Bivalves

Bivalves are a class of mollusc which have shells consisting of two hinged sides known as valves. Bivalve shell shape and size can be highly varied; however New Zealander’s are likely to recognise many of these shellfish due to their importance both culturally and as a food source. Bivalves perform an important role in the food chain in marine systems, with many fish and birds consuming them as part of their natural diet. Most bivalves live in soft sediments below the surface. They are able to move and burrow through sediments using their strong muscular foot. Bivalves generally suspension feed from the water column however some species deposit feed from the sediment surface using their siphons to draw in water and food. The two valves are opened and shut by strong (adductor) muscles.

Bivalves are among the oldest groups of hard-shelled organisms, dating back more than 500 million years (to the lower Cambrian period). There are around 9,000-10,000 species of bivalves worldwide with approximately 740-780 species represented in New Zealand. In estuaries, bivalves are often present in very high densities, reaching several thousand per square metre. Here they perform important functions such as water filtration and nutrient cycling. Many species are sensitive to changes in sediment mud content and water clarity and are therefore useful indicators of ecosystem health. The most common bivalve species we monitor are shown below.

Key features
The most obvious feature of a bivalve is its two sided, hinged shell that open and close by using elastic ligament and (adductor) muscles. Bivalves have no obvious head, but feed through a siphon which draws water in past to the shell (inhalant siphon) in to the body (mantle) of the organism. This water passes across the gills which extract oxygen from the water and also act as a specialised apparatus to filter out tiny food particles. Tiny hair like structures (cilia), trap food particles and pass these on to the mouth and then stomach. Once material has been extracted, water leaves the shell through an exhalant siphon. Bivalve species that live in the sand or mud have a strong muscular wedge shaped foot used for digging.
Life cycle
Bivalves usually have separate sexes, with some hermaphroditic (has both male and female reproductive organs) species. Most species reproduce by releasing millions of sperm and eggs into the water where fertilisation occurs. Other species retain their eggs and fertilise them by drawing water containing sperm into their body cavity. Post-fertilisation, larvae (which lacks a shell) are either retained within the adult bivalve (brooding species) or develop into the water column until they are ready to settle on to substrates such as sand, mud, wood or rock where they continue to grow. The age of adult shellfish can be calculated from the number of rings or bands on its shell, much like counting the rings of a tree.

Descriptions of species commonly found in Waikato estuaries

Common name: Pipi
Latin name: Paphies australis
Scientific group or class: Bivalve
Description: Pipi are large surface suspension-feeding bivalves which can grow up to approximately 80mm shell length. Maturity is being reached around 40mm. They frequently occur in very dense aggregations (for example exceeding 7000 per m² in Tairua Harbour, Coromandel Peninsula). Pipi have a solid white to light brown/orange elongated shell, which is often covered with a thin yellowish periostratum (thin organic coating or ‘skin’). They are highly mobile both as adults and juveniles and are the largest of the bivalves we monitor. Pipi are an important food source for birds and humans.
Habitat: Pipi are often most abundant near subtidal channels where they burrow to 2-3cm below the sediment surface. Juveniles are often found in fine sandy habitats while adults prefer coarser sediment and fast currents. Juveniles are usually more abundant than adults on upper intertidal flats. As they mature, juveniles migrate down the shore to lower intertidal areas.
Distribution: New Zealand wide (North, South, Stewart, Chatham and Subantarctic islands)
Indicator species: Yes
Why is this a good indicator species? Pipi only tolerate a maxium sediment mud content of 5% and are very sensitive to high turbidity. Therefore, they are usually found in sandy habitats. Pipi are also sensitive to zinc contamination. Where sediment becomes muddier (>5% mud) and/or more polluted (particularly with zinc) the abundance of pipi is likely to decline. For this reason Paphies australis are a good indicator species with which to assess changes in the input of sediment and pollutants into our estuaries.
Latin name: Arthritica bifurca
Scientific group or class: Bivalve
Description: Arthritica bifurca is a small deposit feeding bivalve (<6mm shell length) that is dull-white or pale greyish in colour. Its shell is quadrate-ovate in shape and moderately inflated with a smooth surface.
Habitat: Muddy-sandy habitats, occurs in intertidal and subtidal zones to a depth of 110m
Distribution: New Zealand wide (North, South, Stewart, Chatham and Subantarctic islands)
Indicator species: Yes
Why is this a good indicator species? Arthritica bifurca tolerates a sediment mud content of up to 75% with an optimum range of 20-60%. Therefore, it prefers moderately muddy habitats. Where estuarine sediments change from a sandy to muddier type habitat the abundance of Arthritica bifurca is expected to increase. However, where the sediment mud content exceeds its optimum range (>60%),Arthritica bifurca is expected to decrease in abundance. These responses to change in the physical environment make Arthritica bifurca a good indicator species with which to assess changes in the input of sediment into our estuaries.
Common name: New Zealand cockle, tuangi
Latin name: Austrovenus stutchburyi
Scientific group or class: Bivalve
Description: Cockles are one of the most common intertidal bivalves. In North Island estuaries individuals are considered large at 35mm in length, whereas South Island individuals can grow to about 60mm. Densities can exceed 4500 per m² (for example in Otahu Estuary, Coromandel Peninsula). Maturity occurs at 18-20mm. The shell is white-grey in colour, solid, inflated and has strong radial ribs crossed by concentric bands. Cockles are highly mobile surface suspension feeders. They are often found with algae, worm tubes, anemones or barnacles attached to the outer shell.

