Scottish Marine and Freshwater Science Volume 3 Number 5: SOTEAG Rocky Shore Monitoring Programme. TBT Contamination in Sullom Voe, Shetland. 2011 Dogwhelk Survey

2. Methods

2.1 The choice of Survey Sites and Dogwhelk Sampling Procedure

Between 15-26 August 2011, samples of approximately 40 adult dogwhelks (identified by thickened shell rim and the presence of "teeth"; Crothers, 1985) were collected from 20 sites around Sullom Voe and the waters of Yell Sound ( Figure 1, Table 1).

Juvenile dogwhelks were collected at five of the sites (1, 3, 5, 9 and 12), as in previous surveys. The shell length of each animal was measured, and individuals were classified by their shell length according to observations by Moore (1936), i.e. juveniles (10-15 mm shell length), sub-adults (15-21 mm), and un-toothed adults (21‑26 mm and 26-35 mm). At each of the juvenile and sub-adult survey sites, an attempt was made to obtain 20 individuals from each of the above size classes (and 40 toothed adults) although at several sites this was not possible, particularly for the smallest size classes. Due to mortality of the adult samples prior to analysis, less than 40 individuals were analysed from 14 sites, with one site, Scarf Stane, having shown considerable mortality during transportation/storage and only 10 individuals remained for analysis.

In 2011, sites Norther Geo and Breiwick returned no sample (due to inaccessibility or lack of individuals on site) so a new site, Sweinna Stack, in the vicinity was sampled in place. ( Figure 1).

The degree of imposex, as measured by Relative Penis Size Index ( RPSI) and Vas Deferens Sequence Index ( VDSI), was determined using standard internationally approved techniques ( OSPAR, 2002).

2.2 Determination of the Relative Penis Size Index ( RPSI)

The Relative Penis Size Index (or RPSI, Gibbs et al., 1987) was calculated from penis length measurements of the dogwhelks as follows:

The greater the penis growth in females, the higher the RPSI value. An RPSI of 12.5%, for example, indicates that the mean female penis length is half that of the male.

2.3 Determination of the Vas Deferens Sequence Index ( VDSI)

The development of imposex in dogwhelks may be divided into seven stages, depending upon the developmental state of both the penis and vas deferens in the female (Gibbs et al., 1987). Stage 0 is identified where no signs of imposex can be seen. Stage 1 can be identified when the vas deferens begins at the site of the vulva with Stage 2 also showing a small penis behind the right eye tentacle. As imposex progresses, the vas deferens starts to develop from the penis (Stage 3) and will become continuous (Stage 4). Eventually, vas deferens tissue may proliferate over the opening of the vulva (Stage 5), rendering the female incapable of breeding since she can no longer release egg capsules. The trapped egg capsules form a solid mass within the capsule gland. In this final Stage (Stage 6), the capsule gland may eventually rupture, causing premature death of the female. Each of the seven Stages of imposex is known as a Vas Deferens Sequence ( VDS) stage and calculation of the mean VDS for a group of females provides the Vas Deferens Sequence Index ( VDSI) which may be used to compare the reproductive competency of different populations.

The VDS was determined for each female and the mean VDS calculated to provide an estimate of the VDSI of the population.