Tuatara: Volume 9, Issue 3, January 1962
The Lugworm Abarenicola assimilis as a Laboratory Animal
The Lugworm Abarenicola assimilis as a Laboratory Animal
The lugwormAbarenicola assimilis is the creature responsible for many of the large worm casts seen at low tide on many of New Zealand's sandy beaches in both the North and South Island. The animal is chiefly restricted to sand with a high content of organic matter, and thus the animals are often found in great numbers in areas near the outflow from abattoirs, freezing works, or rubbish tips. The worms range in size from 10 to 35 cm. in length and their soft fleshy bodies are extremely attractive to predatory animals and fish. Most fish will strike at a bait of lugworm in preference to anything else — a fact discovered by the Maoris many years ago. Lugworm bait is eagerly taken by snapper, moki, and tarakihi. Petone Beach, Wellington, in the early spring and summer mornings, is the venue where many a keen surf-fisherman can be seen digging vigorously for these quick-burrowing worms.
Abarenicola assimilis is fairly common in the sand at Petone Beach and the description of the species here is from specimens taken from that locality. No other colonies were found in the Wellington area during the course of this study.
Arenicola possesses a single pair of oesophageal caecae, whereas Abarenicola possesses more than two caeca situated in a longitudinal row on each side of the oesophagus. Fig. 3, oes. caec.).
In Arenicola, the gular membrane is muscular and is the primary agent in the process of proboscis extrusion. Moreover the gular membrane has a pair of septal pouches. In Abarenicola, the gular membrane is extremely thin and non-muscular and probably has no part in the process of proboscis extrusion. (Fig. 3, mm.p.ret.). Septal pouches are absent.
Arenicola has a small prostomium which is completely retractable into the nuchal pouch. The prostomium of Abarenicola is broad in front (Fig. 1, D, Pros.) and cannot be retracted into the nuchal pouch.
In Arenicola the neuropodia of the branchiate segments extend ventrally nearly to the ventral mid-line, but in Abarenicola the ventral ends of the neuropodia are separated from the ventral mid-line by a distance roughly equal to the length of the neuropodium (Fig. 1, G, Neurop.).
The lugworms of the Petone colony showed clearly all those features described by Wells (1959) as diagnostic of the genus Abarenicola, and Abarenicola assimilis is now known to be the common lugworm inhabiting the colder waters of the Southern Hemisphere (Wells, 1961).
The first specimens were dug on April 1, 1961, and subsequently at weekly or fortnightly intervals. The greatest concentration of worms is found at the western end of Petone Beach (Fig. 1, A), and the colony extends eastwards to within a few chains of Petone Wharf. In the zone of heaviest concentration, specimens number three or four per square yard of beach and the concentration diminishes east and west of this zone to about six casts per square chain on the fringes of the colony. Specimens taken at random within the colony all appeared to be of a fairly constant size and at a similar stage of sexual development.
The sand containing the greatest concentration of both mature and immature worms is heavy and black, full of decaying organic matter, such as freezing works offal and seaweeds. The seawater is correspondingly polluted with such matter, and there seems to be a direct relationship between the amount of decaying organic matter in the sand and seawater and the number of animals present.
Specimens were taken from the sand by means of a spade, but only about one worm in every twenty was obtained entire as it was page 118 difficult not to cut or tear specimens during extrication from the sand. Sexing of the worms was a relatively simple matter even in the first weeks of study, for at that time the oocytes and sperm plates were sufficiently well developed to be quite easily recognisable. When a fine capillary pipette was pushed through the body wall of a specimen and a small quantity of coelomic fluid syringed out, the sample from a male worm was milky white, and from a female worm it was orange-white, with the individual oocytes discernable with the naked eye.
The burrow of a lugworm is U shaped and is usually about 30 cm. deep. The presence of a worm on the beach is indicated by a coiled casting about 2 mm., in diameter at the anal end, and a countersunk hole at the anterior or head end. The distance between the head and the anal ends of the burrow varies from 10 cm. to 25 cm. (Fig. 1, B). The burrows remain semi-permanent in nature, as the walls of the burrow are lined with mucus secreted by epidermal cells, which binds the wet sand, and prevents the burrow from caving in. When a worm is dug and the burrow exposed, a clear imprint of the worm is left in the sand. When the burrow is covered by water, the worm is in the resting position (Fig. 1, B, A-B). At low water the worm is more usually found in the defaecating position (Fig. 1, B, A1-B1). This position is assumed also at high water, but only temporarily while the worm defaecates.
