The toheroa, Paphies ventricosa, the feeding and diet of, with morphology notes

Abstract

The toheroa, Paphies ventricosa, is a species with a rich history of exploitation but in the last 30 odd years harvesting has been drastically curtailed in attempts to stabilise population numbers. Only two fishing seasons have occurred since 1981, both for one day only and both located at a single beach, Oreti Beach. During these open days each person who wished to try was allowed to collect five toheroa that measured over 10cm in length. For both open days over 10,000 people turned out trying to get their quota, some carted off the beach suffering from hypothermia. The dedication and numbers turning up to open days suggest a still high regard for the protected shellfish. Aquaculture operations, whether for conservation or economic endeavours, have been suggested for the toheroa for these reasons. The suitability of the toheroa, Paphies ventricosa, for aquaculture is therefore examined, with specific investigations into the diet, sedimentological habitat, morphology and feeding ability conducted for toheroa from the Oreti Beach population.

Diet was examined in two ways, by examination of gut contents and water sample counts. The gut content analysis was an abysmal failure with no species recognisable once the samples were returned to the laboratory and dissected and preserved. Water samples proved more successful with five species sharing dominance throughout the year. Two of the five species have been reported as dominant at other toheroa beaches, but cell counts observed were highly variable. No samples coincided with the renowned phytoplankton blooms common to toheroa beaches, so cell counts never reached levels reported in Rapson (1954) or Cassie and Cassie (1960). Organic concentration of the water was also tested and produced levels that were fairly consistent throughout the year with mean organic concentration for the year recorded as around 14mg l-1. This level compares favourably with levels reported for other beaches, but the lower temperatures at the southern beach might inhibit feeding rates and growth to a degree not determined.

Sedimentology data from Oreti Beach revealed that the southern beach has a significantly smaller mean grain size than the northern most beaches. Samples collected at Oreti contained on average around 30% of particles smaller than 0.125mm compared to less than 2% for areas of Dargaville Beach that had good beds of toheroa. Sediment to a depth of at least 10cm was highly sorted, indicating why the toheroa is required to maintain the high burrowing ability reported by Kondo and Stace (1995). Sediment found within the toheroa beds and located higher up the beach profile were statistically similar to a depth of 5cm, below this the sediment located higher up the beach became significantly smaller. Within a sample stratum the mean sediment size and sorting remained statistically similar.

Shell morphology was examined and the results from two methods of measuring length were examined, one used by Redfearn (1974) in examining the toheroa, the other suggested as a standard method for measuring shells between bivalve species by Stanley (1970). The two methods produced results that although different, were easily reconciled using a simple relationship. Length, height and width data were also used to reveal that the toheroa joins a select list of relatively thick shelled species that inhabit hostile environments and have a high burrowing ability. Other relationships length has with other measurable quantities were also reported.

Morphological measurements are also used to establish a method to distinguish between the toheroa and the southern tuatua, Paphies donacina. A score based system similar to that used by Richardson et al. (1982) is devised using ratios commonly used to describe shell shape. Only 4% of southern tuatua tested were misclassified using this system, and no toheroa. While establishing this test for species, it was noted that the score equation reported in Richardson et al. (1982) had an error in it. This equation, used to distinguish between the two species of tuatua, was length biased. Once specimens smaller than 5cm were tested it was highly likely they were misclassified as the northern tuatua, Paphies subtriangulata.

Feeding experiments that used a specialised feeding chamber designed specifically to allow for intertidal simulation for infaunal animals were conducted. Results showed the toheroa to have a slow rate of clearance for the concentrations tested. The clearance rate was related to the size of the individual, with smaller individuals clearing less than larger ones. Clearance rate followed the general formula observed for bivalve filter feeders: CR= aDWb with the b value calculated to be around 1.07 for all three dietary regimes tested. The a value differed significantly between dietary regimes indicating that toheroa regulate their clearance rate, clearing larger volumes in waters with a low phytoplankton concentration. High b values indirectly support Rapson's (1954) observation that toheroa have a high metabolic rate requiring significant amounts of oxygen.

Ingestion rates displayed a significant difference between all dietary levels tested. Finding the relationship between the ingestion rate and phytoplankton cell concentration suggested that at levels below around 350,000 cells l-1 toheroa would not actively feed. This was supported by significantly different levels of clearance for samples that were collected when cell concentration were only 100,000 cells l-1. It thus appears that at low phytoplankton concentrations toheroa may pump water across gills for respiratory purposes mainly, but will ingest what food comes its way at the same time.