Annelid

Lore & Background

The basic annelid form consists of multiple segments called metameres. Each segment has the same sets of organs and, in most polychaetes, has a pair of parapodia that many species use for locomotion. Septa separate the segments of many species, but are poorly defined or absent in others, and Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within blood vessels, and the vessels in segments near the front ends of these species are often built up with muscles that act as hearts. The septa of such species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques—some burrowing species turn their pharynges inside out to drag themselves through the sediment.

Reader's Guide

Earthworms are oligochaetes that support terrestrial food chains both as prey and predators, and in some regions are important in aeration and enriching of soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water. Although bloodletting is used far less frequently by doctors now than it once was, some leech species are regarded as endangered because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are studied by engineers, as they offer an exceptional combination of lightness and strength. Since annelids are soft-bodied, their fossils are rare—mostly jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518 million years ago in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous period, about 299 million years ago. Palaeontologists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 million years ago, are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in the Paleogene period, which began 66 million years ago.

Did You Know?

• The largest annelid, Microchaetus rappi, can grow up to 6.7 m (22 ft) long.
• Cladistic research since 1997 has radically changed the annelid family tree, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes.
• The oldest known annelid fossil identified with confidence comes from about 518 million years ago in the early Cambrian period.
• Ragworms' jaws are studied by engineers because they offer an exceptional combination of lightness and strength.
• Some leech species are regarded as endangered because they have been over-harvested for bloodletting in the last few centuries.

Taxonomic Upheaval and the Shape of the Phylum

The annelid family tree has undergone a dramatic reshuffling in recent decades. For generations, textbooks presented a tidy three-part division: polychaetes (almost all marine), oligochaetes (which include earthworms), and hirudinea (leech-like species). Yet cladistic research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. The ripple effects extended even further: the Pogonophora, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. The Pogonophora, first discovered in 1914, lacked a recognizable gut, making classification difficult; they have been re-classified as a family, Siboglinidae, within the polychaetes. The Echiura were regarded as annelids until the 1940s, then classified as their own phylum, but a 1997 molecular phylogenetics analysis concluded they are annelids. The Archiannelida, minute annelids living between grains of marine sediment, were treated as a separate class due to simple body structure but are now regarded as polychaetes. Today the phylum contains over 22,000 extant species, ranging in size from microscopic to Microchaetus rappi, which can grow up to 6.7 m (22 ft).

Segmented Architecture and the Mechanics of Movement

At the core of every annelid lies a repeating modular design: a chain of segments called metameres, each carrying the same sets of organs. In most polychaetes, each segment also bears a pair of parapodia that many species use for locomotion. Whether these segments are cleanly divided by internal walls called septa or flow into one another with little structural definition varies enormously across the group, and in Echiura and Sipuncula no obvious segmentation is visible at all. In species with well-developed septa, blood circulates entirely within blood vessels, and the vessels in segments near the front ends are often built up with muscles that act as hearts. Those same septa let the animal reshape individual segments, producing the characteristic peristaltic ripples that travel down the body or the undulating waves that make parapodial locomotion more effective. In species with incomplete septa or none, blood circulates through the main body cavity without any kind of pump, and locomotion takes wildly different forms—some burrowing species turn their pharynges inside out to drag themselves through the sediment.

Ecosystem Engineers and Unlikely Allies of Humanity

Annelids punch far above their soft-bodied weight in the ecosystems they inhabit. Earthworms, the familiar oligochaetes, support terrestrial food chains both as prey and predators, while their constant burrowing aerates soil and enriches it with organic material in regions where they are important. In the marine realm, polychaete burrowers—potentially accounting for as much as a third of all species in near-shore environments—churn the sea floor in ways that allow water and dissolved oxygen to penetrate deeper, encouraging the development of ecosystems. Humans have long tapped into annelid biology for practical purposes: they serve as food and fishing bait, and scientists deploy them as living indicators of water quality in both marine and freshwater settings. The leech, once a staple of medical bloodletting, has seen some species regarded as endangered because they have been over-harvested for this purpose in the last few centuries. Meanwhile, engineers study the jaws of ragworms because they achieve an exceptional combination of lightness and strength that inspires new material designs.

A Fossil Record Written in Jaws and Tubes

Because annelids are soft-bodied, they leave behind remarkably few traces in the rock record. What does survive tends to be the harder parts: jaws and the mineralized tubes that some of the species secreted. Some fossils from the late Ediacaran period may represent early annelids, but the oldest specimen identified with confidence dates to about 518 million years ago in the early Cambrian period. From there, the record fills in gradually: by the end of the Carboniferous period, about 299 million years ago, fossils of most modern mobile polychaete groups are already in place. The story for oligochaetes is murkier. Palaeontologists remain divided over whether certain body fossils from the mid Ordovician, about 472 to 461 million years ago, truly belong to that lineage, and the earliest fossils that are beyond serious dispute do not appear until the Paleogene period, which began 66 million years ago.

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