1, Clade 1) include seemingly very different groups of animals. The rotifers, a paraphyletic group in this system, include animals that are free-living (though the two taxa of Seisonida are ectoparasites) and very often dominant members of the freshwater zooplankton and microbenthos. They get their name from the ciliary fields on the anterior end ( coronae (text with tooltip) The corona is the ciliated disc-like organ at the anterior end of Rotifers. Used for locomotion and feeding. ), the motion of which appears to be paired rotating wheels. The ciliary organs surround the mouth and are used for locomotion and for food capture. When feeding, food particles are mechanically broken down with an elaborate “jaw-like chewing mechanism” ( trophi (text with tooltip) Trophi are jaw-like structures of the pharynx (called mastax in rotifers) of Rotifers. ) in the mastax (text with tooltip) Mastax is the pharynx of a rotifer, which contains the trophi. μάσταξ (MASTAX) = MOUTH or pharynx (text with tooltip) The pharynx is the anterior part of the digestive tract, just behind the mouth. (Figure 2). The trophi have been secondarily lost in the Acanthocephala, which are gut parasites of vertebrates with a life cycle that includes multiple larval stages and at least one arthropod intermediate host. All are united by a suite of synapomorphies (e.g. Zrzavy 2001) that include:

• Syncytial integument with an intracellular skeleton in the cellular part of the epidermis
• Cilia are unusual in that the basal bodies are formed of doublets rather than triplets of microtubules
• Sperm flagellum anteriorally-directed

The systematic treatments of the Syndermata are relatively recent (e.g. Garcia-Varela and Nadler 2006, Garey et al. 1998, and Zrzavy 2001) though the particular groups (here given as four classes) have been recognized as distinct for many years (e.g. Buchbaum 1938). The central problem has been the relationship between the rotifers and acanthocephala. If the acanthocephala were an outgroup, then the differences would be at the level of a phylum, and the old taxonomy as illustrated by Margulis and Schwartz (1998) would be appropriate. However, the molecular (e.g. Garcia-Varela and Nadler 2006) and some morphological (Nielsen 2001) characters suggest that the Acanthocephala is an in-group within the rotifer taxa. Nielsen (2001) supports this by citing the following synapomorphies: the same kind of epidermis with an intracellular lamina, and similarities in the embryological development of Asplanchna (a rotifer) and Macrocanthorhynchus (an acanthocephalan). However, Brusca and Brusca (2003) dismiss the relationship because the acanthocephalans have no trace of a mastax.

The relative positions of the four groups within the Syndermata are not entirely clear. Ahlrichs (1995) defines the clade he called Eurotatoria which was Monogonata+Bdelloidea, and he supports it by a set of synapomorphies that include reduced protonephridia, and a vitellarium. Kristensen (2002) and Ruppert et al. (2004) retain the Eurotatoria. Molecular evidence (e.g. Garcia-Valeria and Nadler 2006) shows a strong relationship between the bdelloid rotifers and acanthocephala. The Seisonida may be the sister group to the other Syndermata (Kristensen 2002) or sister to the Acanthocephala+Bdelloidea (Garcia-Valeria and Nadler 2006). However, Zrzavy (2001) makes an argument for the association between Seisonida and Acanthocephala in a taxon called Pararotatoria as sister to the Eurotatoria.

Kristensen (2002) summarizes a body of literature which associates the Gnathostomulids, the Micrognathozoa, and the Syndermata into a larger clade called the Gnathifera, which are unified on the basis of chitinous jaws in the pharynx. The Gnathifera was first suggested by Rieger and Tyler (1995) and shown to be a monophyletic clade (Syndermata + Gnathostomulida) by Witek et al. (2009).

The Monogonata Clade (2)

These are free-living rotifers that mainly inhabit the plankton. Most have a rigid lorica with spines (Figure 3) and a telescoping posterior which terminates in a “foot” with two “toes”. Each animal has only a single gonad (thus the name monogoanta). During most of the growing season, they exist only as females. As conditions deteriorate for any given species, the females begin to produce morphologically-distinct males, usually much smaller, and haploid. The males fertilize females that make haploid eggs and the resulting eggs, which are very resistant to environmental extremes, fall to the bottom and hatch the next year at the onset of the appropriate environmental conditions.

Most of the Monogonata are suspension-feeders and consume small cells like bacteria and small algae. However, a few are carnivores. Asplanchna, which is relatively large and has a thin lorica, is a genus of carnivorous rotifer that specializes on eating other rotifers. When Brachionus, a loricate monogonate, detects Asplanchna, the offspring develop longer spines that make it less easy to capture and eat by Asplanchna.

Though most are individual animals, some are colonial. For example, Conochilus (Figure 4) is a free-floating spherical colony of animals, all genetically identical. A mucilage gland near the foot generates the mucilage around each individual. The mucilage becomes confluent and forms the colonial matrix.

The Seisonida Clade (4)

Seisonida includes only three species, all of which are ectoparasites of Nebalia, a small shrimp-like crustacean. Seison is somewhat elongate with four body regions called: head, neck, trunk and foot (see Figure 5). The corona is vestigial and the differences between the sexes are minor. Sorenson et al. (2005) object to the designation of ectoparasite for two of the taxa because they seem to feed exclusively on bacteria. Only one species actually feeds on the hemolymph and, perhaps, on the eggs of Nebalia.

The Bdelloida Clade (6)

The bdelloid rotifers are vermiform with a telescoping foot that has up to four toes. They are called bdelloid (leech-like) because of their inch-worm movement over the substrate. Very often, they anchor themselves (Figure 6) and feed with their fully deployed corona of two wheels ( trochal disks (text with tooltip) The trochal disks are the paired coronas at the anterior ends of Rotifers. ). The life history of the bdelloids is similar to that of the Monogonata in that they tend to be parthenogenetic and produce males only at the end of the season.

The Acanthocephala Clade (7)

The Acanthocephala or spiny-headed worms are all endoparasites with elaborate lifecycles. The adults generally inhabit the guts of vertebrates while larval stages inhabit arthropods. As in many parasitic forms, the animal has become structurally simplified with a concomitant loss of the gut and most of the nervous system. As the common name implies, the adults have an anterior proboscis (text with tooltip) A proboscis is a tube or tubular process that occurs on the head or the anterior of the gut. that bears hooks which secure it to the intestinal wall of its host (Figures 7 and 8). Most taxa show a characteristic sexual dimorphism with females larger than males (Figure 9).

I have collected Macrocanthorhynchus that was 33 cm long from the gut of a pig. Though most spiny-headed worms are not as big, that genus illustrates the general aspects of the clade very well. The life cycle is fairly typical, too [Life Cycle of Macrocanthorhynchus]. The eggs pass out of the pig with the feces. A beetle grub swallows the egg and the first larva (acanthor) burrows through the wall of the gut and into the hemocoel of the insect where it attaches. After the larva develops, it releases and floats freely in the hemocoel as an acanthella. The acanthella changes to the infective cystacanth, which, after the pig eats the beetle grub, can survive the journey through the stomach to the intestine and develop into a mature worm. In the pig, the female can become fully mature in 2-3 months and begin to shed thousands of eggs per week during its life span.