Introduction to the Earthworms

Unidentified earthworm species, photo by Diane Williamson.

by
John W. Reynolds
Oligochaetology Laboratory
Kitchener, Ontario Canada N2A 2X8

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The following general information is extracted from The Earthworms of Ontario (Lumbricidae and Sparganophilidae) (1977), published by the Royal Ontario Museum, with permission. For additional morphological and other information, please refer to the original publication.

Introduction

Earthworms (Anelida, Clitellata, Oligochaeta) are familiar to almost everyone. In North America, they are one of the most popular forms of live bait for fishing (Harman, 1955); gardeners hold them in high esteem as nature's ploughmen (Darwin, 1881); folklore and scientific accounts tell of their medicinal uses (Stephenson, 1930, Reynolds and Reynolds, 1972), and soil inhabiting vertebrates (moles, voles, etc.) store them as a source of food (Plisko, 1961; Skoczen, 1970). The role of some species in organic matter decomposition and mineral cycling may be important (Bouche, 1972; Edwards and Lofty, 1972), and a great deal has been written concerning earthworm farming (Myers, 1969; Morgan 1970; Shields, 1971). Biology students the world over study their anatomy (mainly Lumbricus terrestris) in great detail (Whitehouse and Grove, 1943). The great amount of literature that has been devoted to a group of organisms that are neither pests nor sources of human nutrition is truly amazing, yet their biology and distribution are still relatively unknown. Many of the world's hundreds of megadrile (= terretrial oligochaetes) species [which includes earthworms] are known only from a limited series of one or a few specimens.

General Activity

The main activities of earthworms that affect the soil involve the ingestion of soil and the mixing of the main soil ingredients of clay, lime, and humus; the production of castings of a fine crumb structure which are ejected on the soil surface by some species; the construction of burrows that enhance aeration, drainage, and root penetration; and the production of a tilth that makes suitable habitats for the smaller scale soil fauna and micro-organisms. It should be remembered, however, that not all Lumbricidae [earthworms] work on the same manner. Some, for example, burrow deeply wheras others do not.

The influence of earthworms on the translocation of soil material may be quite considerable. There have been abundance estimates as high as three million worms per acre and their role in soil fertility is very important. Studying forms that eject casts to the surface, Darwin (1881) estimated that between 7 1/2 and 18 tons of soil per acre per year (about 3 cm per 10 years) can be moved, and the burial of many Roman ruins in Europe has been attributed to the activities of earthworms (Atkinson, 1957).

Earthworms are omnivorous and can utilize many materials in the soil as food, including plant remains, and occasionally animal remains. Lumbricids can withstand considerable starvation and, in L. terrestris at least, a water loss of up to 70% of the body weight. Some species can withstand total immersion in water for many weeks, though normally they avoid waterlogged soils.

Compost worms (Eisenia foetida) mating, photo by Jack Hynes.

Predators

Earthworms are also an important component of the diet of many birds and mammals. In Europe moles may store them as a source of food (Skoczen, 1970; Gates, 1972c), usually after biting off four or fiveof the anterior segments to prevent the worms from escaping (Evans,1948b). In North America they are eaten by many organisms, including some of economic or recreational important. According to Liscinsky (1965), for example, "the diet of the woodcock (Philohelia minor Gmelin) ... is primarily earthworms." From my current surveys, and from gut analyses of woodcock, it appears that in the area bounded by Ontario to Nova Scotia and Minnesota to Maryland, 90% of the earthworms in the diet of these birds are Aporrectodea tuberculata, Dendrobaena octaedra, Dendrorilus rubidus, and Lumbricus rubellus. Snakes, too, may prey extensively on earthworms. This is true especially of our two most common species [in Ontario], the red-bellied snake (Storeria accipitomaculata accipitomaculata Say) and the eastern garter snake (Thamnophis sirtalis sirtalis Linnaeus), and perhaps four or five other species as well (Logier, 1958).

Environmental Requirements

Daylight and ultraviolet light are injurious to earthworms unless the intensity is very low. Temperature relations have been reviewed by Reynolds (1973a), and Gates (1970) quotes interesting accounts of lumbricids [a family of eathworms] studied from the Arctic circle; Eisenia foetida, for example, has been found in snow, even though generally associated with warm habitats such as manure piles, and it remains vigorous below 5 degrees C. In Maine L. terrestris has been seen copulating while bathed with melt water, and other individuals crawled from under the ice and remained active (Gates, 1970).

The pH tolerance of earthworms varies from species to species (Reynolds, 1973d). Usually they occur in soil with pH range of about 4.5 to 8.7 and the earthworm density dimishes as the soil acidity increases. Generally speaking, the greatest earthworm densities are found in neutral soil.

Groups of Earthworms

Little work has been done on the ecology and distribution of Canadian earthworms and it is an open field of study. From a study of 14 species from Mane, 13 of which also occur in Ontario, Gates (1961) concluded that dietary preferences might be an important factor influencing distribution. He divided these species into three groups.

The first group comprised those species that pass much soil through the intestine and hence are termed geophagous, vix., A. chlorotica, Ap. longa, Ap. tuberculata, Ap, turgia, E. rosea, L. terrestris, and O. cyaneum. These species are known from soils of pH as low as 4.5-5.5 and the kinds of soil seem to be of little significance. Within this group only L. terrestris normally and regularly copulates at the surface. Because these species are tolerant of different soil types and occur in a wide reange of habitats they are likely to be widely distributed, aided especially by the activities of man.

The second group, containing El. tetrahaedra, is limiphagous (mud-eating) or limicolous (mud-inhabiting). Outside of Maine other species such as Sparganophlus eiseni and possibly A. chlorotica belong in this group, the members of which thrive thrive in mud well under water or in saturated soils. The fact that on a few occasions Ei. tetraedra has been found to be geophagous suggests an adaptability that would prove advantageous if the animal were introduced to a new area.

