Physiological response of the invasive polychaete Branchiomma boholense (Anellida: Sabellae) to ocean acidification: results of a transplant experiment at a CO2 vent system.

Branchiomma boholense is a sabellid polychaete considered highly invasive in the Mediterranean Sea as in few years it was able to expand its distribution range across the entire basin. Along the Italian coasts, this species has also been reported in venting areas near Castello Aragonese (Ischia Island) where CO2 emissions lower the seawater pH up to 6.00 values. The aim of this study was to test the physiological response of B. boholense to the decrease of seawater pH, measuring the activity of carbonic anhydrase (CA), a zinc metalloenzyme known to be affected by low pH conditions in other taxa. A transplant experiment employing the CO2 vents of Ischia was carried out. Worms were collected from a control pH area in the Mar Grande of Taranto and transplanted in two areas around Ischia, a control and a high pCO2 area. Three sites per area were utilized. Thirty individuals per site were exposed in plastic cages for 30 days. CA activity was measured electrometrically on the tissue homogenate of each specimen. Results of the ANOVA analysis on translocated worms showed no significant differences in CA activity either between individuals exposed to control and acidified conditions, nor among specimens transplanted in different sites within the same area. However, organisms exposed to the acidified area showed a decrease of their wet weight of about 20% compared to animals translocated in control sites. Moreover, CA activity was also investigated in specimens native from normal (Taranto) and high pCO2 environments (Ischia). The analysis of their wet weight revealed a marked decrease (by about 70%) in worms from the acidified area with respect to control animals. Interestingly, protein tissue content showed a significant (50%) increase in specimens native from the vent system with respect to the Taranto control area. Therefore, significant differences in CA activity (U/g wet weight) between specimens naturally exposed to normal and acidified conditions were also observed, with individuals living in acidified environments showing doubled values of enzymatic activity. The results of this study demonstrated that B. boholense is inclined to maintain a great homeostatic capacity when exposed to low pH. However, a marked decrease in body weight was observed under chronic exposure to high pCO2, suggesting the need of further studies to understand if the maintenance of a homeostatic balance come at the expense of other energy demanding physiological processes