Why are Mussels Absent from the Wellington South Coast?

Story and Photos Murray Hosking

Most people in the Wellington region who enjoy a walk by the sea will be familiar with mussels as a feature of the marine life in the intertidal region – that part of the shore which is between the high and low tides. Life in this zone must be hardy to deal with the daily tidal variation, with extreme conditions of wetting and drying. There tends to be a pattern in the marine life within the intertidal zone which is repeated on rocky shorelines virtually anywhere in the world. On the rocks at the top of the intertidal we can see lichens, winkles, barnacles and limpets; mussels inhabit the mid-zone, and at the bottom, the least exposed between tides, we find the large seaweeds. Wellington Harbour rocky shorelines show this ‘universal’ pattern quite distinctly, but on the Cook Strait coasts mussels are generally absent.

Work done by Victoria University marine biologists reads a bit like a detective novel as they sought clues to explain this ‘anomaly’, one of the features which gives the Taputeranga Marine Reserve its distinctive character. The absence of mussels is not just a south coast issue but is general to Cook Strait shores, including the outer Marlborough Sounds and the NI shores from Paekakariki south to Cape Palliser. We should be able to see three mussels in abundance on the South Coast, based on Wellington Harbour patterns– the endemic greenshell mussel (Perna canaliculus), endemic ribbed mussel (Aulacomya maoriana), and the widespread, usually aggressive colonizer, the blue mussel (Mytilus galloprovincialis).

Greywacke rocks in the intertidal zone at Princess Bay. Abundant vacant real estate for mussels, but not a supermarket in sight!

Why should we care? Well, because mussels play a distinctive role on any rocky shoreline. As filter feeders they remove tiny particulate matter from the sea and gain sustenance from single celled organisms within. Mussels then become potential food for other life on these shoreline habitats. Their absence means reduced food for starfish, snails and some fin fish. As they form dense beds mussels trap sands and muds as well as their own waste deposits, habitat for a wide array of small critters.

Mussels are vulnerable in the larval stage, where free-swimming forms may be themselves predated and need to find suitable substrate to settle. The scientists asked some basic questions. Maybe the larval forms are not carried by the current along the south coast, or cannot settle, or maybe there is another factor in growth once settled. Indeed, there are some places where one can see small mussels, especially the normally successful colonizers, blue mussels, forming in isolated clumps. So, why not more?

The scientists addressed each of these questions with a range of experiments and analysis. While larval stages of mussels were clearly more abundant inside Wellington Harbour, there were certainly enough larvae found to be present outside the harbour to colonize the south coast, other factors permitting. Similarly, tests showed that the larvae settled on the rocks in sufficient numbers to establish on the south coast. The dynamic nature of the Cook Strait currents and the associated wave energy and storm events are certainly factors to be taken account in the survival of settled larvae, but there are many other sites around the world of equal or greater energy which show mussel colonies in high numbers.

The greywacke rock type was not found to be a factor as the same rocks comprise the shoreline mussel substrates of the inner harbour where there are abundant mussels. If barnacles could settle and grow, why not mussels? There was no evidence of undue levels of predators, such as starfish, snails or birds, with plenty of bare space in the intertidal zone for mussels to establish.

Blue mussels at Scorching Bay occupying the mid-tide zone above seaweeds. Vulnerable to predating oyster catcher?

The scientists turned their attentions to the quantity and quality of food sources for mussels. The suspended particles upon which mussels feed, called seston, will vary in quantity and quality over time as currents and tides move water around. Like any animals, mussels are sensitive to the levels of both elements in their diet – they may receive an adequate quantity but poor quality of material, and either develop or decline. Laboratory tests of mussels collected from within the harbour and fed on the seawater from both Mahanga Bay and Island Bay showed markedly different results. The harbour water contained sufficient organic components for mussels to flourish but the Island Bay seawater lacked both quantity and quality of seston. In controlled conditions in tanks, with no predation possible, the mussels exposed to south coast nutrient sources lost condition or died. Similar open sea experiments with mussels collected from the same source inside the harbour showed similar results, those re-established within the harbour flourished; those placed at Island Bay declined and died. It appears that both quantity and quality of seston are just plain inadequate on the Cook Strait rocky shorelines to support mussel colonies.

Ultimately, the amount and quality of suspended nutrients will depend on the balance of sources and currents along any shore. The currents within Cook Strait are complex, as a blend of three main sources, the West Coast current via D’Urville island, the Southland Current from the east coast of the South Island, and the remnants of the East Cape subtropical waters which reach the Cook Strait area. High nutrient waters from the Wellington Harbour itself play a part in the mix but the low nutrient level waters from the Tasman Sea, lacking major input from terrestrial or freshwater sources, are probably the main factor in a low quality and quantity of food source for mussels on the south coast.

We may not expect that Taputeranga Marine Reserve will itself change the availability of nutrients over time, but this mussel ‘anomaly’ sets the coast apart from other temperate coastlines of the world – worthy of study in controlled conditions. While we celebrate the high levels of biodiversity expressed in this meeting place of major currents and habitat forming influences, the absence of mussels will remain a special element worthy of continued study here at Taputeranga.