Dark-adaptation in the eyes of a lake and a sea population of opossum shrimp (Mysis relicta): retinoid isomer dynamics, rhodopsin regeneration, and recovery of light sensitivity

We have studied dark-adaptation at three levels in the eyes of the crustacean Mysis relicta over 2–3 weeks after exposing initially dark-adapted animals to strong white light: regeneration of 11-cis retinal through the retinoid cycle (by HPLC), restoration of native rhodopsin in photoreceptor membranes (by MSP), and recovery of eye photosensitivity (by ERG). We compare two model populations (“Sea”, Sp, and “Lake”, Lp) inhabiting, respectively, a low light and an extremely dark environment. 11-cis retinal reached 60–70% of the pre-exposure levels after 2 weeks in darkness in both populations. The only significant Lp/Sp difference in the retinoid cycle was that Lp had much higher levels of retinol, both basal and light-released. In Sp, rhodopsin restoration and eye photoresponse recovery parallelled 11-cis retinal regeneration. In Lp, however, even after 3 weeks only ca. 25% of the rhabdoms studied had incorporated new rhodopsin, and eye photosensitivity showed only incipient recovery from severe depression. The absorbance spectra of the majority of the Lp rhabdoms stayed constant around 490–500 nm, consistent with metarhodopsin II dominance. We conclude that sensitivity recovery of Sp eyes was rate-limited by the regeneration of 11-cis retinal, whilst that of Lp eyes was limited by inertia in photoreceptor membrane turnover.

Further records of the opossum shrimp Neognathophausia ingens (Dorhn, 1870) (Peracarida: Lophogastrida: Gnathophausiidae) from Brazil

Neognathophausia ingens (Dohrn, 1870) is a bathypelagic crustacean distributed worldwide. In the western Atlantic it has been recorded off the La Plata region, Argentina, and off the coast of Brazil, from the state of Bahia to Rio de Janeiro. We provide new records of N. ingens from the state of Rio Grande do Sul, Brazil, about 1188 km south of its previous distribution limit off Brazil, filling the gap between the central West Atlantic and Argentina. Five specimens were analyzed and drawings for all body regions and appendages are presented. Mandible, thoracopods 2, 4, 5, 6 and 8 and pleopods are illustrated and described for the first time. An updated synonym list and a distribution map for this species in Brazil are provided.

Feeding ecology of non-native Siberian prawns, Palaemon modestus (Heller, 1862) (Decapoda, Palaemonidae), in the lower Snake River, Washington, U.S.A.

We used both stomach content and stable isotope analyses to describe the feeding ecology of Siberian prawns Palaemon modestus (Heller, 1862), a non-native caridean shrimp that is a relatively recent invader of the lower Snake River. Based on identifiable prey in stomachs, the opossum shrimp Neomysis mercedis Holmes, 1896 comprised up to 34-55% (by weight) of diets of juvenile to adult P. modestus, which showed little seasonal variation. Other predominant items/taxa consumed included detritus, amphipods, dipteran larvae, and oligochaetes. Stable isotope analysis supported diet results and also suggested that much of the food consumed by P. modestus that was not identifiable came from benthic sources — predominantly invertebrates of lower trophic levels and detritus. Palaemon modestus consumption of N. mercedis may pose a competitive threat to juvenile salmon and resident fishes which also rely heavily on that prey.