T. testudinalis is a cold-water, northern species whose range in Europe has contracted considerably since c. 1920 with warming of the climate. There have been short term fluctuations in numbers and range. A decline in numbers c. 2000 in Northern Ireland with a more recent recovery was noticed (J. Nunn, pers. comm. July 2020).
T. testudinalis was not found in the Isle of Man in the years 2000 – 2005 where it had been previously found in the 1970s and early 1980s and was regarded as ‘loss of species from south of Isle of Man’ (Hawkins in Mieszkowska, 2005). However, it was photographed (images above) in SW Isle of Man by subaqua divers at the Calf of Man in 2014 by P. Lightfoot, and at Niarbyl in 2017 by D. Kipling so it either survived the decline in small, unobserved numbers or re-established from spat fall probably originating from the population in nearby (58 km) Northern Ireland.
Mieszkowska, N., Leaper, R., Moore, P., Kendall, M.A., Burrows, M.T, Lear, D., Poloczanska, E., Hiscock, K., Moschella, P.S., Thompson,R.C., Herbert, R.J., Laffoley, D., Baxter, J., Southward, A.J., & Hawkins, S.J. 2005. Assessing and predicting the influence of climatic change using eulittoral rocky shore biota. M.B.A. Occasional publication No. 20.
www.researchgate.net/publication/281164392_Assessing_and_…
APPENDIX re range advance/retreat: flic.kr/p/2jW74ig