Ultrastructure of the eye of a euphausiid crustacean

The compound eye of the Antarctic euphausiid Euphausia superba is a spherical clear zone eye. The dioptric system consists of a hexagonally-faceted cornea, two corneagenous cells, two crystalline cone cells which form the bipartite crystalline cone, and two accessory cone cells. The dioptric system of each ommatidium is separated from that of adjacent ommatidia by six distal pigment cells and a basement membrane. The proximal tip of the crystalline cone is cupped by the distal ends of the seven retinula cells whose nuclei are arranged in a staggered array slightly distal to the middle of the clear zone. In the distal half of the clear zone, each narrow retinula cell column is surrounded by large proximal extensions of the six distal pigment cells. The pigment cells narrow more proximally and terminate at the proximal basement membrane. A specialized axial channel complex extends from the crystalline cone through the clear zone, and is continuous with a conical refractive element which caps the distal end of the rhabdom. The rhabdom is fused, and made up of alternating highly birefringent layers of orthogonally-oriented microvilli. It is surrounded by a narrow extra-cellular space which is continuous with the distal refractive element and a second conical refractive element at the proximal end of the rhabdom.     

The luminescence and photophores of euphausiid crustaceans

Euphausiid shrimps bear light organs, two pairs in the thorax, and four similar single organs in the abdomen: A pair on the eyestalks differ in structure from these others, and are not considered further by us. In most species the size of the organs is related to that of the animal, but they are relatively smaller in deeper living species. In some species certain light organs are larger in males. The flashes of light emitted begin abruptly, remain steady for several seconds and die away slowly. Euphausiids flash in response to light, including that of another euphausiid, and to certain drugs, notably 5 hydroxytryptamine and certain analogues.
The photophores contain a lantern where the light is generated, and three separate optical components which reflect and refract the light out of one pole of the organ. Our interpretation of the structures suggests that light generation is probably controlled by alteration of the blood flow through the lantern, and that this in turn is under nervous control.     

Growth, metabolism and growth efficiency of a euphausiid crustacean Euphausia pacifica in the southern Japan Sea, as influenced by temperature

Growth (assessed from intermolt period and molt increment) andmetabolism (oxygen consumption) of the post-larva of Euphausiapacifica from the southern Japan Sea were determined at sevengraded temperatures ranging from 1 to 25°C. The intermoltperiod shortened progressively as temperature increased from1 to 20°C, but an effect of temperature on molt-to-moltgrowth increment was not seen. Oxygen consumption rates wereaccelerated by the increase in temperature up to 20°C. Beyond20°C, E.pacifica exhibited reduced oxygen consumption anddied within 1 day without molting. After removing the effectof body size, the relationships between growth rate and temperature,and between oxygen consumption rate and temperature, were established.The carbon budget was calculated as a function of temperature.Because of differential effects of temperature on growth andmetabolism, the net growth efficiency [K₂ growthx100/(growth+metabolism)]changed with temperature. The optimum temperature at which E.pacificaattained the maximum K₂ was 11.4°C, which was derived fromcalculation of cumulative carbon invested in growth and metabolismin this animal. In an alternative method, the optimum temperaturewas obtained mathematically by solving a set of differentialequations. The biological and ecological significance of theoptimum temperature which leads to the maximum K₂ is discussed.     

The gastric mill of euphausiid crustaceans: a comparison of eleven species

The morphology of the gastric mill of 11 species in the family Euphausiidae is described. Two distinct arrangements can be recognized with respect to presence or absence of lateral teeth. One is the well-developed gastric mill of Thsanoessa gregaria, T. inermis, T. macrura and Pseudeuphausia sinica; the other is the gastric mill without lateral teeth of Nematoscelis atlantica, N. difficilis, N. megalops, Nematobrachion flexipes, N. sexspinosum, Stylocheiron abbreviatum, and S. carinatum. This supports the close relationships of the four species of both genera, Thysanoessa and Pseudeuphausia on the one hand and seven species belonging to Nematobrachion, Nematoscelis, and Stylocheiron on the other hand. The hypothesis that species in the same genus have similar gastric mill morphology, which was once rejected in the genus Thysanopoda, is accepted in all five genera examined here. Functional morphology of euphausiid foreguts is discussed.     

