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ANNELIDS Segmented Worms and Nematodes POLYCHAETES The most common and familiar annelids (segmented worms) in marine waters are the polychaete worms. Several species of polychaetes are holoplanktonic, spending their entire life cycles in the plankton, whereas many of the more than 9,000 benthic species currently described have planktonic larvae. Segmentation in this group is usually obvious with paired fleshy appendages called parapodia located on each body segment. The parapodia are commonly tipped with fine bristles called chaetae (formerly called setae). The larvae of benthic polychaetes are among the most morphologically variable of any invertebrate group. Each of the roughly 80 polychaete families has a different larval appearance, bearing only a slight resemblance to corresponding benthic adults. Many benthic polychaetes increase fertilization success through synchronous spawning, resulting in episodic pulses of larvae in zooplankton samples. The earliest larval stage is the ciliated trochophore. The early trochophore larva most often illustrated in zoology texts is representative of the Families Serpulidae and Phyllodocidae. These polychaete trochophores are similar to the mollusc trochophore (see Fig. 26). Segmentation begins in the late trochophore as chaetae develop and cilia disappear. Later, larvae use their parapodia for locomotion as the ciliary bands disappear and segments are added sequentially to the posterior of the now clearly wormlike stages. Some polychaetes have nonfeeding larvae, which live entirely on stored yolk, whereas others have planktotrophic larvae that feed on phytoplankton and other small particles. Streblospio benedicti is unusual because individual females can brood their young, or they can release either feeding or nonfeeding larvae. Larval feeding type in this species is genetically (not environmentally) determined. Late, presettlement polychaete larvae often test several different locations before settling to benthic environments. Three families of benthic polychaetes (Nereidae, Syllidae, and Eunicidae) enter the plankton for mating, which they accomplish during massive aggregations at the surface. In some nereids (e.g., Alitta, formerly Nereis or Neanthes), the reproductive planktonic adults (often called epitokes1) are specially modified and have larger eyes, reduced head appendages, and parapodia altered for swimming. Mating aggregations of some species 268 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON are synchronized so that the time of swarming is highly predictable within days or hours of the event. A variety of environmental cues are involved, including phases of the moon or tide, water temperature, and photoperiod. Epitokes may be numerous in plankton tows made during the spawning period, especially at night or early morning. Some syllid polychaetes (e.g., Myrianida) have complex life cycles that result in planktonic reproductive stages called stolons. Holoplanktonic polychaetes have special adaptations for living in the water column, including large and complex eyes, long sensory antennae, and flattened and, often, transparent bodies. These polychaetes prefer the clear waters of the open sea. The most common nearshore representatives are usually encountered in ephemeral parcels of offshore water. These pelagic polychaetes are predators. OLIGOCHAETES (EARTHWORMS, ETC.) The larger and more familiar oligochaetes are predominately terrestrial or freshwater, but many less conspicuous oligochaetes are abundant in estuarine or marine sediments. They are often indicators of polluted areas. These small (<1 cm), segmented benthic worms are occasionally resuspended in the water column and collected in plankton nets. Oligochaetes lack the parapodia of polychaetes. HIRUDINEA (LEECHES) Leeches are annelids with a sucker at each end. Although leeches are segmented worms, the pseudosegmentation visible on the outside of the body does not reflect true segmentation found internally. Most leeches are good swimmers and are frequently caught in the plankton. Many marine species have associations with other animals, including fishes (especially in boreal and Arctic waters), sea turtles, and a variety of crustaceans, such as blue crabs. There is no free-swimming larval stage. Leeches deposit eggs in cocoons that may be attached to the host or to the substrate. Newly hatched leeches swim immediately and readily attach to their preferred host on contact. Because leeches may leave a host if disturbed , individuals of all sizes may be in the plankton. USEFUL IDENTIFICATION REFERENCES Appy, R. G., Dadswell, M. J. 1981. Marine and estuarine piscicolid leeches (Hirudinea) of the Bay of Fundy and adjacent waters with a key to species. Canadian Journal of Zoology 59:183–192. Bhaud, M., Cazaux, C. 1987. Description and identification of polychaete larvae: Their implications in current biological problems. Oceanis 13:595–753. (A guide to European polychaetes; should be useful to the family level for local species.) Cook, D. G., Brinkhurst, R. O. 1973. Marine Flora and Fauna of the Northeastern United States. Annelida: Oligochaeta. NOAA Technical Report NMFS Circular 374. 