DESCRIPTION OF THE PHYLUM ANNELIDA

DESCRIPTION OF THE PHYLUM ANNELIDA (LAMARCK 1809)

EUKARYA> UNIKONTA> OPISTHOKONTA> ANIMALIA> METAZOA> BILATERIA> PROTOSTOMATA> SPIRALIA> TROCHOZOA> EUTROCHOZOA> ANNELIDA

ANNELIDA LINKS

Synoptic Description

Hierarchical Classification

Annelida (a-NEL-i-da) comes from the Latin word anellus, which means little ring. The reference is to the ring-like appearance of segments on the bodies of the animals in this phylum.

INTRODUCTION TO THE ANNELIDA

The segmented worms are a natural and highly successful group of living things. They each have a segmented body with a ventral nerve cord and a closed circulatory system . The group can be found in the benthic communities of marine and freshwater environments, and terrestrial environments. Some can even be found as components of the plankton. They tend to be free-living (a few are commensals or parasites ) and can be detritivores, herbivores, or carnivores. In general, this phylum has been at the crossroads of discussions about the origin of the coelom and of segmentation (Siddall et al. 2004, Halanych et al. 2002, and Halanych 2004).

An old view, reviewed by Ruppert et al. (2004), considers the Articulata Hypothesis. This view is that the Annelids and Panarthropods are related because of the extreme degree of segmentation in both groups. So, relationships between Annelids (sensu strictu) and sipunculans or echiurids are superficial. In the old view, segmentation is the synapomorphy that is important. However, Giribet et al. (2007) show the annelids within the Trochozoa, but the sister relationship is unclear. Nielsen (2001) proposed several possible sister relationships of the annelids with other groups, particularly with the Pogonophora, Echiura, and Sipuncula. However, Nielsen (2001) reports that some specialists are not convinced of the molecular data and have them as separate phyla sometimes within the deuterostomes. Pechenik (2005) places the Pogonophorans and the Echiurids within Annelida. Brusca and Brusca (2003) take the conservative approach and keep all of the groups separate.

More recently, Struck et al. (2007), Zrzavy et al. (2009), McHugh (1997), Halanych et al. (2002), and Siddall et al. (2004) have exposed an unrecognized diversity in the annelids, particularly among the polychaetes. Also, they have shown convincingly that the former phyla, Echiura, Pogonophora, and Sipuncula are highly derived annelids. Thus, the concept of annelid has broadened to include these three former phyla. The development of the relationships in a hierarchical classification have not been developed, however. In this description we try to show the state of our understanding of the annelids and likely changes in the future. Figure 1 is a great simplification of the current state of knowledge and certainly will become much more complex in the coming years.

The Polychaete Clade (2)

The Polychaetes comprise the largest group within the Annelids, approximately 70% (Ruppert et al. 2004). Analyses of Zrzavy et al. (2009) and Stuck et al. (2007) demonstrate that the diversity of the polychaetes is enormous and all other annelid groups emerge from lines considered to be polychaete. In other words, polychaete likely is an ecological term and in the future many higher-level taxa will emerge from the current group.

Polychaetes are usually marine worms that have lateral biramous parapodia (text with tooltip) with setae in clusters. They have a distinct head made of a prostomium (in front of the mouth) and a peristomium (around the mouth), which together have have the sensory and feeding organs. Also, the mouth may have an eversible pharynx (text with tooltip) . The coelom is relatively large and the septa (text with tooltip) are reduced. Thus, segments may be confluent.

The sexes usually are separate and fertilization is external. Gametes leave the body via simple ducts or rupturing the body wall. A trochophore larva commonly occurs, which allies the phylum with the other trochophore-bearing phyla.

Polychaetes have taken many life strategies. They range also from free-living to commensal to parasitic. As free-living animals, they may be pelagic, but most often they are benthic as sedentaty or errant animals.

The Sedentary Polychaetes live permanently attached to one place, often in a tube. Typically, the sedentary polychaetes are filter-feeders and use the highly modified parapodia and setae to capture suspended food particles. They may make the tube in the mud, in which case it is U-shaped. However, very often the tube is calcareous and attached to a substrate. Such tube worms can look like an undersea flower when it emerges (Figure 2) and the disappear into the tube at a hint of danger. The sedentary taxa usually have a poorly-developed head region.

