PHYLUM BRACHIOPODA

(Summary from University of Florida, Zoology 6207, Spring 1982)

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I. Historical account

A. Fossils known in the early middle-ages.
B. Brachiopods were figured in the late 16th century, under various names.
C. Linnaeus (1758) names them Anomia and places them in the Vermes.
D. Pallas (1766) and Grtlndler (1774) give the name "terebratule".
E. Cuvier (1798) places them in the Class Mollusca, then in a separate Class, the Brachiopoda, "with affinity to molluscs".
F. Huxley (1869) rejects the hypothesis of molluscan affinity. At this time, two theories exist : annelid vs. molluscan affinity.
G. Blochmann (1892) assembles phoronids, brachiopods, ectoprocts and sipunculids together.
H. Schneider (1902) proposes the name Lophophorata, without including the brachiopods.
I. Brachiopods have now the status of a Class or of a Phylum. There are good classifications by Dall (1920) Williams al (1965).

II. Definition of the phylum (or Class)

Solitary, sessile, coelomates with a bivalve shell and a mantle. Fixed to the substrate by a pedicle, or by the cementation of one valve to the substrate itself, or burrowing with an elongated pedicle sheathed in sand. They have a lophophore, an open circulatory system with a dorsal contractile vesicle, and one or two pairs of metanephridia.

III. External features

A. 5 to 80 mm in size (some extinct forms reach 375 mm)
B. Dull yellow or grey in color; some forms are orange or red, or transparent.
C. Shell

1. Two valves bilaterally symmetrical and convex in shape. Dorsal (brachial) and ventral (pedicle) valve, with fine ornamentations (ridges, spines), to which the dorsal and ventral mantle lobes are attached.
2. The mantle secretes the shell, which consists of two layers of different importance in the two groups of brachiopods : a CaCO3 layer or periostracum and an organic, prismatic layer of Ca3(PO4)2
3. Porosity : uniformly distributed vertical channels extend upward through the shell to a point below the periostracum. Channels are filled with mantle tissue, terminating in a cluster of secretory cells (punctate type). Impunctate and pseudopunctate types bear different dissolution properties of the calcitic rods in the shell.
4. Valves dissimilar, the ventral valve larger and shaped to allow for the passage of the pedicle. The fine structure of the opening has taxonomic significance.
5. In burrowing forms, the valves are flattened and more equal in shape.
6. Paleontologists have created a complex nomenclature for the valves.
7. Articulation of the valves along the posterior line of contact (hinge line). The ventral valve bears two hinge teeth fitting into opposing sockets in the dorsal valve. Hinge teeth may be supported by a vertical dental plate.

D. Pedicle :

1. Very diverse in structure, development, and relationships to valves between the two groups.
2. The wall is composed of a thick cuticle, a monostratified columnar epidermis, connective tissue, a muscle layer, and a peritoneum.
3. Lumen in the central part.
4. Rootlike extensions or papillae at the end for adherence to the substrate.
5. The pedicle has been lost in some forms of both classes.

E. Lophophore :

1. Complex filtering apparatus consisting of a tentacular crown surrounding the mouth, occupying two thirds of the space inside the mantle cavity. Suspended from the brachial valve.
2. Two horse-shoe-shaped or spiraled arms, the brachia, project anteriorly. Each of them bears a row of tentacles of varying complexity (phylogeny and ontogeny):

trocholophous type: very short lobes
schizolophous type: a lobed disc with relatively few marginal filaments
ptycholophous type: more numerous indentations
zygolophous type: increase in length of the filaments
plectolophous type: more extended lateral lobes and a coiled median lobe
spirolophous type: no median lobe, but elongated, coiled lateral lobes.

3. Ciliated surface maintaining incurrent and excurrent flows in the mantle cavity.
4. Brachial groove at the base of the tentacle-bearing ridge.
5. The lophophore is supported by a carthilaginous axis and spicules and a fluid-filled canal. Often the dorsal valve bears complex supporting processes with grooves and ridges for the reception of the lophophore.

