Heimo L. Nilsson

Classification of amphipod compound eyes- the fine structure of the ommatidial units (Crustacea, Amphipoda)

SummaryThe ultrastructure of the compound eyes of 13 amphipod species has been investigated. An amphipod type of compound eye can be characterized by the constellation and consistency of a number of morphological features, most of which are also found in other compound eyes. The amphipod eye falls into four sub-categories (types). The ampeliscid type has a tripartite aberrant lens eye; the lysianassid type has a reduced or no dioptric apparatus and a hypertrophied rhabdom; the hyperid type possesses a large number of ommatidial units with long crystalline cones and dark instead of reflecting accessory pigment; and finally, the gammarid type can be interpreted as a generalized amphipod type. The lysianassid type is adapted to low light intensities and demonstrates convergent development with the compound eyes of other deep-sea crustaceans. The ampeliscid type is more similar to the gammarid type. The type characterization of the amphipod compound eye might well serve as a basis and incentive for functional studies also revealing adaptational mechanisms.

A crustacean compound eye adapted for low light intensities (Isopoda)

SummaryThe optical performance of the apposition compound eye of the marine isopodCirolana borealis Lilljeborg (Crustacea) was investigated. The ommatidia comprise large lenses (diam. ca. 150 μm), spherical crystalline cones and hypertrophied rhabdoms. The 7 rhabdomeres are fused distally and open proximally. We have designated this rhabdom type as semifused. Distal pigment cells screen neighbouring ommatidia, and a well developed reflecting pigment layer surrounds the rhabdom. The focal length was determined in situ and refractive index measurements, raytracings, and eye mappings were made. The focus was found to lie well below the distal rhabdom tip. A theoretical acceptance function was constructed and a 50% acceptance angle of 45 ° was estimated. The eye parameter (p, according to Snyder 1977) of different ommatidia was between 44 and 14. This together with the anatomy demonstrate an optimation to extremely low light intensities. TheCirolana eye provides an example where acuity is sacrificed for the eye to be able to see at the low light intensities of the inhabitat.

Eye function of Mysis relicta Lovén (Crustacea) from two photic environments. Spectral sensitivity and light tolerance

Abstract Spectral sensitivities, S(λ), and light tolerances of the eyes of two geographically isolated populations of the opossum shrimp Mysis relicta Loven were studied by recording the electroretinogram. Measurements of the downward irradiance of light in the different water localities revealed a correlation between the spectral sensitivity peak of the mysid eye and the light-transmission properties of the water. Eyes of animals from the “red-transmitting” Lake Paajarvi (transmission maximum 600–700 nm) had a S(λ)max at ≈ 570 nm. The sensitivity of dark-adapted eyes was markedly suppressed by moderate light exposures. Three days of recovery in darkness restored eye sensitivity. Eyes of animals from Pojoviken bay (part of the Baltic Sea, transmission maximum in 565–585 nm) had a S(λ)max of ≈ 550 nm, and upon bright light exposures the sensitivity of the eyes recovered to the sensitivity of the dark-adapted state in ≈ 24 h. These results have a bearing on the adaptional and evolutionary forces acting on the mysid eye, and on the occurrence of sibling species in the Tvarminne area. The importance of spectral sensitivity and light tolerance on vertical migration as well as the impact of light on photoreceptor membrane disruption are briefly discussed.

Spectral and visual sensitivities of Cirolana borealis Lilljeborg, a deep-water isopod (Crustacea : Flabellifera)

Abstract The electroretinogram (ERG) of the dark-adapted compound eye of Cirolana borealis Lilljeborg has a corneal negative on response. The latency period varies between 140 and 8 ms and depends on stimulus intensity. The spectral sensitivity, measured over the range 406–673 nm, has its maximum around 495–528 nm, corresponding to a visual pigment of 514 nm, according to Dartnalls nomogram urves. Only one pigment appears to be present. The ERG visual threshold sensitivity for an extended light source and defined as a 5 μV response at 495 nm, is 2.1 · 10 8 qu·cm −2 ·s −1 ( n =17). One eye was 10 times more sensitive than the others. The electrophysiological results confirm that the eye of Circolana is of the scotopic type and thus is well adapted to perceive light of low intensities.

