Biology of Oyster

By
admin
– October 10, 2010Posted in: Study

Biology of Oyster

Problem Area: Discovering Oyster Biology

Goal: The goal of this problem area is to become familiar with basic oyster biology to be used in
oyster aquaculture.

Learning Objectives: Upon completion of this problem area, students will be able to: describe
taxonomy, morphology, anatomy, reproduction, recruitment, and life cycle of oysters; identify
commercially important oyster species

Introduction

Oysters are bivalve mollusks that exhibit a variety of sizes, shapes, shell textures and colors, and
vary in their mode of reproduction and sexual expression. These biological and physical features
influence such aspects as where they grow and how they reproduce, which in turn influence
commercial aspects such as culture practices and marketing strategy. Individual oysters conform
to the shape of the substrate to which they are attached and are therefore highly variable in shape.
Shape is also influenced by other oysters or substrates pressing on their shells. Shell shape,
texture, and color are all influenced by the oyster’s genetic makeup and the physical environment
such as salinity, attachment substrate, and food. They feed on phytoplankton and nutrient-
bearing detritus by pumping water over their gills, which filters the food material and passes it
into the oyster’s mouth.

Oysters have been eaten by coastal people since before recorded historical times. They are a
staple of many cultures and a gourmet item of many others. Some cultures even consider them an
aphrodisiac. An understanding of basic oyster biology is essential to any successful culture operation.

1. Taxonomy of Oyster
Classification of Oyster

Phylum: Mollusca

Class: Bivalvia (Pelecypoda)

Order: Pseudolamellibranchia.

Family: Ostreidae

Genus: Crassostrea and Ostrea

1. Oysters belong to the phylum Mollusca, class Bivalvia, and family Ostreidae.

2. The two most commonly cultured types are the cupped oysters, genus Crassostrea, and the flat
oysters, genus Ostrea.

Types of Oysters

Table 1. There are two major types of oyster, cupped oyster and flat oyster.

Cupped oysters Flat oysters

Genera: Crassostrea Genera: Ostrea.
Species: Species:

Crassostrea gigas (Pacific or Japanese) Ostrea edulis (European flat oyster)

C. angulata (Portugese oyster) O. chilensis (Chilean oyster)

C. virginica (American oyster) O. lurida (Olympia oyster)

C. eradelie (Sydney rock oyster) Pinctada maxima (Mexican oyster)

C. commercialis (Sydney rock oyster) Pteria penguin (Winged pearl oyster)

C. rivularis (Chinese oyster) P. sterna (Winged pearl oyster)

C. plicatula (Chinese oyster) P. margaritifera (Black-lip pearl oyster)

C. giomerata (Auckland oyster) P. sterna (Rainbow-lipped pearl oyster)

C. eradelie (Slipper oyster). Placenta placenta (Windowpen oyster)

 Fig. 1. Pinctada sp.

N.B.:

Fig. 2. From left to right, at the top, Sydney rock oyster (66–86 mm shell height), coral rock oyster or milky oyster (59–79 mm), and flat oysters
(71–77 mm) and at the bottom, black-lip (73–97) and Pacific oysters (76–104 mm).
 Fig. 3. Valves of blacklip pearl oysters, Pinctada margaritifera mazatlanica, from the Archipielago de las Perlas, Panama. Small
oyster spat, species unknown, are attached to the largest shell, measuring 13.5 cm across.

Fig. 4. Black pearl.

B. External anatomical features of oysters

1. All oyster species exhibit the same general anatomical orientation.

2. Since they are bivalves, they have 2 shells hinged together. The hinge is located at the anterior
end of the animal so the shell opens posteriorly. The left shell is normally the more curved or
cupped of the two. The left shell is also the one normally attached to the substrate.

3. Shell is made up of 3 layers, all produced by the mantle. The outermost layer is a thin
proteinaceous covering called the periostracum (peri = around; ostracum = shell). This can best
be seen around the hinge area. The middle layer is the prismatic layer composed of calcium
carbonate. The innermost layer is the nacreous, or mother-of-pearl layer.