23 pp. 1. Some authorities reserve the term epitoke for the posterior of the reproductive worm in contrast to the anterior part of the worm, the atoke. ANNELIDS 269 Lacalli, T. C. 1980. A guide to the marine flora and fauna of the Bay of Fundy: Polychaete larvae from Passamaquoddy Bay. Canadian Technical Report, Fisheries and Aquatic Sciences 940:1–27. (This guide to Canadian Atlantic fauna is the closest available to our area.) Larink, O., Westheide, W. 2011. Coastal Plankton: Photo Guide for European Seas. 2nd ed. Pfeil Verlag, Munich, Germany. 191 pp. (The polychaete section of this European guide is useful to identify families and genera of western Atlantic polychaetes.) Plate, S., Husemann, E. 1994. Identification guide to the planktonic polychaete larvae around the island of Helgoland (German Bight). Helgoländer Meeresuntersuchengen 48:1–58. (Many of the families, genera, and species also occur on this side of the Atlantic.) Sawyer, R. T., Lawler, A. R., Overstreet, R. M. 1975. Marine leeches of the eastern United States and the Gulf of Mexico with a key to the species. Journal of Natural History 9:633–667. (Contains behavioral and ecological notes.) Shanks, A. L., ed. 2001. An Identification Guide to the Larval Marine Invertebrates of the Pacific Northwest. Oregon State University Press, Corvallis. 314 pp. (Useful to the family level for Atlantic and Gulf fauna.) SUGGESTED READINGS Planktonic Annelids and Polychaete Larvae Giangrande, A. 1997. Polychaete reproductive patterns, life cycles, and life histories: An overview. Oceanography and Marine Biology: An Annual Review 35:323–386. Levin, L. A., Caswell, H., DePatra, K. D., et al. 1987. Demographic consequences of larval development mode: Planktotrophy vs. lecithotrophy in Streblospio benedicti. Ecology 68:1877–1886. Sawyer, R. T., Lawler, A. R., Overstreet, R. M. 1975. Marine leeches of the eastern United States and the Gulf of Mexico with a key to the species. Journal of Natural History 9:633–667. Young, C. M., ed.; Rice, M. E., Sewell, M., assoc. eds. 2001. Atlas of Marine Invertebrate Larvae. Academic Press, New York. 656 pp. (See chapter on polychaetes by Pernet et al.) 270 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON POLYCHAETE LARVAE ID hint: Polychaetes are distinguished by bundles of chaetae coming from the body segments or parapodia and by the characteristic peristaltic movement of annelids. Later-stage larvae usually have definitive heads with eyes, palps, or other structures. The stages shown are snapshots from a continuous process, with changes occurring over hours. The number, size, and shape of segments continue to change as the larva develops. Late Trochophore Larva The trochophore larva is recognized by its ciliary bands. The example shown is typical, but there is considerable variability in appearance among different polychaete families. The ciliary bands gradually disappear. Late trochophores show the beginnings of segmentation . Some trochophores, like this one, have a strong resemblance to mollusc trochophores while others are quite different. Early Segmented Larva The presence of segmentation with early parapodia indicates a transitional phase following the late trochophore stage. The ciliary bands characteristic of the trochophore usually disappear as the body becomes segmented, but these features co-occur in some larvae. This intermediate stage is sometimes called a metatrochophore. Presettlement Larva As larval development continues, many additional segments appear and the parapodia usually become more prominent and complex. At later stages, the larvae usually have distinct heads with eyes, palps, or tentacles characteristic of adults. Again, late larval stages of the many species of polychaetes co-occurring in shallow waters vary greatly in size, complexity , and behavior. Presettlement larvae are usually motile enough to be able to sample and finally select suitable benthic environments for settlement and development into juvenile forms. References. Blake 1969; Dales 1950; Dean 1965; Levin and Creed 1986; Levin et al. 1987. ANNELIDS 271 late trochophore larva early segmented larva presettlement larva 0.1 mm 0.1 mm 0.1 mm 272 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON POLYCHAETES: REPRODUCTIVE ADULTS Alitta spp. (clamworms), formerly Nereis (Neanthes) spp. Occurrence. The clamworm Alitta virens (formerly