Species abundance, sexual encounter and bioluminescent signalling in the deep sea.

The problems faced by deep-sea animals in achieving sexual and other encounters require sensory and effector systems the synergy of which can span the often very substantial distances that separate individuals. Bioluminescent systems provide one of the links between individuals, and the sexual dimorphism of some photophores suggests that they are employed to attract a mate. However, nearest-neighbour values for many deep-sea animals put them beyond the effective range of bioluminescent signals and it is therefore likely that these signals are employed at intermediate ranges, once an initial contact (perhaps olfactory) has been made.

Relationship of dorsoventral eyeshine distributions to habitat depth and animal size in mesopelagic decapods.

Eyeshine distribution patterns recorded from the eyes of 19 mesopelagic decapod species were examined and related to the depths at which the species are found. For most species examined, eyeshine was found to be brighter ventrally than dorsally. Deep-water decapod species that do not undergo diel vertical migrations had brighter dorsal eyeshine than migratory species. Eyeshine intensity increased with body size in five of the species examined and decreased in two. These changes in eyeshine intensity may be an adaptation to variations in depth distributions that occur with increasing body size. It is suggested that the depth and size-related changes reflect the importance of remaining camouflaged in the mesopelagic realm and are an example of ecologically functional development.

Cuticular Photophores of Two Decapod Crustaceans, Oplophorus spinosus and Systellaspis debilis.

The organization, ultrastructure, growth, and development of two types of cuticular photophore in oplophorid shrimps (Oplophorus spinosus and Systellaspis debilis) are described. Photophores located in the third maxilliped consist of a unit structure comprising a single photocyte and associated pigment cells. Reflecting pigment cells contain white pigment and form an apical cap above the photocyte; sheath cells contain red carotenoid pigment and form a light-absorbing layer around the photophore. Photophores located on the pleopods are compound structures comprising many photocytes. They also contain the same types of pigment cell that are found in the unit photophores of the maxilliped. Paracrystalline bodies at the apical ends of the photocytes in both types of photophore are thought to be associated with light generation. Both types of photophore have mechanisms for tilting in the pitch plane. In the maxilliped, the apices of the photophores are connected to a ligament that has its origin in the propodus. Flexion or extension of the dactylus displaces the ligament, which tilts the photophores synchronously. The cuticular window beneath each photophore remains stationary. The tilt mechanism of the pleopod photophores is quite different, and depends upon muscular contraction. A main and an accessory longitudinal muscle cause backwards rotation of the photophore by deforming the cuticle surface. A loop muscle that passes around the anterior face of the photophore causes forward rotation. The two mechanisms optimize the use of the photophores in ventral camouflage. They allow photophore rotation to compensate for changes in the shrimp's orientation in the plane of pitch and thus maintain the ventral direction of the luminescence.

Comparative Morphology of the Eyes of Postlarval Bresiliid Shrimps From the Region of Hydrothermal Vents.

The structure and ultrastructure of the eyes of postlarval vent shrimps provisionally designated `Alvinocaris' and `Chorocaris' are described. The eyes of the postlarval `Alvinocaris' are cylindrical, borne on short stalks, and contain closely packed rhabdoms. The ommatidia lack dioptric apparatus and have rhabdoms extending almost to the cornea. The rhabdoms consist of orthogonal layers of microvilli typical of crustacean rhabdoms. The eyes of the `Chorocaris' are similar, but the rhabdom layer extends back through the reduced eyestalks and covers some of the dorsal surface of the cephalothorax. The rhabdoms from both the anterior and the thoracic regions consist of layered microvilli. The eyes of a slightly smaller postlarval vent shrimp, termed `Type A', differ. Although clearly related to the other vent shrimps, Type A has stalked eyes held at an angle to the head. The eye displays a gradient of ommatidial development, with the older ommatidia closely resembling those seen in the other postlarval types. Between the cornea and the rhabdom layer, the youngest ommatidia possess quadripartite crystalline cones similar to those seen in related families of caridean shrimps; these are absent in the more mature ommatidia. The external structure of the anterior and thoracic eyes of juvenile Rimicaris exoculata (after settlement at the vent site) is also described. Juveniles up to 9 mm in carapace length have anterior corneas similar to those seen in postlarvae, whereas in larger specimens the corneas are progressively replaced by an ocular plate.

The widespread occurrence and tissue distribution of the imidazolopyrazine luciferins.

Bioluminescence has been reported to occur in 17 phyla and at least 700 genera. However, the luciferin chemistry of the majority of luminous organisms has yet to be determined. The most common chemistry which is known to occur in deep sea bioluminescence is imidazolopyrazine bioluminescence. The main aim of this study was to examine the phyletic and tissue distribution of imidazolopyrazine luciferins. This will facilitate analysis of imidazolopyrazine bioluminescence at the cellular and molecular levels and, in particular, how and when its chemistry is controlled and expressed in vivo. Assays for both known imidazolopyrazines were established and a range of fresh organisms and tissue were analysed, i.e. fish, cephalopods, copepods, ostracods, amphipods and euphausiids. The main findings were that the number of genera in which coelenterazine has been detected has been increased from 52 to about 90. Also, for the first time, the other known imidazolopyrazine luciferin, Vargula-type luciferin, was quantified in the ostracod Cypridina dentata, but was not detected in any of its potential predators. Neither imidazolopyrazine luciferin was found in several luminous stomiiform fish assayed. Coelenterazine was measured in the livers and photophores of a number of cephalopods and it is apparent that coelenterazine is responsible for both modes of luminescence.

Systematic distribution of bioluminescence in living organisms.

A list of the genera of living organisms known or believed to contain luminous species is provided in the Appendix, in a systematic context. The constraints on the accuracy of such a list and some aspects of the apparent distribution of bioluminescence are discussed.

How to survive in the dark: bioluminescence in the deep sea.

Bioluminescent tissues in marine organisms may take the form of point source emitters, internal or external glandular organs or glands containing bacterial symbionts. In many cases additional accessory optical structures have been evolved to increase the efficiency of emission, to restrict the angular direction, to focus or collimate the light, to alter its spectral distribution or to guide it from the source to a distant point of emission. This variety of structure is matched by a variety of locations of luminous tissues and organs over the body of different animals. The time course, intensity and spectral nature of bioluminescence are equally variable. Information can be encoded in the spatial pattern, time course and spectral characteristics of bioluminescent signals and the recognition of this information depends upon the visual abilities of the target organism. The known characteristics of the bioluminescence of certain marine organisms are compared with those that would be predicted for different functional interpretations. It is probable that each type of bioluminescent signal in deep-sea organisms is but one factor in the suite of activities which make up a particular behavioural pattern.

Far red bioluminescence from two deep-sea fishes.

Spectral measurements of red bioluminescence were obtained from the deep-sea stomiatoid fishes Aristostomias scintillans (Gilbert) and Malacosteus niger (Ayres). Red luminescence from suborbital light organs extends to the near infrared, with peak emission at approximately 705 nanometers in the far red. These fishes also have postorbital light organs that emit blue luminescence with maxima between 470 and 480 nanometers. The red bioluminescence may be due to an energy transfer system and wavelength-selective filtering.