4. Located at the hinge is the umbo, the portion of the shell formed first. Growth lines surround
the umbo concentrically. These lines are similar to those found in trees. These lines are not put
down uniformly. Many factors such as temperature, food, stress, and disease affect the oyster’s
ability to produce shell.

5. The hinge is made up of two parts:

a. Flexible ligament, which is the axis of movement for the two shell halves.

b. Resilium, which is a pad between the shell pushing the shells apart. This is needed because the
muscle in the oyster can only pull the shells together, not push them apart.

External Anatomy of Oysters

C. Internal anatomy

1. The mantle of the oyster is the colored portion closest to the ventral and posterior edges of the
shell. It covers the internal organs and may also function to hold gonadal tissue during reproduction. Its primary
function is to secrete the layers of the shell.

2. The single large adductor muscle is centrally located. Its function is to close the shell or keep the shell
closed. It is comprised of 2 regions, the quick and the catch. The quick is made of striated muscle and closes the
shell by contracting or by relaxing, allowing the resilium to push the shells open. The catch is made of smooth
muscle and holds the shells shut for a long period of time.
 Fig. Pearl in situ in dissected back-lip oyster. Photo: Idris Lane

3. Gills: The gills run roughly parallel to the mantle from the ventral margin to the middle of the
visceral mass. Although the gills function for respiration, they are better suited for capturing and sorting food.
They are heavily ciliated and produce large quantities of mucous. Food particles are trapped in the mucous and
moved by ciliary action to the labial palps.

4. Labial palps: The labial palps are next to the dorsal end of the gills where final sorting and
selectiing food particles takes place. Food particles move to the mouth, which is located at the extreme dorsal
attachment of the labial palps. Food then passes through a short esophagus and into the visceral mass.
 Fig. 5. An oyster viewed from the right side with the right valve removed but with the right mantle skirt still in place (from Galtsoff,
1964).

5. The visceral mass lies under the mantle, dorsal to the gills, and anterior to the adductor
muscle.

a. It is composed of the stomach, crystalline style, digestive gland, intestine, kidney, and gonad.

b. The food travels from the esophagus into the stomach where further sorting and digestion occur.

c. The crystalline style, a gelatinous rod, projects into the stomach and stirs the contents as well as
produces digestive enzymes to aid digestion.

d. The digestive gland is dark green and further digests food particles. The intestine Ioops several times in
the visceral mass and terminates at the anus.

e. Waste particles are carried through the intestine and expelled from the anus into the excurrent opening.
The kidney functions to rid nitrogenous wastes from the oyster.

f. The gonads occupy the dorsal-most area of the viscera and adjacent parts of the mantle. Eggs or sperm
are produced here for reproduction.

6. The heart is in a pericardial cavity immediately dorsal and anterior to the adductor muscle. It
is composed of one auricle and one ventricle. The heart circulates the blood through the gills and
other organs and body parts.
7. The nervous system is simple, consisting of two paired ganglia.

a. The major sensory area is located along the edge of the mantle where single photoreceptor cells are
located in the epidermis.

b. These cells can sense light and dark.

D. Feeding and Breathing in Oysters

Process of water flow and feeding in Oysters.

1. Oyster are filter feeders. They feed on microscopic algae or phytoplankton.

1. Water enters the oyster on the ventral side.

2. The oyster pumps water through the incurrent opening, allowing water to flow over the gills dorsally and
anteriorly toward the mouth.
3. In the ventral portion of the gills are partitions between the gills that form channels guiding the water
through the oyster. The water then flows posteriorly and dorsally, exiting the excurrent opening on the
dorsal side.

2. The water going through the oyster contains oxygen, which is taken up by the gills, absorbed
into the blood, and distributed to necessary organs and body parts. Food particles are processed
in the same manner.

3. As food particles enter the oyster, they are trapped by the gills. The gills are ciliated and
produce large quantities of mucus.

1. Food particles are moved by ciliary action toward the labial palps.
2. Particles that are too large or unsuitable for digestion are coated with mucus and periodically discharged
out the incurrent opening by a sudden, strong contraction of the adductor muscle.
3. The internal faces of the labial palp have folds where the final sorting of food takes place.
4. Heavier particles settle into the grooves formed by the folds, where they will be expelled.
5. Lighter particles flow over the ridges of the grooves to the mouth. Food then passes through the esophagus
into the stomach.
6. The stomach wall is also grooved to provide a greater surface area for intracellular digestion.