The Errant Polychaetes actively move about, sometimes by swimming, crawling, or digging. The locomotion usually is accomplished by the parapodia which act like legs (Figures 3 and 4). These animals tend to be scavengers or predators and have a well-developed head and associated sensory organs. Also, these very often have an eversible pharynx which shoots out a mouth with chitinous jaws.

The Pogonophora (Siboglinidae) Clade 3

The beardworms and vestmentiferans are bottom-dwelling marine organisms (Figure 5). They live in chitinous tubes that they anchor to the bottom. Beardworms have no digestive tract and and feed by direct absorption of dissolved substances, presumably the beard-like tentacles serve as absorptive structures (Figure 6). McHugh (1997) proposed that the pogonophorans were derived annelids. Since then, they have been subsumed into the Polychaetes. Their position within the annelids seems very stable (e.g. Zrzavy et al. 2009 and Stuck et al. 2007) even if the relative positions of the major groups are, at present, unstable. The beardworms contain two separate groups: the Perviata and the Obdurata (also called Vestmentifera).

Perviata has a short glandular region which has a raised ridge called a frenulum (text with tooltip) , which runs obliquely around it. The trunk usually has setae and they can have 1->200 tentacles. The animals live in tubes that are anchored in soft sediment.

Obdurata has no frenulum or glandular region, instead it has two vestmental “wings” that meet at the dorsal mid-line and extend toward the anterior end. A plug of hardened tissue (an operculum?) among the tentacles is used to close tube opening. They have no setae on the trunk and the animals may have more than 1000 tentacles. Their tubes are attached to a hard substrate.

Riftia (Figure 5) is an obdurate tubeworm that commonly occurs in association with deep ocean vents called black smokers, which can be a mile deep and very hot. As worms go, these are giants. They can reach lengths in excess of 2 meters. The top of the worm has a red beard, which is colored by hemoglobin. The worms do not feed directly. In fact, they have no gut at all. Inside the worm, a whole community of chemosynthetic bacteria reside and convert the hydrogen sulfide, carbon dioxide, and water to organic matter which the worm uses for food.

The Echiura Clade 4

Echiurans are called spoon worms because of the spoon-like appearance of the proboscis (text with tooltip) (Figure 7). They have characteristic hooks and setae on the outsides of their bodies. The echiurans are marine worms that resemble most annelids except that they have no evidence of the annelid metameric segmentation (text with tooltip) . Like other annelids, echiurans have trochophore larvae, a closed circulatory system, paired metanephridia, and setae that are chemically similar Brusca and Brusca (2003).

The Clitellata Clade 6

Animals in this clade have a clitellum (text with tooltip) , they tend to be hermaphrodites, have internal fertilization, allow the zygote to develop in a cocoon. Furthermore, they have no trochophore larva.

The Oligochaetes

The Oligochaetes are the worms of terrestrial environments and some in freshwater. Very few taxa live in marine environments. The name, oligochaete, means few setae. That is, they have relatively few setae per body segment.

The actions of earthworms (Figure 8) can have a profound impact on the landscape. Darwin (1881) used them to demonstrate how a small and seemingly insignificant organism could change a landscape and completely turn over the topsoil given enough time, an obvious parallel with Natural Selection. Interestingly, North America did not have large terrestrial earthworms before Europeans came and released them inadvertently. The subsequent actions of the invaders brought about substantial changes in the “naive” temperate and boreal forest biomes of North America (Frelich et al. 2006).

Some aquatic worms are also quite interesting. Tubifex (Figure 9) lives in tubes in the mud of ponds and quite streams. They have adapted to low oxygen environments by increasing the amount of hemoglobin and adopting the behavior of moving increase their contact with the water when oxygen tension gets low. They are small worms (~1-2cm) yet they can live in populations dense enough to color the bottom of the pond red. However, the bottom returns to brown with a heavy stomp on the ground and the worms retract into their tubes.