Rhabdom breakdown in the eye of Cirolana borealis (Crustacea) caused by exposure to daylight

SummaryThe effect of daylight on the compound eye was investigated in the deep-water crustacean isopod Cirolana borealis Lilljeborg. The animals were captured and fixed at night (‘dark-exposed’, i.e. not exposed to light) and day (‘daylight-exposed’), respectively. Changes in light and darkness have an effect on the retinula cells; the ultrastructure of dark-exposed eyes is characterized by well-preserved organelles and cytoplasm. The photoreceptor membranes covering the microvilli are regularly aligned, and the outline of the villi is smooth. Electron-dense pigment granules are evenly distributed in the cytoplasm of the retinula cell outside the rhabdom. Daylight-exposed eyes differ from the dark-exposed eyes in the following aspects: (i) the microvilli are disrupted, (ii) retinula-cell pigment is found in the rhabdom, and (iii) the cytoplasm of retinula cells is vesiculated. These results are interpreted as retinal damage caused by excess exposure to light.

Changes in behaviour and eye-morphology of BoreomysisMegalops G.O. Sars (Crustacea : Mysidacea) following exposure to short periods of artificial and natural daylight

Abstract Boreomysis megalops G.O. Sars were collected from 240 m depth using a protective cod-end, which shielded the eyes from light exposure. Animals were divided into groups which were exposed to darkness (DE) or to natural or artificial daylight (LE) for periods of 1–15 min or 4 h. Some animals were thereafter kept in aquaria under simulated habitat light conditions (light/dark cycles). The animals were observed in infra-red (IR) light. DE animals showed a vertical zonation behaviour in the laboratory similar to that obtained from sledge data: i.e. the animals stayed close to the bottom during daytime (light) and spread out vertically at night (darkness). LE individuals showed putative pathological changes both in zonation behaviour in the laboratory and in eye-morphology. The photoreceptor membranes of the six main retinula cells were damaged. The seventh cell was unaffected by light exposures. A correlation was found between the extent of damage to the eye and the degree of modification of vertical zonation behaviour. The groups exposed to the least light showed the smallest changes in zonation behaviour and eye-morphology. The light induced changes were not reversed even after 4 days in the aquarium under simulated habitat light conditions. In conclusion, deep living animals with well-developed and very light sensitive eyes should be protected from daylight during sampling and handling. IR light should be used for observation purposes.

Fine structure and convergent development of the Cirolana compound eye (Crustacea Isopoda)

SummaryEach sessile eye comprises about 60 ommatidia with large lenses (diameter 100–150 μm), spherical crystalline cones, and hypertrophied rhabdoms (diameter about 100 μm). The cones are formed by two main cone cells, and in addition two accessory cone cells are present. Seven retinula cells contribute to the rhabdom, which is fused distally and open proximally (separated rhabdomeres). A special cell type, previously designated ‘hyaline cells’ are shown to be reflecting pigment cells. These cells form a well-developed tapetal layer. Distal pigment cells screen neighbouring ommatidia. Further, the eye is delimited by two membranes homologous to the fenestrated and eye capsule membranes present in several other isopod groups. The hypertrophied rhabdoms and the elaborate tapetal layer separate the morphology of the Cirolana eye from that of other isopod eyes. These structural features makes the Cirolana eye a case of convergent development with several other deep water living crustaceans.

Eye camouflage in the isopod crustacean Astacillalongicornis (Sowerby)

Abstract The stick-like Astacilla longicomis (Sowerby) (Isopoda, Crustacea) is found mainly on the anthozoan Funiculina . The unusual development of its eyes is interpreted as a part of a refined overall camouflage. The apposition compound eyes have two functional features related to camouflaging. 1. (1) The major part of each protruding eye-bulb is transparent owing to the lack of screening pigment around the crystalline cones. Optical isolation of the ommatidia is maintained by a proximal refractive index gradient in the crystalline cones. 2. (2) The proximal cone tips are surrounded by a layer of reflecting pigment cells resulting in a golden hue in the depth of the eye. Probably, neither of these properties has any impact on vision. Thus concealment is the only functional explanation. Both adaptations together prevent the eyes from appearing as large dark spots on the otherwise relatively well camouflaged animal.