First visual record of rare purple dogwhelks (Nucella lapillus) on the Atlantic coast of Nova Scotia, Canada

Introduction

The dogwhelk Nucella lapillus (L. 1758) is a common predatory gastropod in the rocky intertidal of the North Atlantic (Crothers, 1985; Etter, 2007). Individuals vary in shell color. White and brown are common colors (Berry & Crothers, 1974; Crothers, 1983; Crothers, 1985; Etter, 1988), with darker dogwhelks being more affected by physiological stress under high temperatures than lighter-color conspecifics caused by higher energy intake from sunlight (Etter, 1988). Other shell colors reported are black, mauve, pink, yellow, and orange on European coasts (Berry & Crothers, 1974; Moore, 1936), red and grey from the Bay of Fundy coast of New Brunswick and Nova Scotia (Canada) (Colton, 1922, Crothers, 1983), and black, purple, gray, yellow, and orange from the coasts of Maine (Colton, 1922; Crothers, 1983) and Massachusetts (USA) (Etter, 1988). Purple is considered to be a rare color in dogwhelks (Colton, 1922; Etter, 1988). On the Atlantic coast of Nova Scotia, dogwhelks are active from April until November (Hughes, 1972; Hunt & Scheibling, 1998), but information on dogwhelk shell colors is missing for this coast.

Methods

On 16 June 2016, we collected 1000 dogwhelks along 300 m of coastline in the mid-to-high intertidal of a moderately wave-exposed rocky shore near Duncans Cove (44°29’41.22”N, 63° 31’26.66”W), Halifax on the Atlantic coast of Nova Scotia. The dogwhelks were collected for manipulative field experiments to examine nonconsumptive effects (NCEs) of these predators on their prey. Equal dogwhelk quantities were collected by one of us (JAE) for related research projects on dogwhelk NCEs (e.g. Ellrich et al., 2015; Ellrich et al., 2016) in several locations, with similar levels of intertidal elevation and wave-exposure, along the Atlantic coast of Nova Scotia: in Glasgow Head (45°19’12.61”N, 60° 17’34.15”W) in May and June 2011, in Deming Island (45°12’44.31”N, 61° 10’25.99”W) in May 2012, and in Deming Island, Halfway Cove (45°20’58.98”N, 61° 21’46.58”W), and Half Island Cove (45°21’19.77”N, 61° 11’23.73”W) in May and June 2013.

During field surveys for another research project near our dogwhelk collection site in Duncans Cove, dogwhelk colors were observed regularly (on 12 August 2014, 1 September 2015, and 21 August 2016). To observe dogwhelk colors, 30 quadrats (25 cm × 25 cm) along a ^~150 m transect parallel to the coastline were sampled at random on each survey date. Throughout the entire survey period, intertidal temperature was measured continuously every 30 minutes by two submersible loggers (HOBO Pendant Logger, Onset Computer Corp., Pocasset, MA, USA). Using temperature data from those loggers, we calculated the average intertidal temperature for the all the dates when dogwhelks were collected or observed in Duncans Cove.

Results & discussion

In our collection of dogwhelks near Duncans Cove on 16 June 2016 (n= 1000 dogwhelks), we found two dogwhelks of purple shell color. Our results provide the first visual record of purple dogwhelks on the Atlantic coast of Nova Scotia (Figure 1). The other dogwhelks collected on that day were of common white and brown shell colors. We did not find any other purple dogwhelk during any of our five collections of equal dogwhelk quantities along the Atlantic Coast of Nova Scotia (n= 5000 dogwhelks of brown and white shell color in total) or three field surveys near Duncans Cove (n= 82 dogwhelks of brown and white shell color in total) indicating that purple dogwhelks are rare in that region.

Figure 1. A purple dogwhelk, Nucella lapillus (L. 1758).

Picture taken near Duncans Cove (44°29’41.22”N, 63° 31’26.66”W), Halifax on the Atlantic coast of Nova Scotia, Canada on 16 June 2016 (picture credit: Julius A. Ellrich).

Comparing the average intertidal temperatures of the dogwhelk collection day (12.3 ± 0.4°C, mean ± se, n = 96 temperature measurements, 16 June 2016) and the three dogwhelk observation days (19.5 ± 0.7°C, 12 August 2014; 18.2 ± 0.5°C, 1 September 2015; 22.3 ± 0.5°C, 21 August 2016) in Duncans Cove revealed that the purple dogwhelks were found on a relatively cool day. This suggests that purple dogwhelks may find it less thermally stressful to venture out of crevices and macroalgal cover under relatively cool temperatures. Darker dogwhelks show stronger physiological responses to heat, such as faster desiccation, than lighter-color conspecifics (Etter, 1988). Future experiments could, thus, examine if dogwhelk behavioural responses to temperature are related to shell color, which may contribute to the rareness in the observed purple dogwhelks.

Data availability

F1000Research: Dataset 1. Intertidal temperatures 12-Aug-2014, 10.5256/f1000research.9707.d137308 (Ehlers & Ellrich, 2016a)

F1000Research: Dataset 2. Intertidal temperatures 01-Sep-2015, 10.5256/f1000research.9707.d137309 (Ehlers & Ellrich, 2016b)

F1000Research: Dataset 3. Intertidal temperatures 12-Jun-2016, 10.5256/f1000research.9707.d137310 (Ehlers & Ellrich, 2016c)

F1000Research: Dataset 4. Intertidal temperatures 21-Aug-2016, 10.5256/f1000research.9707.d137311 (Ehlers & Ellrich, 2016d)