Rockpool survey, Prussia Cove, fourteen low tides

Bessy's Cove is the second of the four coves at Prussia Cove, on the east side of Cudden Point in Mount's Bay. The Site of Special Scientific Interest that runs from the point along to here was designated for the greenstone, not for anything living on it, which I think about most times I climb down.

Between 8 November and 20 March I went down at low water fourteen times and counted things. This is not a monitoring programme and I will not pretend it is one. It is a notebook, and the holes in it are the interesting part.

Method

Timed search, forty-five minutes nominal, starting about an hour before predicted low water. Same stretch of lower shore, same six pools, same order. Individual counts where the animal is countable, cover by eye where it is not. Boulders turned and put back the same way up and the same way round. Water temperature from a hand thermometer held two minutes in the largest pool, which is a pool temperature and not a sea temperature.

Tide heights are predicted low water for Penzance (Newlyn), metres above chart datum. Chart datum at Newlyn sits 3.05 m below Ordnance Datum, and Ordnance Datum is itself defined from mean sea level at Newlyn between 1915 and 1921, so the figures are more circular than they look.

The visits

#	Date	LW	LW (m CD)	Pool °C	Weather	Note
1	8 Nov	12:51	0.86	13.8	flat, grey, windless	first Aeolidia, on a beadlet
2	19 Nov	10:26	1.38	13.1	SW 4, drizzle, mild	poor light, poor counts
3	22 Nov	12:05	1.38	12.9	bright, cold, NE breeze	Codium clean, no Elysia
4	6 Dec	11:54	0.80	12.2	clear, hard frost, still	three Elysia at last
5	20 Dec	11:17	1.40	11.6	grey, heavy swell, brown	Codium stripped out
6	4 Jan	11:48	0.79	11.0	calm, high cloud, cold	two clingfish, one slab
7	10 Jan	16:10	1.73	10.4	dusk, still, bitter	25 minutes, lost light
8	18 Jan	11:04	1.34	10.6	W 5, hail, foul	three velvet crabs
9	31 Jan	09:57	1.21	10.1	overcast, mild, damp	sea hare, 18 mm, red
10	15 Feb	10:05	1.49	9.7	rain, windless, dark	worst of the fourteen
11	20 Feb	13:06	0.68	9.4	clear, cold, offshore	Semibalanus, shaded gully
12	22 Feb	14:18	0.98	9.9	sun, thin cloud, calm	Asterina eggs, boulder
13	4 Mar	12:00	0.58	9.6	grey, flat calm, cold	best water, 55 minutes
14	20 Mar	12:06	0.53	10.3	bright, still, cold	see below

What was in them

Counted animals only, visits out of fourteen. Barnacles, breadcrumb sponge, coralline turf and wireweed are cover estimates and are not here.

Species	Name I use	Visits	Peak	Total
Steromphala umbilicalis	flat topshell 1	14	117	921
Phorcus lineatus	toothed topshell	14	88	704
Actinia equina	beadlet anemone	14	63	549
Anemonia viridis	snakelocks	14	31	262
Littorina obtusata	flat winkle	13	40	246
Porcellana platycheles	porcelain crab	11	26	143
Carcinus maenas	shore crab	14	19	131
Nucella lapillus	dog whelk	12	22	118
Lipophrys pholis	shanny	13	9	58
Aplysia punctata	sea hare	4	41	57
Elysia viridis	green elysia	6	14	39
Asterina gibbosa	cushion star	10	7	34
Gobius paganellus	rock goby	9	4	21
Actinia fragacea	strawberry anemone	8	4	19
Necora puber	velvet crab	6	3	11
Lepadogaster purpurea	Cornish sucker	5	2	7
Aeolidia sp.	grey sea slug 2	3	2	4

What the numbers say

Toothed topshells on every visit, most of them fat adults. Phorcus lineatus is a warm water indicator species that the MarClim work has tracked at its northern edge for two decades, and that edge has moved along the North Wales coast within living memory. My fourteen counts at one site over one winter say nothing whatsoever about that. They say there are a lot of toothed topshells at Bessy's Cove, which anybody could have told you.

The more interesting result is an absence. Chthamalus montagui holds the upper rock here in continuous sheets. Semibalanus balanoides, the animal most of Britain means when it says barnacle, I could only find on visits 11 and 13, both times in the same shaded gully at the back of the shore. That fits the coast: it thins out towards the far south west and can be missing altogether from parts of Cornwall and from Scilly. It is still odd to stand on a Cornish shore and struggle to find the commonest barnacle in the country.

The slug counts track seaweed rather than the calendar. No Elysia viridis for the first three visits, three of them on 6 December, none on 20 December because the swell had taken the Codium out of that pool entirely, then nothing until the weed grew back in February, after which they were on it every time and nowhere else. That is a seaweed signal with slugs attached.

Sea hares came late. One on 31 January, three on 22 February, twelve on 4 March. On 20 March the shallow pool at the top of the gully, which had held nothing but coralline turf and two shore crabs all winter, had forty-one juvenile sea hares in it, most under 25 mm and still red from a diet of Ceramium, and I stopped writing and sat down on the wet rock for a while.

Where this falls down

No fixed quadrat. "Peak count" is therefore a statement about how much shore was uncovered and how long I stayed, not about how many animals were present. Look where the peaks fall: visits 11, 13 and 14, the three lowest waters of the winter. If I drew a line through those and called it a spring increase I would be lying with real numbers.

Effort ran from 25 minutes to 55. Detectability is neither constant nor knowable. Elysia viridis is 10 to 20 mm and pigmented to match its food, and I recorded none on the first three visits. Either they were absent or I had not yet learned to see them, and nothing in my data separates those accounts. @dust/limiting-magnitude-from-first-principles is the same problem with a better instrument: you cannot report an absence without stating your detection limit, and I never measured mine.

Turning boulders finds porcelain crabs and cushion stars and misses anything with the sense to swim off. Then there is the cold, which does not sound like a source of error until you look at the handwriting. Visit 10 has four legible lines for the whole session.

Done properly this wants fixed quadrats on a marked transect, SACFOR classes for the encrusting taxa, two observers, and a written record of effort per visit. I do a version of that all week for a living. The point of Sunday is that nobody is checking.

Footnotes

1. This was Gibbula umbilicalis in every book I own. The genus was split on molecular and morphometric grounds and the western European species went to Steromphala, so half my notebooks now disagree with the other half. ↩

2. Recorded as Aeolidia sp. rather than Aeolidia papillosa, because Kienberger and colleagues showed in 2016 that British papillosa is two species, papillosa and the newly described filomenae, living side by side here. I cannot separate them in a rockpool in February with wet hands, so I do not claim to. ↩