Nereis virens) is common in northern , high-salinity coastal systems. Another clamworm, A. succinea (formerly N. succinea), is abundant across a wide range of salinities in Atlantic and Gulf Coast marshes and estuaries . Biology and Ecology. Nocturnal or dawn-breeding aggregations of the specially modified reproductive worms of A. succinea take place periodically at the surface of both coastal and estuarine waters throughout our region. At breeding time, the benthic worms transform into a planktonic reproductive form often called an epitoke in which the posterior region is filled with eggs or sperm. Once at the surface, these fast-swimming worms with enlarged parapodia release their gametes. The resulting larvae may be locally abundant in the plankton . The precise timing of the aggregations is in response to lunar cycles. In A. virens only, the males become planktonic and mate with females on the bottom. Both A. succinea and A. virens were recently in the genera Nereis and Neanthes. Much of the literature on these species can be found under those genus names. References. Bass and Brafield 1972; Chatelain et al. 2008; Dales 1950; Ram et al. 2008; Wilson and Ruff 1988. Myrianida spp., formerly Autolytus spp. Occurrence. Many species of syllid polychaetes (Family Syllidae) occur in shallow water systems along the Atlantic and Gulf Coasts. Reproductive individuals are commonly encountered in plankton collections in the warmer months, especially in high-salinity regions near the coast. Different species of Myrianida seem to be the most common syllids in nearshore plankton collections. Biology and Ecology. Juveniles living on the bottom become male or female “stolons” through an extraordinary form of asexual budding. Stolons are active swimmers and leave the bottom to enter the water column where reproduction occurs. Mature planktonic female stolons carry their eggs in a ventral pouch, which bursts to release free-swimming young. References. Franke 1999; Gidholm 1965; Schiedges 1979a, 1979b. ANNELIDS 273 Alitta succinea epitoke Myrianida sp. stolon stolon with eggs • body with 2 distinct sections; the anterior end is narrower (1) • head has 4 eyes, 1 pair of large palps (mouthparts), and 4 pairs of tentacles (2) • posterior section is tapered with large chaetae on the front half (3) and short chaetae along the tapered end (4) • similar species: other species of clamworms and other types of polychaetes produce similar epitokes, and many are difficult to distinguish from one another • distinct head with 2 or 4 eyes, no palps, and single (often long) central tentacle (1) • most parapodia in the middle of the body have single or multiple chaetae on each side (2) • posterior segments are simple with short chaetae (3) • side view of a stolon with eggs carried on the ventral body surface (1) • body is usually seen curled around egg mass (2) • similar species: many species of syllids produce free-swimming stolons, and they are difficult to identify to the species level 1 mm 1 mm 10 mm 1 2 1 2 3 1 2 3 4 Myrianida sp. 274 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON Polygordius spp. larvae Occurrence. Polygordius jouinae ranges from Massachusetts south to at least Chesapeake Bay in coastal bays and harbors and along the coast out to the continental shelf. Other species may occur farther south. Biology and Ecology. Adults live in coarse, sandy sediments and release their gametes into the water column where fertilization occurs. This genus exhibits two types of planktonic larval forms. The exolarva of P. jouinae begins without segments as an early trochophore, but in later stages, the worm trunk is gradually elongated by addition of new segments to the posterior end. Polygordius spp. are unusual because both larvae and adults lack parapodia and chaetae typical of polychaetes. Polygordius larvae have been raised successfully on diatoms. Multiple larval stages and young juveniles of P. jouinae are most commonly found in the plankton from the end of May to September. References. Cowles 1903; Ramey 2008. HOLOPLANKTONIC POLYCHAETES Tomopteris spp. Occurrence. Tomopteris helgolandica is a cold-water species found from Maine to the Chesapeake Bay, especially in colder months. Tomopteris septentrionalis ranges from Maine to South Carolina. Although it is most often found in clear ocean water, Tomopteris may occur in open coastal bays, in salinities as low as 24. Tomopteris is frequently caught but never abundant. Biology and Ecology. These truly pelagic polychaetes are often transparent, with broad, flattened bodies. Tomopteris is a fast-swimming predator that uses its eversible proboscis to capture fish larvae, chaetognaths, larvaceans, and siphonophores. References. Jiménez-Cueto and Suárez-Morales 1999; Rakusa-Suszczewski 1968. ANNELIDS 275 