IV. Internal morphology

A. Body wall

1. Cuboidal or columnar epithelium
2. Secretes the shell and produces the mantle papillae in punctate shells.
3. Attached dorsally and ventrally to the inner surface of the shell
4. Layer of connective tissue.
5. Where the body is free of the shell, a layer of longitudinal muscle-fibers.
6. Internally lined with peritoneum.

B. Mantle

1. Bears long chitinous setae, with probable protective and sensory functions, on the inner surface.
2. Since mantle lobes are extensions of the anterior body wall, each of them is composed of a double wall with coelom in between.
3. Has mucous cells.

C. Coelom : similar to that of other lophophorates

1.Divided longitudinally into two compartments by a mesentery.
2. Divided transversally into protocoel (which disappears during development), mesocoel, and metacoel. The latter forms the main body cavity.
3. Extends into the pedicle lumen and into mantle lobes (mantle canals), as well as into the lophophore.
4. Has free coelomocytes, sometimes hemerythrin.

D. Digestive system

1. Mouth at the base of the lophophore.
2. Esophage extending dorsally and forward, then turning posteriorly.
3. Stomachal pouch or "liver", a bunch of digestive glands for intracellular digestion.
4. In Articulata, a blind intestine extends posteriorly. In Inarticulata, intestine opens through a rectum and anus.

E. Excretory system

1. One or two pairs of metanephridia.
2. A funnel-shaped nephrostome opens into the metacoel, then ends in a posterior nephridiopore. The nephrostome is glandular and ciliated.
3. Coelomocytes (amoeboid cells) ingest waste particles.

F. Musculature is complex :

1. 1 pair of adductor muscles, striated or smooth, close the valves
2. 1 pair of abductor muscles open the valves.
3. Muscles are attached to the shell by means of an altered epidermis of stiffened, striated cells containing tonofibrils. Muscle scars on valves.
4. Pedicle is muscular in Inarticulata. In Arriculata, muscles within the valves, inserted on the pedicle base, allow for flexions and rotations.

G. Circulatory and respiratory system

1. Closed system with one (or more) contractile cardiac vesicle(s) in the dorsal mesentery
2. From the "heart", anterior and posterior channels. The anterior lead to sinuses in the wall of the digestive tract, then to the arms of the lophophore and to the tentacles. The posterior channel irrigates the mantle, the gonads, and the nephridia.
3. Separated from coelomic channels.
4. Vesicle is contractile, but the role of this vascular system is not well known. It possibly involves circulation of food material
5. No particular gas exchange organ are present. The lophophore and the mantle seem to be sites of gas exchanges, since they develop a large surface. Probably the coelomic fluid circulating in the lumina is more important in respiration than the blood circulation.

H. Nervous system and sense organs

1. Esophageal nerve ring and "cerebroid" (supraenteric) ganglia, plus a subesophageal ganglionic mass.
2. From there, anterior and posterior nerves lead to the lophophore, mantle muscles.
3. In articulates, the nervous system is closely associated with the epidermis, and all brachiopods have a subepidermic nerve plexus.
4. Two palleal statocytes with numerous vibrating statolithes.
5. Sensory setae on the mantle margins.

I. Reproductive system

1. Dioecy, though some species of Argyrotheca are hermaphroditic, sometimes protandric.
2. Sexual organs are 4 proliferations in the metacoelomic peritoneum.
3. Little sexual dimorphism, except that ripe male gonads are white or pale yellow, while female gonads are deep-yellow or red-brown.
4. At maturity, genital products pass into the coelom and are expelled through the nephridia, which thus act as Ronoducts.
5. Some brachiopods brood youngs up to the free-swimming larval stage in a median brood pouch.,?lhis pouch may occupy a depression of the ventral valve, with consequent sexual dimorphism.
6. Fecundation is external in most forms, and internal in the ones that have brood pouches.
7. Some species have a breeding-season, while others breed throughout the year.