4. Another form of digestion, microphagy, is performed in the stomach by wandering

amoebocytes.

1. The crystalline style projects into the lumen of the stomach and performs extracellular digestion.
2. The style stirs the contents of the stomach and releases a digestive enzyme called amylase which breaks
down carbohydrates.
3. As the finer food particles enter the digestive gland, tubules made up of phagocytic cells ingest them and
complete the digestive process.
4. Waste products and undigestible particles move through the intestine and out the anus where they are
expelled out the excurrent opening.
 Water Flow and Feeding Process in Oyster

E. Reproduction in Oysters

1. Oysters are protandrous. They can change their sex between spawning seasons (Crassostrea)
or even within seasons (Ostrea).

a. Generally they develop first as males, then change to females.

b. This adaptation occurred probably because sperm cells are smaller than eggs and can be
produced efficiently by smaller animals, whereas eggs are produced more efficiently and in large
enough numbers by larger animals.

c. A female oyster can produce 1-10 million eggs depending on size.

2. Gradually warming water temperatures cause oysters to start production of gametes, or ripen.
Spawning occurs when oysters are ripe and water temperatures are optimal. Water temperatures
vary between species.

3. In Crassostrea, fertilization and larval development take place externally.

a. Both males and females release their gametes into the water column where fertilization occurs.

b. After fertilization, the larvae develop as plankton, then settle to the substrate as post-larvae.

4. In Ostrea, fertilization and larval development take place internally.

a. Once the male releases sperm cells, the female draws them into the pallial cavity near the gills
where the eggs are fertilized.

b. Egg development and hatching occur there and further larval development may take up to 3
weeks.

c. The larvae are then expelled from the female into the water column.
F. Life Cycle

Show TM B5 and discuss the life cycle of oysters.

1. After the oyster egg has been fertilized by the sperm, larval development takes place. On an
average, development takes about 3 weeks, depending on temperature.

2. The first stage of development occurs within hours of fertilization. The trochophore larvae
have cilia at one end that make them motile. Although they can move, they are still planktonic
and subject to the whims of the currents.

3. Within about 4 days, the next larval stage is reached. The larvae assume a characteristic “D”
shape, and become veliger larvae.

1. The veligers have complete digestive systems and eat voraciously to aid their development.
2. The veliger stage lasts about 15 days.

4. The larval oysters now resemble adults and are sometimes referred to as eyed larvae. They
have an eye spot that is light sensitive and aids the larvae in dropping from the water column to
the bottom.

1. At this point they will settle to the bottom and search for a suitable place to attach themselves.
2. The material to which they attach is called cultch.
3. The attached oyster is referred to as spat. Spat will attach to most hard surfaces, but prefer a calcareous
material.

5. Upon finding a suitable site to spend the rest of their lives, the larvae will crawl along the
material with their foot to survey the surroundings.

1. Once the exact location of attachment is chosen, the larvae will lay on their left (cupped) shell and hold on
with the tip of their foot.
2. The byssus gland is located at the tip of the foot and is responsible for secreting the cement that will hold
the oysters permanently to the substrate.

6. After adhesion, the foot and eye spot are adsorbed because the oysters will never need them
again.

1. Shell formation and growth are now the main concerns of the spat.
2. Within about 3 months the spat will reach the size of a dime.

7. If food supply and growth conditions are favorable, the oysters will reach sexual maturity in
about a year.

1. The growth rate is highest for the first 3 years, after which it tapers off. The highest fecundity occurs
between the 4th and 7th years, after which it again tapers off.
2. The life cycle is now complete and begins again every year.
Life Cycle of the Oyster
 Figure 1. Life cycle of the eastern oyster, Crassostrea
virginica.

G. Biological requirements of oysters

Variance of water quality requirements between different oyster species.

1. The water that oysters live in must meet certain quality requirements such as temperature,
salinity, and turbidity. When water quality does not meet the individual tastes of oysters, they
will usually shut themselves

tight and wait until conditions improve.

