Life history and demography of the pipi, Paphies australis (Bivalvia: Mesodesmatidae) in northeastern New Zealand

Abstract

This thesis examines the life history characteristics of the New Zealand pipi (Paphies australis) from gametogenesis, through spawning, larval development, settlement, juvenile recruitment patterns, distribution and abundance patterns, population size structure and growth characteristics from the Whangateau Harbour, in northeastern New Zealand. The thesis concludes by integrating this information into a model, based on hydrographic information and post settlement movement in order to help explain the observed patterns of distribution and abundance of juvenile and adult pipi in the Whangateau Harbour. The temporal pattern of the reproductive cycle of pipi was investigated over two years (May 1991 - April 1993). Samples of pipi gonads were processed using standard histological techniques, and the resultant sections placed into six developmental categories (early active, late active, mature, partially spawned and indeterminate). Gametogenesis was similar in both years, beginning in autumn with most pi pi in early active stages of development. By late winter many pipi were mature. Spawning commenced in early spring and continued through spring and summer. From a sample of 104 pi pi collected in spring 1992 for investigation into the length at sexual maturity, all pipi were found to be sexually mature above 40 mm shell length, but some individuals between 30-40 mm also had gonads with sex cells present. This sample had a sex ratio significantly biased towards females. Further analysis revealed that juveniles (below 40 mm shell length) had a significantly higher proportion of females than males, but adults (above 40 mm shell length) had a sex ratio of approximately 1: 1. No evidence of hermaphroditism was observed. The spatial pattern of the reproductive cycle of pipi was also investigated over one year (February 1992 to January 1993) at three subtidal sites using the same techniques as for the temporal study, but with one further category (parasitised). The reproductive cycle was similar between sites, with an extended spawning period . of late winter to summer. It was concluded that the reproductive cycle of pipi in the Whangateau Harbour was synchronised at all three sites sampled. A visual maturity index gave the same overall pattern of reproductive cycle as the histological analysis, but was found to be inaccurate mainly due to a poor recognition of the mature stages. It was concluded that the visual index was of limited value in accurately assessing the maturity of an individual. The sex ratio of adult pipi was not significantly different from 1: 1. A small percentage (0.87%) of pi pi were infected by a digenetic trematode fluke that, in most cases, totally castrated the individual. The larval development of pipi was described from larval culture experiments. Pi pi were conditioned, spawned and the resultant larvae reared to settlement in the laboratory. Adult pipi were successfully conditioned after 23 days at 22°C. Spawning was accomplished using a combination of increased temperature and a dilute sperm solution. Settlement occurred 18-22 days after spawning at a mean shell length of 264 Ilm . After settlement and metamorphosis pi pi gradually began to take on the adult shape. Cultured juveniles were grown to 37 mm shell length both in the laboratory and later in the wild. Once in the wild, cultured pipi grew from 13 mm to 37 mm in 17 months with a strong seasonal component to their growth. Microscopic examination revealed slight differences in shell morphology between pi pi larvae and the closely related tuatua (Paphies subtriangu/ata) and toheroa (Paphies ventricosa). Pi pi were more rounded and smaller than both tuatua and toheroa at a similar stage of development. The results of scanning electron microscope examination of larval shell hinge structure confirmed previous preliminary findings for pipi. The larval shell ligament is posterior to the centre of the provinculum in pipi but central in both the tuatua and toheroa. These differences are sufficient to enable larval pipi to be distinguished from larval toheroa and tuatua. The spatial and temporal patterns of distribution and abundance of pipi were measured from November 1991 to January 1994. Previous information had suggested that pipi were a mainly intertidal species, but this study found that they can occur in large numbers subtidally. Temporal sampling was carried out by systematically measuring the abundance of pipi found within three subtidal grids. Abundances were compared within and between the grids. The subtidal population was extremely dense (up to 4400 m-2 ), with well defined boundaries. Pipi densities at the edges of the beds dropped from many hundreds to zero m-2 within 2 to 3 m. Large numbers of juvenile pi pi «30 mm) were present at the start of sampling and the density of these decreased during the sampling period. Juvenile pipi occurred at high density at the harbour entrance site, but were rare or absent at the other two sites further up the harbour. Small scale distribution (ie. within a grid) at the harbour entrance showed high variability which was linked to large swells from storms, observed only at this site. The advantages of systematic sampling compared with random or haphazard sampling are discussed. The population size structure, growth and small scale movement patterns of pipi, were also studied. Pi pi formed a continuous bed from the harbour entrance up the main subtidal channel, and extended approximately one kilometre upstream. Pi pi had different shell morphologies at different sites within the harbour, and population size structure differed among sites. Small pi pi were found at either end of the subtidal pi pi bed but were absent from the middle. Analysis of length-frequency distributions indicated that there had been a large recruitment of pipi prior to the commencement of the study, but little further recruitment occurred during 1992 to 1994. Pipi took approximately three to four years to reach the population maximum size of 55-60 mm shell length. Tagged pipi, constrained in suspended nets grew more slowly than predicted from population length-frequency analysis, but tagged pi pi that were returned to the sediment grew as predicted. Localised movement of tagged pipi showed that pi pi gradually move into the centre (deeper part) of the channel and towards the harbour entrance. Adult and juvenile pipi, were observed drifting in mid-water, in the Whangateau Harbour. The animals were buoyed up by the secretion of long mucus threads which extended out through their siphons. Quantitative information from netting experiments over a period of five months from May 1993 to September 1993, collected a total of 509 drifting pipi. These were mainly juveniles «15 mm shell length) but a small number of adults (up to 58 mm shell length) were also collected. The numbers of pi pi caught was highly variable, however, there was a trend for more drifting pipi to be caught on the flood tide (n=386) than the ebb tide (n=123). Overall there was little difference in the number of pipi caught between spring (n=263) and neap tides (n=246), however, this was heavily influenced by one neap tide in May 1993 (n=207). Three other bivalve species (So/etellina sp, Gari stangeri, Macomona . lilliana) were also caught drifting. Pipi were also observed to attach themselves to debris, such as dead branches of trees, which then moved them with the tidal currents. Large epiphytic algal growths on the shell also acted to buoy individuals which moved them around in tidal currents. It is hypothesised that mucus drifting in pipi is an active process, adopted for movement to other areas triggered by very high densities (over 4000 m2 ) in the harbour entrance. Finally, the distribution and size structure of the pi pi population was described by a simple model, based on the post-settlement movement of pi pi and tidal current patterns. Pipi recruits «5 mm shell length) were found to be largely restricted to a small strip, mid-intertidally in clean coarse sediments. Juvenile pi pi (5-35 mm) were found at a low intertidal site directly downstream of the recruitment site. Further downstream of this site, at a subtidal site at the harbour entrance both juvenile and adult pi pi were found. A similar population size structure was found at the innermost extent of the pi pi bed in the main channel. Adult pi pi were abundant in the central part of the pi pi bed, but no juveniles were found at this site. It was concluded that post-settlement movement patterns could account for the distribution patterns of pipi in the Whangateau Harbour. If subsequently found to be of more general applicability, this model provides a powerful, predictive tool for managing pipi population and possibly other infaunal clams. This study provides, for the first time, a detailed description of the distribution and abundance of an entire pipi population from one harbour. This information allows a more informed management of pi pi populations around New Zealand. It also provides a framework in which more indepth questions about the ecology of pipi populations (and other bivalve populations) can be more clearly addressed.