V. Embryology

A. Clusters of freely-shed eggs, enclosed in mucus, are often fertilized in the water.
B. Holoblastic, subequal, radial cleavage, forms a coeloblastula. The latter gastrulates by invagination, and in one case by delamination.
C. In contrast to the mesoderm formation in protostomes, e? occurs in Inarticulata. Mesoderm may proliferate from the archenteron and later from a coelomic cavity, or arise as pouches to form the archenteron, then divide as in echinoderms (enterocoely)) in Articulata.
D. The blastopoa closes and elongates in a groove from which the stomodaeum originates. Then the embryo divides into two (inarticulates) or three (articulates) regions, separated by a mantle fold.
E. Dorsal and ventral mantle folds separate.
F. A median tentacle is sometimes formed, from which the tentacles of the lophophore originate.
G. Free-swirmning, planctonic larva. Feeding occurs. Locomotor function of the lophophore, coiled pedicle. Formation of the shell causes the larva to sink. This larva, of the rrocophore type, is similar to that of deuterostomes. It bears no divisions of the coelom, which is continuous with the cavity of the lophophore.
H. The settled larva develops only gradually adult traits, without metamorphosis. Its tentacles develops, and coelom extends into them. Ontogenic differences occur in the structure of the lophophore, the skeletal elements, the pedicle opening, the mantle folds.

VI. Ecology and physiology

A. Marine, mostly within the shelf, but some forms are abyssal.
B. Benthic : sessile or burrowing. Substrates: rock, shells, algal stems, soft sediment.
C. Not colonial but tend to aggregate intraspecifically and interspecifically, which seems to depend on the larval settlement.
D. Inhabit largely the cold waters, but are present in all latitudes. Usually highly endemic.
E. Feeding biology

1. Feed on fine phytoplankton (diatoms), and dissolved and colloidal material.
2. Adjustable ciliary current created by the lophophore ciliation. Distinct inhalant and exhalant apertures. Particles are trapped on the filaments and converge in the groove down to the mouth. There is mucus, but it seems not to play a major role here.
3. In burrowing species, the mantle setae prevent fouling by the sediment. Mucus is secreted by the gland zone of the mantle lobes.
4. Some species can reverse the water current when particles accumulate within the lophophore.
5. Ingestion is controlled by peristaltic movements of the esophagus and stomach.

F. Predators : fish, men.
G. Parasites : gregarines.

VII. Classification

A. 280 species
B. Ecardina or Inarticulata schizocoelic, oviparous, no hinge, ? ,subequal valves hold together by the muscles only. No brachidium. Direct development of the mantle without reversal. Muscular, retractable peduncle. Vertical burrows in sand or mud. Valves of chitinophosphatic type.

C. Testicardina or Articulata :

Order Rhynchonellacea : spirolophous, with skeletal support. Two pairs
of nephridia.
Order Terebratulacea : short supporting loop in the lophophore.
Order Terebratellacea : long, recurved loop, plectolophous type
Order Terebratellidae : complex, long loops.
Examples : Terebratula, Magellania, Megathrys, Terebratulina.

VIII. Phylogeny

A. Brachiopods are the last of three lophophorate classes (phyla)
B. Originate in the Paleozoic Era. Peak of differenciation in the Devonian, with more than 200 genera, 30,000 species. Then supplanted by other forms, notably bivalves.
C. The bivalved condition suggested molluscan affinities, bur the soft anatomy is very different. The valves are dorsal and ventral, not lateral as in Mollusca.
D. Inarticulates are more primitive, but both groups are present in the earliest fossil records.
E. The common presence of the lophophore of similar anatomy and histology and the body plan proves affinity with Phoronida, Ectoprocta.
F. The trocophore type of larva relates brachiopods to the protostomes, despite the enterocoelous formation of the coelom. Affinities with deuterostomes, and in particular with hemichordates, are the division of the body region in three parts. Brachiopods are therefore considered as the connecting link between protostomes and deuterostomes.

IX. References

Gosner K.L., 1971 : Guide to the identification of marine and estuarine invertebrates : Cape Hatteras to the Bay of Fundy. Wiley-Interscience, NY. pp 245-248.

Grasse P.P., 1959 : Precis de Zoologie. Masson, Paris.

Hyman, L.H., 1959 : The Invertebrates, Vol V. McGraw-Hill, NY. pp 516-609.

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