2. Temperature requirements of individual oyster species vary widely.

1. Temperate zone oysters can range from 4 to 30°C, whereas tropical zone oysters can range from 15 to
35°C.
2. The temperature at which most oysters start to spawn is around 20°C.

3. Salinity requirements also vary widely. Ranges of salinity tolerance are from below 10 ppt to
40 ppt. Optimum salinities range from 15 to 35 ppt.

4. In general, the tropical species are more tolerant of silty or turbid water than temperate. Silt
poses problems because it plugs up oysters’ gills and is difficult to sort from food particles due to
their small size.

5. Oysters’ food consists mainly of microscopic algae: flagellates and diatoms.

1. Larval oysters need smaller food particles than adults so their choices of phytoplankton vary.
2. Some common genera of flagellates that oysters eat include isochrys and monochrysis. Some common
genera of diatoms include chaetoceros, dunaliela, and phaeodactylum.

H. Diseases and Parasites

 Predators of Oysters

1. Pathogens, parasites and diseases of pearl oysters Pinctada maxima Northern Australian
waters: (Humphrey et al., 1998?)

A health survey, based primarily on gross and histopathological examinations of pearl oysters
Pinctada maxima, was undertaken between 1994 and 1997 from Queensland, Northern and
Western Australia. The study included 4767 mature animals with no history of disease from wild
harvest and pearl culture farms, together with batches of spat for interstate movement and cases
of diseased mature and juvenile oysters.

The study was undertaken to improve knowledge on diseases confronting the pearl oyster
industry, to facilitate regional and national quarantine, to enhance diagnostic capabilities and to
identify pathogenic agents for further investigation. The study established the occurrence,
prevalence and distribution of a taxonomically diverse range of microbial, protozoan and
metazoan agents associated with pearl oysters and evaluated the pathogenic significance of these
agents. Over 57% of the mature oysters were normal and free from infectious agents, whilst
many carried agents not considered significant pathogens.

Pathogenic or potentially pathogenic agents identified in apparently normal P. maxima included

a papovalike virus of the palp, viral-like inclusion bodies in the digestive gland epithelium,
rickettsiales-like agents in the digestive gland and gill, enigmatic protozoan-like bodies in the
digestive gland, metazoa including copepods in the digestive gland, a copepod Anthessius
pinctadae in the oesophagus and a Haplosporidian sp. in the digestive gland. Bivalve molluscs,
sponges and polychaetes commonly invaded the shell matrix.
Vibrio sp., the enigmatic protozoan-like agent and sub-optimal environmental conditions were
associated with mortalities in mature and juvenile oysters. Differences in regional occurrence
were evident with some agents, providing a basis for implementation of quarantine.

Normal histological criteria for P. maxima were established and host responses to injury
described, providing a basis on which the normal structure of the pearl oyster may be
differentiated from the structure altered by disease.

The study indicates that Australian P. maxima are relatively free of serious pathogens. At the
same time, a need exists to clarify the taxonomic status and establish the pathogenic significance
of a number of the agents recorded. The study provides baseline data on the occurrence and
prevalence of potential pathogens and provides a basis for the diagnosis of infectious and non-
infectious diseases of P. maxima.

2. Cliona: An enemy of the pearl oyster, Pinctada maxima in the west Australian pearling
industry (Moaseet al., 1999)

With the next millennium fast approaching, and an ever-increasing change in aquaculture
technology, there are many issues that affect the successful operation of pearl oyster farms, and
pearl production in the Southern Hemisphere. One such issue, facing the future of most, if not all
farms, is a substantial increase in the presence of Cliona (phylum Porifera), a boring sponge
which penetrates the outer prismatic and inner nacreous layers of the pearl oyster Pinctada
maxima, resulting in high mortalities over a relatively short period of time. The term ‘parasite’ is
used frequently to illustrate the lifestyle of Cliona, however, its structure and physiology is
consistent with all other free-living sponges.