In the laboratory, a glass U tube was partially filled with sand, topped up with seawater, and set up on a stand. A living Abarenicola assimilis was placed in the tube, and as the worm burrowed, its digging mechanism was observed.page 119
(A) Location Map.
(B) Nature of Burrow and Response to Tidal Variation. A — B, resting position; A1 — B1, defaecating position.
(C) External Features. An. — Anus, Br1 - Br13 — Branched gills, Chaet. annl-Chaet. ann19 — Chaetigerous annuli, Mth. — Mouth, Neph P1 - Neph P6 — Nephridiopores, Neurop — Neuropodium, Noto — Notopodium, No. chaet — Notopodial chaeta, Pros. — Proboscis, Se IV Segment IV (a1-a5 — annuli), Tail Seg. — Tail segment, V.M.L. — Ventral midline, A - A - T.S. — Fig. 2A, B - B - T.S. — Fig. 2C.
(D) External Features of Proboscis and Anterior Segments. Chaet. ann2 — Chaetigerous annulus II, Mth. — Mouth, No. chaet. — Notopodial chaetae, Nuch. p. — Nuchal pore, Pap. — Papillae, Pros. — Proboscis.
(E) Mature Ovum. Cyt. — Cytoplasm, Memb. — Outer Membrane, Nuc. — Nucleus.
(F) (1) Side View of Mature Sperm Plate. (2) Top View of Mature Sperm Plate. Sp. tails — Sperm tails.
(G) Transverse section through a parapodium. Br. fil — Gill filament. Circ. mm. — Circular muscles, Coel. — Coelom, Int. — Intestine, Long. mm. — Longitudinal muscles. Neurop. — Neuropodium, No. chaet. — Notopodial chaetae, No. chaet. sac. — Chaetigerous sac., Noto. — Notopodium, V.M.L. — Ventral midline.
The proboscis is extruded by delivery of coelomic fluid under pressure, and is pressed into the sand, displacing it laterally. The anterior segments also become quite turgid and help to enlarge the burrow laterally. Then a wave of contraction passes posteriorly along the length of the animal and by means of the longitudinal muscles and the gripping action of the notopodial chaetae, it draws itself into the sand. The proboscis is again everted, and at the same time a wave of relaxation passes along the length of the body. As the worm attains a greater length, the cross-section is diminished and water moves rapidly down the burrow, bathing the gills and breaking up the tightly packed sand grains in the path of the worm. A further extension of the proboscis and subsequent wave of contraction draws the worm deeper.
Periodically a vertical channel is made to the surface, by a jet of water from the mouth. This, together with the waves of relaxation passing from the anus to the prostomium while the animal is resting, further irrigates the gills.
Observations showed that large local changes in body diameter employed in active burrowing were brought about by the combined action of the somatic longitudinal and circular muscle layers, and fluctuations in coelomic fluid pressure.
The proboscis was observed to have two distinct and independent functions: (a) feeding — a pumping action which actively admits water and sand; (b) digging — sand is laterally displaced, and none appears to enter the mouth.page 121
(A) Transverse section of tail on A — A of Fig. 1, C. Circ. mm. — Circular muscles, Coel. — Coelom, Conn. t. — Connective tissue, Dig. epi. — Glandular epithelium, Dors. B.V. — Dorsal blood vessel, Epid. — Epidermis, Ga, V. — Gastric vessel, Int. lu. — Lumen of intestine, Long. mm. — Longitudinal muscles, N.C. — Ventral nerve cord, Nev. B.V. — Neural blood vessel, Obl. mm. — Oblique muscle, Perit. — Peritoneum, Vent. B.V. — Ventral blood vessel.
(B) Vertical longitudinal section of proboscis, 0.5 mm. to left of midline. Bra. — Brain, B.V. — Blood vessels, Com. — Circum - oesophageal commissure, Dphm.1 — First septum, H. Coel. — Head coelom, Mth. — Mouth, Nuch. P. — Nuchal pore, Oes. — Oesophagus, mm. P. ret. — Retractor muscle of proboscis (gular membrane). Other abbreviations as for Figure (A).