The third group consisted of litter feeders: E. foetida, D. octaedra, Dd. rubidus, L. castaneus, and L. rubellus. Litter includes all types of accumulation of organic matter such as leaves, manure, compost, etc. The activities of farmers and horticulturists greatly affect the distribution of these forms, and also, of course, of geophagous species that may stray into organic matter. In contrast, prior to hibernation, litter feeders become geophagous when they burrow down to the levels where they hibernate.

General Morphology

The Oligochaeta are defined as annelids with internal and external metmeric segmentation throughout the body, without parapodia, but possessing setae on all segments except the peristomium and periproct, with a true coelom and closed vascular system, generally hermaphroditic with gonads few in number in specific locations, with special ducts for discharge of genital products, with a clitellum that secretes cocoons in which ova and spermatozoa are deposited, and which are fertilized and develop wtihout a free larval stage.

The following brief discusson refers primarily to the Lumbricidae, which make up nearly all of the Canadian megadrile fauna...

External Structure

Terrestrial oligochaetes vary greatly in size. Bimastros spp. are less than 20 mm long, the largest tropica species are over 1000 mm (Glossoscolex, Megascolides), and some Australian forms may reach 3000 mm in length. The largest species in Canada is Lumbricus terrestris, which varies from 90 to 300 mm when mature. The body shape is generally cylindrical though usually flatteneed dorso-ventrally in the posterior region in the case of burrowing species.

The entire body is divided along the longitudinal axis into segments separated by intersegmental furrows and septa. This is primary segmentation. These are also secondary annuli, or furrows, whihc appear to subdivide some of the indivdiual segments, usually in the anterior region...the primary segments are numbered by Roman numerals. There is a loss of uniformity in segmentation at the anterior end ofthe earthworm; this condition is referred to as cephalization (cf. Gates, 1972c). The first body segment, containing the mouth, is known as the peristomium, and may have a tongue-like lobe projecting anteriorly. The prostomium is located above the mouth, and is not a true segment. Its appearance is often important in species identification. The last, or caudal, body segment is referred to as the periproct.

Sometimes a swelling may be seen around the body, the clitellum. The layman frequently mistakes this for the scar of a regenerated animal. In fact it is an epidermal modification of sexually mature specimens where gland cells secrete material to form the cocoon.

Characteristic of all earthworms are the short bristles or setae, retractile structures that add to the worm's grip during tunneling and locomotion. The setae are produced by cells in the body wall. In the Lumbricidae and Sparganophilidae there are four pairs of setae per segment, except for the peristomium and perproct, which are asetal. The type and position of these setae have been used as taxonomic characters.

The colour of the megadriles is primarily a result of pigment in the body wall. But it may be a secondary result of lack of pigment and the red colour of some forms is due to haemoglobin in the blood. Some colour is due to the presence of yellow coelomic corpuscles near the surface, but the presence of chloragogen cells near the surface is rarely, if ever, an influence on colour. Preliminary results of current North American studies indicate that the physical and chemical properties of the soil are a possible influence on earthworm colour.

The body wall, upon which the excretory, genital, and reproductive apertures all open, comprises six layers. From the outside, these are: cuticle, epidermis, nerve plexus, circular muscle, longtitudinal muscle, and peristoneal layer. The well-developed muscle layers are important in locomotion. The body wall is the foundation for many glandular swellings such as the clitellum, tubercula pubertatis, and genital tumescences, all of which have long been employed as taxonomic characters.

Internal Structure

The annelids have often been characterized as possessing a "tube-within-a-tube" body style. The outer tube is formed by the body wall and the inner tube by the alimentary canal. Between these two tubes is the secondary body cavity, or coelom, which is divided at each setment by a septumat the intersegmental furrow. Non-segmental alignment may occur anteriorly in some species as a result of cephalization. The coelmic cavitiy is filled with fluid that varies in composition interspecifically, and also intraspecifically for those species that are euryecious in that they tolerate a wide range of habitat conditions. Pores in the septa permit the coelmic fluid to pass freely between segments.

The alimentary canal or digestive tract is essentially a tube extending from mouth to anus. The anteriormost part of the tract consists of a muscular buccal cavity, followed by a pharynx which has sucking action during feeding, the oesophagus, the crop, a crushing organ known as the gizzard, and finally the intestine. The intestine may posses a dorsomedian fold, the typhlosole, that serves to increase the absorptive surface. Many associated structures are connected to the alimentary system, vix., blod glands, chloragogen cells, calciferous glands, and salivary glands. An extensive account of the alimentary canal is found in Gansen (1963).

The circulatory system is closed but there is an extensive sinue between the intestinal epithelium and the choragogen cells. Extending almost the total length of the body are three main vessels: the dorsal vessel, closely associated with the alimentary canal for most of its length, and two ventral vessels (ventral and sub-neural vessels). The ventral vessel is located between the nerve chord and the alimentary canal, while the sub-neural vessel is located between the nerve chord and the body wall. These main vessels are connectedin each segment by paired connectives. In several anterior segments these connectives, termed 'hearts', are enlarged and contractile, and posses valves. There are other trunks and branches which anastomose throughout the body. The circulatory or vascular system has not yet achieved its proper position in oligochaete systematics. It's importance has been discussed by Gates (1972c) and Reynolds (1973e).

There is no formalized respiratory system in earthworms; exchange of oxygen and carbon dioxide takes place through the moist cuticle. Respiration normally occurs in air but earthworms can exist in water for long periods of time (e.g. for six months) if the water is oxygenated (Brown, 1944; Roots 1956).