An extreme heatwave in August 2021 eradicated the Perna population at the sampling sites -
empty shells washed ashore at all sites, with the exception of the Haifa B site, where dead
shells still attached by their byssus to the concrete surface were collected in-situ. Specimens
are deposited at Steinhardt Museum of Natural History, Tel Aviv University (SMNH – MO
100507–100511).
Results
A total of 5691 live P. perna were collected. The number of individuals and population size
structure reveal considerable differences among the four sites, abundance ranging from
2155 to 8022 per m². Average shell size was greater for individuals collected in Haifa sites
than in the southern sites. Alone among the sample sites, Haifa B samples were clearly bimodal, the largest size cohorts measuring 22.6–25.0, 57.1–60.0 mm and 27.6–30.0, 67.6–
70.0 mm, respectively. Mussels of > 50 mm shell length were extremely rare in Ashdod A,
B and Haifa A sites (7 of 5303 specimens), but were well represented at Haifa B samples,
where a third of the mussels attained this size. A total of 147 dead shells were collected off
the concrete slabs at Haifa B following the August 2021 heatwave. The sample size–
frequency distribution was unimodal with the largest size cohort (≥ 10%) 77.6–80.0 mm,
and largest shell length 101.8 mm. Shell sizes differed greatly between June and August: in
June 66% of the shells were < 50 mm (average size 38 mm, ±19.7 SD, n = 194), while in
August only 3% were < 50 (average size 75 mm, ± 12.5 SD, n = 174).
Discussion
The decimation of the Israeli population within 2.5 months of its first and only livesampling left us with fragmentary but tantalizing data. Yet, the remaining in-situ shells
were significantly larger than those collected in June – the largest specimens being
101.8 in shell length compared to 82.7 mm shell length in the earlier sample. The
postulated growth rate is high but aligns with figures from other warm water locales.
Marine heatwaves (MHWs) in the Mediterranean Sea were linked to extirpations of
hard substrate macroinvertebrates. Mortalities of marine intertidal mussels have led to
studies on thermal Exposure, though few documented heatwave-induced mortalities in
introduced mussels. The sole documented case of heat-induced mortality in introduced
P. perna concerns the population in Texas, USA, which collapsed when mean summer
seawater temperature rose in 1997, but reappeared in 2000 when the average summer
surface-water temperature was 28.5 °C. Over the past two decades, concurrent with
long-term persistent warming, the mean MHW frequency and duration in the Eastern
Mediterranean increased by 40% and 15% (Garrabou et al., 2022), respectively, and is
projected to increase further, favoring thermally-tolerant species and those adapted to
warmer waters. It is likely that native stenothermal biota unable to shift their range to
deeper and/or colder water may endure increasing stress and demographic attrition, and
replacement by warm water invasives, likely introduced through the Suez Canal.
Bibliography
GALIL B.S., MIENIS H.K., MENDELSON M., GAYER K., GOREN M. (2022) - Here today,
gone tomorrow-the Levantine population of the Brown mussel Perna perna obliterated by
unprecedented heatwave. Aquat. Invasions 17(2): 174-185.
GARRABOU J., GOMEZ-GRAS D., MEDRANO A., CERRANO C., PONTI M., SCHLEGEL
R., BENSOUSSAN N,…, HARMELIN J.-G. (2022) - Marine heat waves drive recurrent
mass mortalities in the Mediterranean Sea. Global Change Biology. DOI: 10.1111/gcb.16301
2nd Mediterranean Symposium on the Non-Indigenous Species (Genoa, Italy, 22-23 September 2022)
96
empty shells washed ashore at all sites, with the exception of the Haifa B site, where dead
shells still attached by their byssus to the concrete surface were collected in-situ. Specimens
are deposited at Steinhardt Museum of Natural History, Tel Aviv University (SMNH – MO
100507–100511).
Results
A total of 5691 live P. perna were collected. The number of individuals and population size
structure reveal considerable differences among the four sites, abundance ranging from
2155 to 8022 per m². Average shell size was greater for individuals collected in Haifa sites
than in the southern sites. Alone among the sample sites, Haifa B samples were clearly bimodal, the largest size cohorts measuring 22.6–25.0, 57.1–60.0 mm and 27.6–30.0, 67.6–
70.0 mm, respectively. Mussels of > 50 mm shell length were extremely rare in Ashdod A,
B and Haifa A sites (7 of 5303 specimens), but were well represented at Haifa B samples,
where a third of the mussels attained this size. A total of 147 dead shells were collected off
the concrete slabs at Haifa B following the August 2021 heatwave. The sample size–
frequency distribution was unimodal with the largest size cohort (≥ 10%) 77.6–80.0 mm,
and largest shell length 101.8 mm. Shell sizes differed greatly between June and August: in
June 66% of the shells were < 50 mm (average size 38 mm, ±19.7 SD, n = 194), while in
August only 3% were < 50 (average size 75 mm, ± 12.5 SD, n = 174).
Discussion
The decimation of the Israeli population within 2.5 months of its first and only livesampling left us with fragmentary but tantalizing data. Yet, the remaining in-situ shells
were significantly larger than those collected in June – the largest specimens being
101.8 in shell length compared to 82.7 mm shell length in the earlier sample. The
postulated growth rate is high but aligns with figures from other warm water locales.
Marine heatwaves (MHWs) in the Mediterranean Sea were linked to extirpations of
hard substrate macroinvertebrates. Mortalities of marine intertidal mussels have led to
studies on thermal Exposure, though few documented heatwave-induced mortalities in
introduced mussels. The sole documented case of heat-induced mortality in introduced
P. perna concerns the population in Texas, USA, which collapsed when mean summer
seawater temperature rose in 1997, but reappeared in 2000 when the average summer
surface-water temperature was 28.5 °C. Over the past two decades, concurrent with
long-term persistent warming, the mean MHW frequency and duration in the Eastern
Mediterranean increased by 40% and 15% (Garrabou et al., 2022), respectively, and is
projected to increase further, favoring thermally-tolerant species and those adapted to
warmer waters. It is likely that native stenothermal biota unable to shift their range to
deeper and/or colder water may endure increasing stress and demographic attrition, and
replacement by warm water invasives, likely introduced through the Suez Canal.
Bibliography
GALIL B.S., MIENIS H.K., MENDELSON M., GAYER K., GOREN M. (2022) - Here today,
gone tomorrow-the Levantine population of the Brown mussel Perna perna obliterated by
unprecedented heatwave. Aquat. Invasions 17(2): 174-185.
GARRABOU J., GOMEZ-GRAS D., MEDRANO A., CERRANO C., PONTI M., SCHLEGEL
R., BENSOUSSAN N,…, HARMELIN J.-G. (2022) - Marine heat waves drive recurrent
mass mortalities in the Mediterranean Sea. Global Change Biology. DOI: 10.1111/gcb.16301
2nd Mediterranean Symposium on the Non-Indigenous Species (Genoa, Italy, 22-23 September 2022)
96