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ROTIFERS Rotifers are among the smallest of the true metazoans; most are less than 1 mm in length. The Phylum Rotifera (or Rotatoria) includes about 2,000 described species. Most rotifer species occur in freshwater, a few occur in seawater, and some are abundant in brackish waters, especially at river-estuarine transition regions. Most rotifers have a circular corona of cilia at the anterior end that resembles a rotating wheel, which is used for both locomotion and feeding. The biology of marine and estuarine rotifers is understudied, especially in the United States. This is probably due to their small size and to difficulties in preserving them for later identification rather than to an absence of rotifers. The common rotifers covered here have a complex and variable life cycle similar to the cyclic parthenogenesis seen in Daphnia and other cladocerans (see “Cladocerans and Ostracods” section for details). Females dominate most populations and usually reproduce asexually by parthenogenesis. Sexual reproduction, with a brief appearance by males, occurs intermittently, seemingly brought on by specific environmental cues. Sexual reproduction results in fertilized “resting” eggs (actually zygotes) that remain dormant until conditions signal the initiation of hatching and a new cycle. (See Wallace and Snell [2001] or Smith [2001] for an overview.) Rotifers have rapid life cycles. Females can reach reproductive maturity in one day and produce two to eight eggs daily during their one to two-week life span. Thus, an individual could give rise to more than 100,000 offspring in one week under ideal conditions. Many rotifers reproducing parthenogenically release their eggs directly into the water, but a few brood their eggs. The coronal cilia propel rotifers through the water. Some rotifers are entirely planktonic , while others cement themselves to the substrate using adhesive produced in special glands in the foot. Most rotifers are omnivorous and will consume whatever particles or prey of the appropriate size that they encounter. Some (e.g., Brachionus) feed primarily on small particles in suspension: phytoplankton, detritus, or small protozoans brought to the mouth by the whirling anterior cilia. The raptorial predators project their jaws from the mouth to grab their prey, including smaller rotifers. Synchaeta can use either method, depending on the type of food available. Rotifer diversity and abundance decreases with increasing salinity, but some occur in both coastal and oceanic waters. Information on rotifer ecology along our coasts is limited, with the exception of ongoing research in the Chesapeake Bay where huge concentrations of rotifers can appear and disappear within a few days. When abundant, rotifers play a crucial role in the food web, especially in the spring when other microphagous (small particle) ROTIFERS 131 grazers may be virtually absent. Thus, rotifers can form an important link between the microbial loop and higher trophic levels. They are particularly important, perhaps essential, as food for the many larval fishes that are abundant at the freshwater-estuarine interface in spring. IDENTIFICATION HINTS Living rotifers are best for identification, but they need to be relaxed and immobile. When preserved, rotifers tend to contort into unrecognizable blobs unless special precautions are taken to relax them. Appendix 3 gives detailed suggestions for narcotizing and preserving rotifers. Because the species most common in the plankton are <400 μm in length, a compound microscope is recommended. Dark-field illumination may produce stunning images. Rotifers often occur in different “morphs,” so your specimen may not be an exact match for any of those depicted. Definitive identification may require examination of the hard internal feeding structures (trophi). The rotifer body generally contains three regions (Fig. 16): The head contains the corona (wheel organ) at the anterior end made of two ciliated rings. Muscles may contract the corona, especially if the rotifer is disturbed. Check for rudimentary eyespots: one, two, three, or none. Hard jaws (trophi) in the pharynx are always present and are often used by specialists. Unfortunately, the jaws are not readily seen without a compound microscope after first dissolving the tissue. The trunk of many rotifer species, including Keratella, is encased in a shell called a lorica adorned with hooks, spikes, and plates. Other species have a thin, flexible body corona corona flexible foot flexible foot lorica with spikes lorica with spikes eyespot eyespot toes toes foot foot head head trunk trunk Fig. 16. Anatomy of a rotifer (Branchionus sp.). The presence of a corona is shared by most rotifers . The general shape of the body, presence of a lorica, projections on the lorica, eyespot(s), and a foot vary considerably between species. In some rotifers without a lorica, internal organs may be visible. Some rotifers carry eggs externally. 132 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON wall. Some species have a flexible foot, often with toes that attach temporarily to the substrate (e.g., Brachionus). One or more large eggs may be attached where the body meets the foot. Some entirely planktonic species lack feet; presence or absence of a foot is a good diagnostic feature, but be sure to check several specimens because the foot can be retracted. USEFUL IDENTIFICATION REFERENCES No guides specifically address the brackish or marine fauna from the western Atlantic or the Gulf of Mexico. However, most species will be found in the following references. Jersabek, C. D., Segers, H., Dingmann, B., et al. 2003. The Frank J. Myers Rotifera Collection: An Illustrated Catalog. Philadelphia, Academy of Natural Sciences of Philadelphia. CD-ROM. Koste, W. 1978. Rotatoria. In: Die Rädertiere Mitteleuropas. Bestimmungswerk begründet von Max Voigt. 