Visual evidence indicates that Cliona displays a preference to infestation of larger pearl oysters,
many of which have entered the operation phase of their lifecycle on the farm. However, due to
the rapid growing phase of juvenile oyster shells, the sponge may still be present, but not appear
to have penetrated its host. Once mature, the shells growth decreases, with the sponge continuing
development at a faster rate. The result of infestation is discernible both internally and externally.
Externally the shell becomes excavated with holes forming a ‘honeycomb’ pattern, often bright
red or orange in colour. Internally, the shell deposits thickened nacre around visible darkened
lesions beneath nacreous layers where penetration into the muscular cavity appears inevitable.
As P. maxima concentrates its energy on fighting the sponge, it neglects to deposit nacre on the
previously inserted nuclei. From this stage forward the pearls display physical imperfections and
discoloration, resulting in a substantial decrease in quality. Often P. maxima‘s own method of
defence becomes futile, as infestation of the shell reaches a capacity far greater than it can
handle. At the stage, the shell becomes weak, brittle, its mantle retracts, and the animals dies.

Considering that colour, shape, and weight determine the value of a pearl, Cliona infestation on a
large scale within a farms lease (and even from wild caught shell) can cost a pearling company
millions of dollars each year.

Maxima Pearling Co. is an Organisation, which places enormous emphasis on providing the best
environmental conditions in order to produce potentially the world’s finest South Sea Pearls.
Ongoing experiments relating to Cliona infestation have shown that ‘suffocation’ of the sponge
appears to halt its growth and cause its eventual death. Various techniques have been trialed with
some astonishing results.

Biology of Black-lip Pearl Oyster (Pinctada margaritifera)

Black-lip pearl oysters are bivalve molluscs, which means that they have two shells (also called
valves) that house and protect their body parts (Figure 1). They are found attached to hard
substrates as deep as 40 m, usually in association with reef habitats. Pearl oysters start life as
males and change into females after 2-3 years. Each female can release millions of eggs into the
water, which are fertilized externally by sperm from the male. Eggs hatch and the oysters pass
through various larval phases during which they remain swimming freely in the water. At
between 25 and 35 days of age the larvae start to spend more time crawling on the bottom and
finally metamorphose into a juvenile pearl oyster that attaches itself to the substrate. Black-lip
pearl oysters feed by filtering water across their gills to trap plankton and other digestible
materials.

Black-lip pearl oysters are widely distributed throughout the tropical Indo-Pacific and are hence
native to all the U.S. Affiliated Pacific Islands. Water temperatures should range between 25-30
°C and salinity should remain above 33 ppt. Black-lip pearl oysters can withstand some silt in the
water but too much can interfere with their feeding.

Description: Black Lipped Pearl Oysters grow to about 10cm. They have rough, sharp-edged shells which are
usually grey to black (Crassostrea belcheri). This species ranges through sheltered coastal waters in
Queensland south to about Gladstone to the New South Wales Border. They have been collected by hand
from banks of the intertidal zone or in shallows at low tide, however, are only found in low abundance.
Cooking Black Lipped Pearl Oysters: Oysters have a rich and distinctive flavour. Their flesh is grey when
raw and brownish grey when cooked. Oysters are often served raw, but baking, deep-frying, shallow-frying
and grilling are also popular methods of preparation. Oysters are often served in lemon or lime juice, or with
ginger, shallots or herbs.

Notes: Black Lipped Pearl Oysters were naturally abundant until about 1910. It is cultured successfully in the South Pacific to produce
black pearls, but is not cultured commercially in Australia. As this oyster produces a high quality pearl suitable for selected markets,
there is interest in developing commercial production techniques. Some production of this oyster may occur in the future if the
techniques can be refined.

Biology of Crassostrea virginica (Eastern Oyster)

An understanding of basic oyster biology is essential to any successful culture operation. Under natural conditions,
oysters spawn as water temperatures rise in the spring. The temperature at which spawning occurs varies from north
to south. Northern oysters spawn at temperatures between 60 and 68ºF (15.5 and 20ºC), while southern oysters
spawn at temperatures above 68ºF (20ºC). Spawning can occur throughout the warm months.

Sperm and eggs are released synchronously and fertilization occurs in the water column. A fertilized egg develops
rapidly into a microscopic swimming trochophore (Fig. 1). After 24 to 48 hours, the non-feeding trochophore
develops into the feed-

References

Cahn, A.R., 1949. Pearl Culture in Japan. Fisheries leaflet. United States Department of the Interior, Washington
DC.