FISH TECHNOLOGY: TYPES OF SHELLFISH
Diagram
Two bivalve shellfish
Oysters
Bivalves with a rough, uneven shell. They sit on the sea bed or on ropes, and they are often eaten raw on the half shell.
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Mussels
Bivalves with a smooth, dark, wedge shaped shell. They cling to rocks and ropes with threads, and they are usually cooked.
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Both bivalves
Both have two shells, both filter feed on plankton, and both are farmed. Spencer covers oyster cultivation and mussel cultivation in separate chapters.
Figure 1: Oysters have rough, uneven shells and mussels have smooth, dark, wedge shaped shells. Drawn in our own style from Brian Spencer, Molluscan Shellfish Farming, Wiley, 2008, chapter 6, Oyster cultivation, and chapter 7, Mussel cultivation, and from Swift, Aquaculture Training Manual.
Introduction
Oysters and mussels are the two best known bivalve shellfish, and both are eaten raw or cooked, but they differ in shell, habitat, farming and flavour. Oysters have rough, uneven shells and sit on the sea bed or on ropes, while mussels have smooth, dark, wedge shaped shells and cling to rocks and ropes with threads. Spencer covers both in Molluscan Shellfish Farming, with oyster cultivation in chapter 6 and mussel cultivation in chapter 7, and Swift covers the farming of both in the Aquaculture Training Manual.
The core difference is the shell and the life: oysters are rough shelled and often eaten raw, while mussels are smooth shelled and usually cooked.
Quick comparison
Table 1: Oysters compared with Mussels
| Aspect | Oysters | Mussels |
|---|---|---|
| Shell | Rough, uneven, often grey or brown | Smooth, dark blue or black, wedge shaped |
| Shape | Round to oval, irregular | Long, narrow wedge |
| How it holds on | Cements itself to rock, bed or rope, or lies free | Clings with byssus threads to rocks, ropes and poles |
| Habitat | Sea beds, estuaries and oyster beds | Rocks, ropes, poles and mussel beds in the sea and estuaries |
| Feeding | Filter feeder: strains plankton from the water | Filter feeder: strains plankton from the water |
| Farming | Grown on beds, trays or ropes; hatchery reared young are set in the sea | Grown on ropes, poles or beds; young mussels settle on ropes |
| Eaten raw or cooked | Often eaten raw on the half shell | Usually cooked: steamed, in soups or in sauces |
| Flavour | Briny, mineral, like the sea | Mild, sweet, softer |
| Size | Larger, a single oyster is a mouthful | Smaller, eaten by the handful |
| Price | Usually dearer per piece | Usually cheaper per kilo |
Scientific explanation
Definition
Oysters and mussels are both bivalve molluscs: animals with two shells that filter feed on plankton. Spencer’s Molluscan Shellfish Farming covers the general biology of bivalves and gives oyster cultivation its own chapter (chapter 6, p. 123) and mussel cultivation its own chapter (chapter 7, p. 147).
An oyster is a bivalve with a rough, uneven shell that cements itself to a surface or lies on the sea bed. A mussel is a bivalve with a smooth, dark, wedge shaped shell that clings to surfaces with byssus threads.
Working principle
Both feed the same way: they open their shells, draw in water, and strain out plankton and tiny food particles. That is why both are sensitive to the water they live in, and why both can gather up what is in the water, which is why shellfish waters are watched for safety.
The farming differs because the animals hold on differently: oysters are grown on beds, in trays or on ropes, and mussels are grown on ropes or poles where their threads can grip. Both start from hatchery reared young or from wild spat that settles on the ropes and beds.
Apparatus
Oyster farming uses beds, trays, bags and ropes, and a hatchery to rear larvae. Mussel farming uses ropes, poles and rafts, with boats to harvest the ropes. The books used here describe the methods rather than naming one standard instrument for the comparison itself.
Measurement procedure
For oysters, young oysters are set on beds, in trays or on ropes, grown in the sea, and harvested when they reach size, then graded and kept cool. For mussels, young mussels settle on ropes or are set on them, grow in the sea, and are harvested by lifting the ropes, then cleaned and graded.
There is no single standard measurement procedure for this comparison in the books used here: the two are told apart by their shell, their shape and their label.
Factors affecting readings
Water quality matters most for both: clean, plankton rich water gives good growth and good flavour, and polluted water makes the shellfish unsafe to eat. Temperature and season matter too: both grow faster in warm water and spawn in season, which changes the flesh. The site matters for farming: a sheltered estuary suits oysters and mussels alike.
Formulas and calculations
The books used for this article give no formula for this comparison, so none is given here. The comparison is qualitative: it rests on definitions, materials and method, not on a calculation.
Industrial applications
- Oysters are used for oysters on the half shell, eaten raw with lemon or sauce.
- Oysters are used for cooked dishes such as oyster stew and fried oysters.
- Mussels are used for steamed mussels, mussels in white wine sauce and mussel soups.
- Mussels are used for farmed shellfish that is cheap, plentiful and easy to cook.
- Both are used in shellfish farming to supply markets with fresh bivalves.
Advantages and limitations
- Oysters, advantage: prized raw, with a briny flavour no other shellfish gives.
- Oysters, limitation: dearer, and eating them raw needs very clean water and careful handling.
- Mussels, advantage: cheap, plentiful and quick to cook, with a mild sweet flesh.
- Mussels, limitation: usually eaten cooked, so they do not suit raw dishes, and they need thorough cleaning before cooking.
FAQ
Are oysters and mussels the same animal?
No: both are bivalves, but they are different animals. Oysters have rough, uneven shells and mussels have smooth, dark, wedge shaped shells.
Can mussels be eaten raw like oysters?
It is not usual: mussels are normally cooked, steamed or in sauces, while oysters are often eaten raw on the half shell. Raw shellfish of any kind needs very clean water.
Why are oysters more expensive than mussels?
Because oysters grow more slowly, take more handling on beds and trays, and are sold one by one as a prized food. Mussels grow fast on ropes and are harvested in bulk, so they are cheaper.
Do both filter the water?
Yes: both are filter feeders that strain plankton from the water. That is why the water they live in must be clean, and why shellfish beds are watched.
How do mussels hold on to ropes?
With byssus threads: strong threads the mussel makes to grip rocks, ropes and poles. Oysters hold on differently, by cementing their shell to the surface.