2 vols. Borntraeger, Berlin, Stuttgart. 673 pp. (A guide to European species but still useful since many rotifers occur on both sides of the Atlantic.) Nogrady, T., Segers, H., eds. 2002. Guides to the Identification of the Microinvertebrates of the World. Vol. 18, Rotifera 6: Asplanchnidae, Gastropodidae, Lindiidae, Microcodidae, Synchaetidae , Trophosphaeridae and Filinia. Backhuys, Leiden, The Netherlands. 304 pp. Smith, D. G. 2001. Pennak’s Freshwater Invertebrates of the United States: Porifera to Crustacea. 4th ed. Wiley & Sons, New York. 638 pp. Wallace, R. L., Snell, T. W. 2010. Rotifera. In: Thorp, J. H., Covich, A. P., eds. Ecology and Classifications of North American Freshwater Invertebrates. 3rd ed. Academic Press, New York, 173–235. SUGGESTED READINGS Arndt, H. 1993. Rotifers as predators on components of the microbial food web (bacteria, heterotrophic flagellates, ciliates): A review. Hydrobiologia 255/256:231–246. Edmondson, W. T., ed. 1959. Fresh-water Biology. Wiley & Sons, New York. 1248 pp. Nogrady, R., Wallace R. L., Snell, T. W. 1993. Guides to the Identification of the Microinvertebrates of the Continental Waters of the World. Pt. 4, Rotifera. Vol. 1, Biology, Ecology and Systematics . SBP Academic, The Hague. 142 pp. (Provides an extensive introduction to the group.) Park, G. S., Marshall, H. G. 2000. The trophic contribution of rotifers in tidal and estuarine habitats. Estuarine, Coastal and Shelf Science 51:729–742. Pourriot, R., Snell, T. W. 1983. Resting eggs in rotifers. Hydrobiologia 104:213–224. Smith, D. W. 2001. Pennak’s Freshwater Invertebrates of the United States. 4th ed. Ronald Press, New York. 803 pp. (See chapter on rotifers.) Wallace, R. L., Snell, T. W., Ricci, C., et al. 2006. Rotifera. Vol. 1, Biology, Ecology and Systematics . 2nd ed. Vol. 23, Guides to the Identification of the Microinvertebrates of the Continental Waters of the World, Segers, H., ed. Backhuys, Leiden. 299 pp. (Provides an extensive introduction to the group.) This page intentionally left blank 134 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON Brachionus spp. Occurrence. Many brackish water Brachionus species occur in our area. Brachionus calyciflorus can be dominant in early spring blooms in oligohaline regions of Mid-Atlantic estuaries, primarily near the surface. It is less abundant in mesohaline areas. Brachionus plicatilis may also occur in higher salinities and was found in the hypersaline Laguna Madre, Texas. Biology and Ecology. Brachionus plicatilis is widely used in aquaculture as a food for both larval fishes and shrimps. It is also used extensively in ecotoxicology assays. This species has a life cycle with both sexual and asexual phases and resting eggs. Brachionus plicatilis is an especially variable species and can differ considerably in both size and morphology from place to place and seasonally. References. Gilbert 2007; Hagiwara et al. 1989; Hlawa and Heerkloss 1995; RicoMartinez and Snell 1995; Snell et al. 1983. Keratella spp. Occurrence. Keratella cochlearis may be present all year and can be abundant in early spring blooms at the freshwater/oligohaline interface of Atlantic estuaries, especially in creeks and tributaries. Biology and Ecology. The diet includes phytoplankton. Predatory rotifers eat Keratella when they co-occur. Synchaeta spp. Occurrence. Synchaeta baltica is abundant in colder months from Woods Hole, Massachusetts , south to at least the Chesapeake Bay, where it can be a winter dominant in salinities of 5–22. Synchaeta curvata, S. stylata, S. cecilia, and S. fennica (among others) are reported from this region, with S. fennica more common in oligohaline areas. Synchaeta stylata and S. cecilia are typically the more common summer rotifers and can be locally abundant. Biology and Ecology. Synchaeta, a major phytoplankton grazer, is found mainly in periods of high phytoplankton production. Synchaeta feeds on bacteria, nanophytoplankton , and heterotrophic microflagellates using both raptorial and filter feeding. It is eaten by some copepods, including Acartia tonsa. References. Badylak and Phlips 2008; Dolan and Gallegos 1991, 1992; Egloff 1988; Heinbokel et al. 1988; Sellner and Brownlee 1990. ROTIFERS 135 100 µm 100 µm 100 µm Brachionus calyciflorus Keratella cochlearis Synchaeta sp. • occipital spines on dorsal margin below corona (1) • spines near centerline of lorica not always present (2) • flexible foot with toes (3) • internal organs visible (4) • occipital spines on dorsal margin below corona (1) • rigid body covering (lorica) has a patterned surface (2) • pointed rigid foot (3) • internal organs not visible • sensory setae above the corona (1) • lateral palps with cilia for swimming (2) • internal organs visible (3) • fairly rigid foot with toes (4) 2 3 1 2 3 1 2 1 4 3 4 ...

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Additional Information

ISBN
9781421407463
Related ISBN
9781421406183
MARC Record
OCLC
814454605
Pages
432
Launched on MUSE
2012-11-16
Language
English
Open Access
No
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