A method for measuring the size of early euphausiid larvae

A simple method to measure early euphausiid larvae is tested on 35 specimens of Euphausia superba obtained in the Weddell-Scotia Confluence region (ranging from Calyptopes I to Furcilia III) against the measures obtained on the same specimens with a graduated eyepiece. The arrangement of the test includes two observers, four magnifications (from 7.5× to 62.5×) whenever feasible and two replications of each measurement. A total of 953 measurements were analyzed in an incomplete random blocks ANOVA design not finding significant differences between magnifications, methods, observers and their interactions. It was found that the relative differences between methods were of the same magnitude as the differences between replications (approximately 5%). The proposed method is less demanding on laboratory work, thus allowing the measurement of the large number of specimens needed to estimate size frequency distributions.     

Optical parameters of euphausiid eyes as a function of habitat depth

Summary The relationships between habitat depth, eye diameter relative to body length, and the dimensions of rhabdoms and crystalline cones have been examined for 13 species of three oceanic euphausiid genera with habitats ranging from near-surface waters to the deep-sea. Rate of eye growth decreases with depth. Longer rhabdoms may increase the visual sensitivity to point and extended light sources by an eye of a particular size with depth. Larger interommatidial angles suggest that visual acuity decreases at depth. Depth-related changes in euphausiid eyes are considered with respect to the probable roles of vision and bioluminescence in the deep-sea. Unusual features of the eyes of several species are described.     

The visual pigment of a stomatopod crustacean,Squilla empusa

Summary Stomatopod crustaceans are visually active animals which have large, mobile compound eyes of unique design. Aspects of their ecology and behavior suggest they may be able to discriminate hues. Isolated rhabdoms of the squillid stomatopod,Squilla empusa, were investigated using microspectrophotometry and fluorometry. A single rhodopsin, of _max507 nm, exists in the main rhabdom. Its stable metarhodopsin, with _max503 nm, possesses typical arthropod fluorescence characteristics. No evidence was found for a visual pigment with peak absorption in the ultraviolet. Vision in this animal might therefore be monochromatic.Abbreviation ASW artificial sea water     

The evolutionary history of crustacean segmentation: a fossil-based perspective

The evolution of segmentation in Crustacea, that is, the formation of sclerotized and jointed body somites and arrangement of somites into tagmata, is viewed in light of historical traits and functional constraints. The set of Early to Late Cambrian 'Orsten' arthropods have informed our current views of crustacean evolution considerably. These three-dimensionally preserved fossils document ancient morphologies, as opposed to purely hypothetical models and, because of the unusual preservation of larval stages, provide us with unparalleled insight into the morphogenesis of body somites and their structural equipment. The variety of evolutionary levels represented in the 'Orsten' including lobopodians, tardigrades, and pentastomids also allows phylogenetic interpretations far beyond the Crustacea. The 'Orsten' evidence and data from representatives of the Lower Cambrian Chengjiang biota in southwestern China, including phylogenetically earlier forms, form the major source of our morphology-based review of structural and functional developments that led toward the Crustacea. The principal strategy of arthropods is the simultaneous development of head somites, as expressed in a basal "head larva," and a successive addition of postcephalic somites from a preterminal budding zone with progressive maturation of metameric structures. This can be recognized in the developmental patterns of extant and fossil representatives of several euarthropod taxa, particularly crustaceans, trilobites, and chelicerates (at least basally). The development of these taxa points to an early somite-poor and free-living hatching stage. Embryonic development to a late stage within an egg, as occurring in recent onychophorans and certain in-group euarthropods, is regarded as achieved several times convergently.     

The composition of crustacean muscle fibres

• 1.1. The ionic composition and amino acid content of the flexor muscles of seven species of decapod crustaceans have been investigated under defined conditions.
• 2.2. Haemolymph of the same animals has also been analysed.
• 3.3. The results show that muscle tissue of these animals has substantially similar intracellular ionic concentrations but widely varying water contents.