(C) Transverse section of branchial region on B — B of Fig. 1, C. Br.8 — Gill VIII, Chloro. tiss. — Chlorogogenous tissue, Eff. aff. br. a. — Efferent and Afferent branchial arteries, Lat. B.V. — Lateral blood vessel, Noto.14 — Notopodium XIV, No. chaet. — Notopodial chaeta, Ooc. — Oocyte, St. lu. — Lumen of stomach. Other abbreviations as for Figure (A).
Specimens in seawater were anaesthetised by slowly adding 70% alcohol, and preserved in 5% formaldehyde solution. Serial sections were cut at 10μ on the microtome and stained with Heidenhain's haematoxylin and eosin. Observations of the external features and general behaviour were made on living specimens both in the field and in the laboratory.
The body is divided into an anterior chaetigerous region including the prostomium, a middle branchial region, and a posterior caudal region (Fig. 1, C).
The anterior region is made up of the prostomium or proboscis (Fig. 1. C, D — Pros.), and six chaetigerous segments — each segment consisting of the chaetigerous annulus (Fig. 1, C, D — Chaet. ann.) together with three preceding annuli, and one following annulus. Evidence for this is based on the position of the diaphragm (Fig. 3 —Dphm. 2 and Dphm. 3).
The proboscis is top-shaped when fully extended and covered by short papillae which extend into the oral zone (Fig. 1, D; Fig. 2, B — Pap.). The proboscis cannot be completely withdrawn as in the genus Arenicola.
The specimen sketched (Fig. 1, C) shows that proboscis in a fairly advanced stage of contraction, while in Fig. 1, D, the proboscis is fully extended. A large nuchal pore (Fig. 1, D — Nuch. p.) is located on the mid-dorsal line just behind the proboscis, and opens into a cavity lined with glandular cells (Fig. 2, B — Nuch. p.). The chaetigerous section possesses six neuropodia (Fig. 1, C, G — Neurop.), appearing as transverse slits extending from beneath the notopodia to within about 2 mm. of the ventral mid-line. Each notopodium bears a double row of notopodial chaetae (Fig. 1G, No. chaet.). These are stiff, bristle-like structures which may be articulated in an anterior-posterior direction so as to aid in movement of the animal. A small hooded nephridiopore (Fig. 1, C — Neph. p.) is located just below the fourth to the ninth notopodium on each side. The epidermis of both the anterior chaetigerous and the branchial regions consists of raised polygonal areas of mucus-secreting columnar cells separated by shallow grooves (Fig. 2, C —Epid.).
The brancial region, in addition to neuropodia. notopodia, and setae, is distinguished by the presence of thirteen pairs of contractile gills (Fig. 1, C— Brl -Brl3), which are attached just behind each notopodium. The front pair of gills is always smaller than the rest but still has the same well-developed feathery structure. Both the page 123 gills and parapodia (notopodium and chaetae) will retract on application of strong light, interference or immersion in a foreign non-anaesthetic medium.
The neuropodia in this region are separated from the strongly marked ventral mid-line (Fig. 1, C — v.m.l.) by a distance roughly equal to the length of the neuropodia themselves (Fig. 1, G).
The caudal region is typified by the absence of any specialised external structures (Fig. 1, C — Tail). The tail is very variable in length. Living specimens were found with twenty to thirty tail segiters, while others had as few as two or three. It is probable that specimens with very short tails have at some time been damaged by predators.
The worm brings its anus to the surface of the sand in order to defaecate, and consequently the tip falls easy prey to flounders, or other bottom feeding fish and to the hundreds of herring gulls patrolling the beach at low water. Many gulls were observed to pounce on defaecating worms and plunge their beaks into the burrow. In general, the tail segments are longest in the region of the anus.
Dissection was performed on a freshly-killed specimen which had been previously relaxed in alcohol so that the pressure of the coelmic fluid would not force the internal organs through the primary incision in the body wall. The body cavity was opened by a mid-dorsal longitudinal incision from the proboscis to a short distance down the tail, and the flaps of the body wall were pinned back (Fig. 3). The gut was slightly displaced to the left and pinned.