It has been difficult to separate the Oligochaetes according to a particular structural synapomorphy. In general, they are defined by exclusion. That is, they are the Clitellata that are not leeches. Ruppert et al. (2004) suggest the muscular pad on the dorsal wall of the pharynx could be the synapomorphy for the group.

The Hirundinoids

The leeches are terrestrial or freshwater ectoparasites or predaceous worms with particular synapomorphies. They have an anterior and a posterior sucker with the anus opening just dorsally to the posterior sucker. The annulations are superficial (no internal septa) and the coelomic space is filled with mesenchyme. Furthermore, they have no setae. They get about by swimming in an undulating ribbon pattern or by creeping over the surface by alternating attachments by the anterior and posterior suckers.

Most leeches are adapted to feeding on the blood of animals, mainly vertebrates. Some of them have an eversible pharynx with jaws, but most have three toothed jaws inside the mouth with which they make an incision. The medicinal leech Hirudo medicinalis (Figure 10) injects hirudin, an anticoagulant, into the wound. The medicinal leech once a mainstay of 18th century medicine, is now making a comeback because of its ability to remove excess fluid following reattachment of a finger or ear or in the treatment of snake bite.

The predaceous leeches (Figure 11) are quite common in freshwater environments. They are flattened and feed primarily on snails. The flattened leaf-like adult also broods its young and protects them until they are old enough to leave and feed on their own.

The Sipunculan Clade 7

The sipunculans, the peanut worms (Figure 12), resemble echiurids in that they, too, are unsegmented benthic worms with an introvert. Like other annelids, however, the sipunculans have trochophore larvae, and metanephridia (Brusca and Brusca 2003 and Nielsen 2001). The sipunculans also seem to share features (a specialized trochophore with the characteristic molluscan cross) with a group that Nielsen (2001) calls the Articulata (a group that includes all of the segmented protostomes as well as the molluscans). Despite the structural similarities, Struck et al. (2007) and Zrzavy et al. (2009) show clearly that the sipunculans emerge within the annelids.

Thoughts on Segmentation

Sipunculans and the echiurids are unsegmented, but their inclusion within the annelids suggests that segmentation, can be lost. Furthermore, many of the polychaetes, pogonophorans, and hirundinoids have septa that are incomplete. The earliest segmented worms, likely polychaetes, were richly segmented with two pairs of setae in tufts on each segment (Conway Morris and Peel 2008). Thus, segmentation is a primitive state in the annelid (sensu latu) line and was lost multiple times. Furthermore, segmentation as seen in other protostome groups like the arthropods, likely evolved independently.

LITERATURE CITED

Barnes, R. D. 1980. Invertebrate Zoology. Saunders College/Holt, Rinehart and Wilson, Philadelphia.

Barnes. R. S. K. 1984a. Kingdom Animalia. IN: R. S. K. Barnes, ed. A Synoptic Classification of Living Organisms. Sinauer Associates, Inc., Sunderland, MA. pp. 129-257.

Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass.

Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago.

Conway Morris, S. and J. S. Peel. 2008. The earliest annelids: Lower Cambrian polychaetes from the Sirius Passet Lagerstätte, Peary Land, North Greenland. Acta Palaeontol. Pol. 53(1): 137-148.

Darwin, C. R. 1881. The Formation of Vegetable Mould, Through the Action of Worms, With Observations on their Habits. John Murray. London.

Frelich, L., C. Hale, S. Scheu, A. Holdsworth, L. Heneghan, P. Bohlen, and P. Reich. 2006. Earthworm invasion into previously earthworm-free temperate and boreal forests. Biological Invasions. 8(6): 1235-1245.

Giribet, G., C. W. Dunn, G. D. Edgecombe, and G. W. Rouse. 2007. A modern look at the Animal Tree of Life. Zootaxa. 1668: 61-79.

Halanych, K. M. 2004. The new view of animal phylogeny. Annu. Rev. Ecol. Evol. Syst. 35: 229-256.

Halanych, K. M., T. G. Dahlgren, and D. McHugh. 2002. Unsegmented annelids? Possible origins of four lophotrochozoan worm taxa. Integ. and Comp. Biol. 42: 678-684.

Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis.

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company. New York.

McHugh, D. 1997. Molecular evidence that echiurans and pogonophorans are derived annelids. Proc. Nat. Acad. Sci. USA. 94: 8006-8009.

Meglitsch, P. A. and F. R. Schramm. 1991. Invertebrate Zoology. Oxford University Press, New York, Oxford.

Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford.

Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill. New York.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX.

Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole. New York. pp. 1-963.

Siddall, M. E., E. Borda, and G. W. Rouse. 2004. Toward a tree of life for Annelida. In: Cracraft, J. and M. J. Donoghue, eds. Assembling the Tree of Life. Oxford University Press. Oxford, New York. pp. 237-251.

Storer, T. I. and R. L. Usinger. 1965. General Zoology. 4th Edition. McGraw-Hill Book Company. New York.

Struck, T. H., N. Schult, T. Kusen, E. Hickman, C. Bleidorn, D. McHugh, and K. M. Halanych. 2007. Annelid phylogeny and the status of Sipuncula and Echiura. BMC Evolutionary Biology. 7:57 doi: 10.1186/1471-2148-7-57

Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.

Walker, J. C. and D. T. Anderson. 2001. The Platyhelminthes, Nemertea, Entoprocta, and Gnathostomulida. In: Anderson, D.T., ed. Invertebrate Zoology. Oxford University Press. Oxford, UK. pp. 59-85. [L]

Valentine, J. W. 2004. The Origin of Phyla. University of Chicago Press. Chicago. 614 pp.

Zrzavý, J., P. Ríha, L. Piálek, and J. Janouskovec. 2009. Phylogeny of Annelida (Lophotrochozoa): total-evidence analysis of morphology and six genes. BMC Evolutionary Biology. 9:189 doi: 10.1186/1471-2148-9-189

By Jack R. Holt. Last revised: 04/10/2013


FIGURE 8. Lumbricus, an oligochaete worm with an obvious clitellum. Image from http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/anne1.htm	FIGURE 9. Tubifex, oligochaete worms that inhabit water enriched with organic matter and depleted in oxygen. Image by F. Lamiot, Wikimedia Commons	FIGURE 10. Hirudo, the Medicinal Leech. Image from Systematics Biodiversity Image Archive	FIGURE 11. A predaceous leech in a stained preparation. Image from http://cas.bellarmine.edu/tietjen/images/phylum_annelida.htm

	MAJOR CLADES OF THE ANNELIDA 1. THE ANNELIDA 2. POLYCHAETA 3. POGONOPHORA (SIBOGLINIDAE) 4. ECHIURA 5. CLITELLATA + SIPUNCULA 6. CLITELLATA 7. SIPUNCULA
FIGURE 1. MAJOR CLADES OF THE ANNELIDA (SHADED BOX) WITHIN THE PROTOSTOMES. The overall topology of the cladogram is from Edgecombe et al. (2011), but the details are informed by Struck et al. (2007), Zrzavy et al. (2009), McHugh (1997), Halanych et al. (2002), and Siddall et al. (2004). Click on the terminal taxa within the shaded box to go to descriptions of the respective clades.


FIGURE 2. A sedentary polychaete worm. Image from http://biodidac.bio.uottawa.ca/	FIGURE 3. An errant polychaete worm. Image from http://cas.bellarmine.edu/tietjen/images/phylum_annelida.htm	FIGURE 4. Nereis, an errant polychaete worm. Image from http://cas.bellarmine.edu/tietjen/images/phylum_annelida.htm


FIGURE 5. Riftia, a tubeworm that commonly occurs in deep-sea habitats, particularly those associated with deep ocean vents. Image from http://www.ucmp.berkeley.edu/annelida/pogonophora.html	FIGURE 6. The anterior beard-like absorptive structure that gives the Pogonophora the common name, beard worm. Image from http://orion1.paisley.ac.uk/courses/Tatner/biomedia/units/mino14.htm

DESCRIPTION OF THE PHYLUM ANNELIDA (LAMARCK 1809)

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