Tomopteris helgolandica adult • wide and flat body with broad parapodia (1) • one pair of very long tentacles extend from the head (2) • posterior segments are simple, and they are often lost in collections (3) • body transparent, although prey are often visible in the axial gut 1 mm 1 2 3 Polygordius jouinae exolarva • umbrella-shaped form with stalk becoming longer as the larva develops (1) • reddish-brown margin around the translucent cap (2) • cap not attached to stalk; organs visible under cap (3) • distinct segmentation on trunk (4) • similar species: distinctive larval form among temperate coastal polychaetes 0.5 mm 1 2 3 4 276 IDENTIFICATION AND BIOLOGY OF COMMON ZOOPLANKTON Leeches ID hint: Leeches appear segmented and have a sucker at either end. They move by extension of the body and alternating attachment of the anterior and posterior suckers. Occurrence. Cystobranchus vividus, formerly Calliobdella vivida, occurs from Massachusetts to the Gulf Coast, primarily in the coldest months and especially in salinities of 5–20. Mysidobdella borealis is most common in colder months from New England to southern New Jersey. Myzobdella lugubris occurs from Massachusetts to the Gulf in brackish and freshwater. Many other species of small leeches are known from the Atlantic and Gulf Coasts. Biology and Ecology. A variety of free-living and parasitic leeches may be in the plankton , but they are seldom numerous. Those listed next are strong swimmers and are often taken in plankton nets, especially during the coldest months. Cystobranchus occurs on a variety of fish hosts but is especially common in or on the mouths of menhaden. It leaves the host to deposit its eggs on oyster beds, especially near the green seaweed Ulva. Cystobranchus has chromatophores and can change color rapidly. Mysidobdella borealis occurs in the plankton, especially at night and may be collected near the bottom with epibenthic sleds. It is common in areas with an abundance of the mysids Neomysis americana and Mysis stenolepis , the only known hosts for this ectoparasitic leech. Myzobdella lugubris leaves its fish hosts to deposit its egg cocoons on crustaceans, including blue crabs and grass shrimps. Fish hosts include mullets, killifishes, and catfishes. References. Burreson and Allen 1978; Burreson and Zwerner 1982; Daniels and Sawyer 1975; Sawyer and Hammond 1973; Sawyer et al. 1975. Oligochaetes Marine oligochaetes are small, cylindrical segmented worms; however, segmentation may not be obvious. Parapodia are absent. Careful examination at high magnification reveals minute chaetae imbedded in the body wall. Peristaltic movement and a tendency to become twisted distinguish them from the similar roundworms. Biology and Ecology. Oligochaetes are widespread, often abundant, free-burrowing benthic worms. They are commonly caught in shallow nearshore and estuarine waters, where the currents sweep them from their benthic habitats. Marine oligochaetes do not produce larval stages, and juveniles resemble adults. Reference. Attrill et al. 2009. Nematodes Although nematodes are not annelids, they look much like oligochaetes and some polychaetes . However, they are not segmented and move with a characteristic lashing motion rather than with peristalsis. Their cuticles have no chaetae. Most are less than 3 mm and are long and thin. The posterior end has a firm, sharp point. Unlike oligochaetes and polychaetes, nematodes are often colorless. Identification to species is not practical. Biology and Ecology. Although best known as internal parasites, most roundworms are free living. These minute and abundant benthic worms are frequently swept into the plankton , especially in shallow areas with high turbulence, including saltmarsh creeks. Only adults are likely to be found in plankton collections. ANNELIDS 277 Cystobranchus vividus oligochaete nematode adult adult adult • small, oral sucker with 2 pairs of small, slit-shaped eyes on dorsal surface (1) • large, caudal sucker is joined to the last body segment by a narrow neck (2) • segmentation is less conspicuous in living specimens (3) • body cylindrical, elongate, and pigmented with head end less pointed than posterior (1) • symmetrical segments usually visible; very fine chaetae are not usually seen (2) • body can extend and twist in life; usually contorted in preserved specimens (3) • body cylindrical and elongate, usually with firm, sharply pointed posterior end (1) • smooth cuticle and no segmentation (2) • limited flexibility possible but no twisting or complex convolutions possible • body often colorless or transparent 1 1 2 2 3 0.1 mm 1 mm 1 mm 1 2 3 ...

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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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