The coelom is very spacious, and extends from one end of the body to the other. Anteriorly the coelom is partially transversely divided by the retractor muscles of the proboscis (Fig. 3 — mm. p. ret.) and more completely so by three septa (Fig. 3 — dphm. 1), which arise from the groove behind the first annulus posterior to each of the first three chaetigerous annuli. The septa are perforated to allow passage of the coelomic fluid so, in effect, the continuity of flow of the coelomic fluid is not appreciably interrupted. Dorsal and ventral mesenteries support the oesophagus and anterior portion of the gut from the first to the third septum.
From the third septum to the base of the tail, the body cavity is undivided. No mesenteries support the gut which is slightly larger than the portion of coelom in which it lies, and it tends to swing freely with movements of the body. Segmental blood vessels, however, give some elastic support to the mid-gut. Arrangement of repeating organs such as nephridial funnels (Fig. 3 — Neph. f.) and somatic segmental afferent and efferent blood vessels, indicate the segments in this body region.
From the base of the ‘tail’ region to the anus, transverse septa (Fig. 3 — T. dphm.) divide the body cavity into clearly recognisable segments.page 124
The musculature of the body wall is arranged in an outer circular band (Fig. 2, C — Circ. mm.) and a thick inner longitudinal muscle sheath (Fig. 2, C — Long. mm.) which is divided into one dorsal and two ventrolateral sections by the left and right oblique muscle layers (Fig. 2, C — Obl. mm.). The oblique muscles, which divide the coelom longitudinally into three compartments, commence behind the third septum and lose their identity after the first few ‘tail’ segments.
The retractor muscle of the proboscis (Fig. 2, B — mm. P. ret.) is a very thin, weakly developed modification from the longitudinal muscle layer. The retractor muscles of the parapodia (Fig. 3 — mm. Para. ret.) are also derived from the longitudinal muscle, and they extend almost to the mid-ventral line. The parapodial proctractor muscles (Fig. 3 — mm. Para. prot.) are usually eight in number — four each side of the chaetigerous sac, and these muscles move the parapodium in an antero-posterior direction.
The proboscis lined with vascular papillae (Fig. 1, D, and Fig. 2, B — Pap.) which trap sand grains and draw them into the mouth when the proboscis is withdrawn.
An oesophagus interiorly lined with glandular epithelium and possessing up to twenty-one oesophagael sacs (Fig. 3 — Oes. caec.) opening into the oesophagus by wide ducts.
These caeca or sacs are arranged in two longitudinal rows, one on each side of the oesophagus, and in fresh specimens are red in colour owing to their large blood supply. The anterior pair of caeca are always much larger and longer than the following caeca. The most posterior pairs are in general the smallest of all. One specimen was found to have only eleven pairs of oesophageal caeca and the largest number noted was twenty-one. Figure 3 illustrates a specimen with nineteen pairs of caeca.page 125
Abarenicola dissected to show major features of internal anatomy. Ant. oes. caec. — Anterior oesophageal caecum, Br. aff. — Afferent branchial vessel, Br. eff. — Efferent branchial vessel, Br. Nn. — Annulus branch nerve, Chloro. tiss. — Chlorogogenous tissue, Dors. B.V. — Dorsal blood vessel, Dphm. — first 2 anterior septa, Long. mm. — Longitudinal muscle, mm. Para. prof.— Parapodial protractor muscles, mm. Para. ret. — Parapodial retractor muscles, Mth. — Mouth, Neph. f. — Nephridial funnel, (Nephrostome) Neph. sac — Nephridial sac, N.C. — Ventral nerve cord, New. B.V. — Neural blood vessel, Oes caec. — Oesophageal caeca, Para 1 -Para XIX — Parapodia, Phar. — Pharynx, Seg. eff. B.V.— Segmental efferent blood vessel, T. dphm. — Tail septum, Vas. gut. ret. — Vascular reticulum of gut, Vent. B.V. — Ventral blood vessel.
The stomach region (Fig. 3 — Stom.) extends from the level of the heart to the base of the tail where it merges into the intestine. As previously discussed, the stomach has no attachments to the body wall other than the afferent and efferent blood vessels in each segment. The stomach is lined with glandular epithelium and the exterior is a striking mustard/yellow colour from cholorgogenous cells arranged in broad patches, the latter, however, becoming smaller towards the tail. The stomach is richly supplied by blood vessels forming a reticulum (Fig. 3).
The intestine is brownish in colour with a poorly developed blood supply in comparison to the stomach. It is supported by numerous septa (Fig. 3 — T. Dphm.) as well as a ventral and dorsal mesentery. It opens at the terminal anus.