Cockles play a key role in estuarine ecosystems. Large, high density cockle beds are often a dominant feature of intertidal flats and provide a valuable food source for birds and fish, such as oystercatchers and eagle rays. Also, cockle beds can filter many thousands of litres of water per tidal cycle and therefore may influence nutrient dynamics in estuaries. In some regions reductions in abundance and size have been attributed to pollution and over harvesting.
Habitat: Cockles are very common on tidal mud and sand flats, from mid-tidal zone to depths of 5m. Larvae settle near the high tide mark and migrate down the beach as they grow. Individuals burrow just below the surface (2-4cm).
Distribution: Endemic to New Zealand (North, South, Stewart, Chatham and Subantarctic islands)
Indicator species: Yes
Why is this a good indicator species? Cockles tolerate mud content up to 85% with an optimum range of 0-10%. They are sensitive to long term exposure to high levels of mud. Therefore, they prefer sandy habitats with a small amount of mud. Cockles are also sensitive to copper contamination. Where the sediment mud content increases (exceeding their optimum range) and/or sediments become polluted (particularly with copper) the abundance of cockles is likely to decline. This makes the New Zealand cockle a good indicator species with which to assess changes in the input of sediments and pollutants into our estuaries.
Common name: Nut shell
Latin name: Linucula hartvigiana
Scientific group or class: Bivalve
Description: Linucula hartvigiana is a small to moderately large shellfish (6-8mm in length) with an ovate inflated shape. The shell is olive/bronze to light brown coloured with small concentric ridges. Linucula hartvigiana is a highly mobile deposit feeder. It has evolved from an ancient lineage that began about 250 million years ago.
Habitat: Muddy sand to sandy mud habitats (intertidal and subtidal to a depth of 20m) in unpolluted environments
Distribution: New Zealand wide (North, South, Stewart, Chatham and Subantarctic islands)
Indicator species: Yes
Why is this a good indicator species? Linucula hartvigianatolerates a sediment mud content up to 60%, with an optimum range of 0-5%. Therefore it prefers more sandy habitats. Linucula hartvigiana is also sensitive to organic enrichment and copper contamination. Where the sediment mud content increases (exceeding its optimum range) and/or becomes organically enriched or polluted with copper, the abundance of Linucula hartvigianais likely to decline. This makes Linucula hartvigianaa good indicator species with which to assess organic enrichment and changes in the input of sediment and pollutants into our estuaries.
Macomona liliana (Photo courtesy of Dr. Barry O'Brien, University of Waikato)

Common name: Wedge shell
Latin name: Macomona liliana
Scientific group or class: Bivalve
Description: Macomona liliana is a large surface deposit feeder with an ivory white ovate shell that is roughly triangular in shape and slightly inflated. Individuals can grow up to 70mm in length (more common around 40mm) and mature around 20-22mm. Adults can have a strong effect on many of the other sediment-dwelling organisms living nearby. Macomona are an important food resource for fish and birds.
Habitat: Intertidal and shallow subtidal sand and mud flats in estuaries and sheltered harbours. Prefers sandy to muddy sand habitats. Individuals burrow in the sediment to depths of 5-15cm. Adults live at around 10cm sediment depth.
Distribution: New Zealand wide (North, South, Stewart and Chatham islands)
Indicator species: Yes
Why is this a good indicator species? Macomona liliana tolerates mud content up to 75%, with an optimum range of 0-30%. Therefore it prefers sandy habitats with some mud. Macomona liliana is also sensitive to copper contamination. Where sediment mud content increases (exceeding its optimum range) and/or sediments become polluted (particularly with copper), the abundance of Macomona liliana is likely to decline. This makes Macomona liliana a good indicator species with which to assess changes to the input of sediment and pollutants into our estuaries.
Theora lubrica (Photo courtesy of Peter J. Bryant, University of California)

Common name: Asian semele
Latin name: Theora lubrica
Scientific group or class: Bivalve
Description: Theora lubrica is a selective deposit feeder. Its shell is very thin and almost transparent, smooth and elongated in shape (7-15mm in length). Theora lubrica is an invasive species that arrived in North Island harbours about 1972, probably in ballast water from its home in Japan. It has since spread to most other ports in the country and rapidly colonises disturbed and muddy habitats.
Habitat: Theora lubrica occurs subtidally and on lower intertidal flats where it burrows to 50mm below the sediment surface.
Distribution: North Island (east and west coast) and South Island
Indicator species: Yes
Why is this a good indicator species? Theora lubrica is very mud tolerant. It can tolerate a mud content of up to 65% with an optimum range of 45-50%. Therefore Theora lubrica has a preference for muddy habitats with some sand. Where estuarine sediments change from a sandy to muddier type habitat the abundance of Theora lubrica is expected to increase. However, where sediment mud content exceeds their optimum range (>50%) Theora lubrica is expected to decrease in abundance. Theora lubrica is also tolerant of pollution and organic enrichment. Consequently, it is often found in organically enriched or polluted sediments. This makes Theora lubrica a good indicator species with which to assess organic enrichment and changes in the input of sediment and pollutants into our estuaries.

Bivalves

Key features

Life cycle

Descriptions of species commonly found in Waikato estuaries

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