Ammonia accumulation in the brood chamber of the estuarine gastropod Crepipatella dilatata: How big a problem for mothers and brooded embryos?

Abstract

Crepipatella dilatata is an estuarine gastropod that broods its encapsulated embryos in the pallial cavity for several weeks before releasing juveniles. When environmental quality declines (e.g., in response to decreased salinity), females seal themselves against the substrate and isolate the pallial cavity from surrounding seawater, altering the chemistry of the pallial fluid surrounding the embryos. We documented rates at which total ammonia concentrations (NH 4 +–N + NH 3) increased in the pallial cavity during these extended periods of isolation, as well as excretion rates for both adult females and developing embryos (veligers). Also, we quantified the effects of elevated ammonia concentrations on both adults (mortality, egg capsule abandonment, and neck-radular activity) and veligers (mortality, and activity of the velum and velar cilia). In the pallial fluid, ammonia concentration increased about 150% during the first 16 h of female isolation from about 7–17 μg ammonia l −1, while excretion rates decreased nearly 85%, from an average of 5.5 to about 0.85 μg ammonia h −1 g −1 after 12 h of isolation. Brooded veligers also decreased their individual excretion rates over time, from 0.0035 to 0.0007 μg ammonia h −1 veliger −1 over 12 h. Ammonia accumulation eventually caused females to abandon their egg capsules and to detach from the substrate; in the field, this would probably lead to the death of both the encapsulated embryos and the detached females, but this happened only at concentrations of ≥ 0.1 mg ammonia l −1. In contrast, veligers withstood concentrations up to 34 mg ammonia l −1 for 72 h without dying, although ciliary activity and velar lobe activity decreased at concentrations of ≥ 0.1 mg ammonia l −1. Ammonia tolerance did not seem to change with continued development. Although previous studies have shown that prolonged isolation causes dangerous declines in oxygen and pH in pallial fluid for this species, the accumulation of ammonia in itself seems too low to cause direct harm to adults or developing embryos.