The general features only of the blood system of Abarenicola assimilis are noted here.
There are two chief vessels — one above and one below the alimentary tract, running from one end of the body to the other (Fig. 3). The gut is served by a reticulate system of capillaries in turn which are connected via a pair of red bulbous hearts (Fig. 3 — H.) to a ventral vessel. The dorsal vessel originates near the anus and gives off a pair of segmental vessels (Fig. 3 — Seg. B.V.) at the beginning of each caudal segment. This blood is collected by the ventral vessel. In the gastric region, the blood from the last seven pairs of gills returns by way of afferent vessels to the dorsal vessel, but anteriorly from the level of the sixth gill the dorsal vessel receives numerous branches from the gastric reticulum, but no efferent branchial or nephridial vessels. These are returned via the gastric reticulum. In front of the heart the dorsal vessel receives branches from the oesophagus and each oesophageal caecum, and after many branches are received from the brain it finally disappears in the tiny capillaries of the proboscis.
The ventral vessel is large and covered by chlorogogenous tissue (Fig. 2, C — Chloro. tiss.) and sends efferent branches to the brain, oesophageal caeca, each of the gills, and the caudal septa. It does not appear to have any direct connection with the ventral part of the gut other than through the heart. Other vessels present are lateral vessels supplying the body wall (Fig. 2, C — Lat. B.V.), neural vessels lying each side of the nerve cord along the length of the body (Fig. 2, C; Fig. 3 — Neu. B.V.), and efferent and afferent branchial vessels shown in the section of the gills (Fig. 2, C). The blood flow is: The heart draws blood from the gastric vessels into the highly elastic ventral vessel, from whence it circulates through the body.
Nervous System and Sense Organs
The nervous system consists of the brain (Fig. 2, B — Bra.), the circum-oesophageal commissures (Fig. 2, B — Com.) and the ventral page 127 nerve cord (Figs. 2, B, C — N.C.). The brain is a small, oval-shaped, richly vascularised knot of nervous tissue situated just anterior to the nuchal organ on the dorsal aspect. The circum-oesophageal commissures originate from the brain and form the ventral nerve cord from their ventral convergence. As far as could he traced, the ventral nerve cord gives off a pair of nerves in each annulus of the body, thus giving each segment five pairs of branch nerves (Fig. 3 — Br. Nn.). The ventral nerve cord is not obviously ganglionated.
In close apposition to the brain is the nuchal sense organ (Fig. 1, D; 2, B — Nuch. P.). Dorsally it appears as a dark pit divided by a single median papilla (Fig. 1, D) and in longitudinal section the cells are large and distinct (Fig. 2, B). The parapodia and gills are extremely sensitive to light and touch.
There are six pairs of nephridia in all (Fig. 3 — Neph.) located in the fourth to the ninth chaetigerous segment. They are quite large and readily seen. Each nephridium consists of a funnel opening into the body cavity, an elongate central portion, and a small end sac, which opens to the exterior through the nephridiopore situated just below the notopodium. Each nephridium is supplied with an efferent and afferent blood vessel.
Gonads are present in conjunction with each of the six pairs of nephridia in the breeding season. The oocytes and spermatoblasts break free to complete their development in the coelomic cavity. On April 1, 1961, a few worms had liberated their gonadial contents into the coelom, and by August 1, 1961. the breeding season was in full swing. Spermatozoa are aggregated in plates up to 0.75 mm. in diameter until fully mature (Fig. 1, F, 1, 2). When fully mature, the individual spermatazoa break free. Each measures about 1/200 in. in length. When spawning, ripe ova surrounded by a thin membrane, or spermatazoa, are drawn by the action of the ciliated nephrostomes into the nephridia. and liberated through the nephridiopores into the seawater, where fertilisation takes place.
At fertilisation (which was achieved in the laboratory) the diameter of a mature ovum was found to be approximately 0.25 mm. (Fig. 1, E). The nucleus of the ovum is conspicuous and spherical and the cytoplasm is very granular.
After fertilisation the zygote shrinks within the fertilisation membrane and then undergoes a spiral system of cleavage to form a morula.
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——, 1951. How Lugworms Move, The Cell and the Organism, Cambridge University Press, pp. 209-33.