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.