4.2 RESULTADOS VARIABLE MÉTODOS DE ENSEÑANZA
4.2.3. Resultados para las hipótesis específicas
Two methods of marking dog-whelks in the field were employed. Bright enamel paint
(Humbrol Ltd., U.K.) was painted on the shell apex; the colour permitted quick recognition in
the field and distinguished samples obtained on different occasions. A numbered 'beetag'
(Christian Graze Inc., W. Germany) was also attached to the apical whorl using cyanoacrylate
glue. Both marking methods had previously been tested at Whitsand Bay (O.S.ref. SX390523)
and found to persist for at least six months in the field with less than 5% losing both shell
marks (pers. obs.). The Whitsand Bay site has a high density of N. lapillus and is a 3km wave-
swept shore, which is predominantly sandy with isolated rock masses reducing the movement
of whelks over large areas (Harris, 1988). Dog-whelks were therefore easily recovered from the
field and the abrasive action of sand and debris was considered an extreme test of the
permanency of the marking techniques. No deaths occurred in a sample of labelled animals
maintained in the laboratory for one week, but the long-term effects upon survival and
mortality in the field are unknown. However, several whelks were feeding within a few days of
being labelled suggesting that disturbance had been minimal for these individuals.
At Peartree Point and Prawle Point (fig. 3.1), all whelks visible within an area of approx. 20m^
in the mid-tidal zone were labelled and measured for shell length and aperture (total per site
>280). The two sites were sampled between 25th to 27th May, two weeks later (llth-13th
June) and then every subsequent month (see section 3.3.3.2 for exact dates). Sampling occurred
during low water spring tides over the summer period between May and September, during
which time additional whelks were measured and labelled to ensure an adequate number of
smaller sized juveniles as the season progressed (Sainsbury, 1980). Length and aperture
measurements taken on each occasion provided six monthly growth rates per site and length-
dry weight relationships (see below and section 3,3.3.1) were used to distinguish between shell
and tissue weight for experimental animals on the shore.
A size range of whelks (n=30) was collected from each sample site in both May and
September. These animals were used to determine length-dry weight relationships for shell and
tissue weights, which may differ between sites due to site-specific shape variation (section
3.3.2.1) and possible differences in nutritional and reproductive status in the field. The sample
obtained in September was used to determine whether these relationships had altered over the
summer months for both within- and between-site comparisons. In previous experiments shell
and tissue weights were measured using the methods devised by Palmer (1982, section
2.2.1.3.1), but it was not considered appropriate in this experiment since the large number of
individuals that would have to be measured would require animals to be removed from the field
and maintained in the laboratory for approximately two days. This in itself would cause a
disturbance effect upon feeding and behaviour (pers. obs.), in addition to those incurred by
using the Palmer (1982) methodology as previously noted in the laboratory (chapter two).
An increasing proportion of apparently non-growing whelks are found if the growth of a subset
of a population is monitored over a period of time. This presumably results from the onset of
reproduction and approach to the asymptotic size for an increasing number of individuals.
Previous attempts to overcome these problems have included the omission of whelks with a
growth rate below a certain limit or whose size is over the mean asymptotic size determined by
one of a number of growth models (eg. Etter, 1989). In this study, a dry tissue weight of 160mg
was chosen as the upper size limit of whelks to be included in the growth rate analyses, rather
than using a standard shell length due to differences in shell shape; it corresponds to a shell
3.3 RESULTS
3.3.1 Environm ental variables
3.3.1.1 Maximum wave force
No site-specific differences in maximum wave force were found. Both Peartree Point and
Prawle Point had similar variation in the calculated 24 hour maximum wave velocities
(fig.3.3.1).
3.3.1.2 Temperature and humidity
I
The temperatures recorded at Prawle Point were consistently and significantly higher than
those recorded at Peartree Point (fig.3.3.2a) (Wilcoxon signed- rank test; d = -2.666, p<0.01).
Prawle Point also experienced lower relative humidities than Peartree Point (fig. 3.3.2b)
(Wilcoxon signed-rank test, d = -2.52, p<0.01). Humidities at both sites reached their highest
values at approximately 13.00 BST.
3.3.1.3 Desiccation rates
The sites did not differ significantly in the rates of desiccation from oases attached inside the
pot (fig. 3.3.3) (t =-0.91 18 d.f. p=0.377). However, the oases attached to the lid had a
significantly higher rate of evaporation at Prawle Point than those at Peartree Point (t=-6.107
18 d.f. p=0.00). These differences also occurred two days later (fig. 3.3.4) when additional
sites were sampled; rates of water loss at all the intermediate sites were higher than those
measured at Peartree Point. The highest desiccation rate recorded was at site 3; sites 2, 4 and 5
were the same, and the desiccation rates at Prawle Point were lower than at both sites 2 and 4.
3.3.2 L ab orato ry physiology of field juveniles
3.3.2.1 Size and shape analysis
The following analyses were undertaken on whelks collected from Peartree Point and Prawle
Point for the destructive sample (unless indicated otherwise). The regressions relating
immersed to actual shell weight, and also between shell length and dry weight were
significantly different between populations (p<0.0005, section 2.2.1.3,1). The coefficients
given are mean values + 2SE:
Figure 3.3.1 Maximum wave force recorded at Peartree Point (closed squares) and Prawle Point (open circles) over eight 24 hour periods. Mean values ± SE are presented (n=4 per mean).