Otoliths are dense structures in the ears of fishes that function in hearing and gravity perception. Otolith (sagitta) diameters, as percentages of standard length (% SL), are calculated for 247 marine fish species in 147 families and compared by taxonomic group (usually order), habitat and presence or absence of luminescence. Otolith sizes range from 0.4-31.4 mm and 0.08-11.2% SL. The eel and spiny eel orders Anguilliformes and Notacanthiformes have small to very small otoliths, as do the triggerfish order Tetraodontiformes, pipefish order Gasterosteiformes, billfish suborder Scombroidei and many of the dragonfish order Stomiiformes. The soldierfish order Beryciformes has moderate to very large otoliths. The perch order Perciformes has a wide range of otolith sizes but most have small to moderate otoliths 2-5% SL. Only 16 out of the 247 species have the relatively largest otoliths, over 7% SL. Seven out of these 16 species are also luminous from a variety of habitats. Luminous species have slightly to much larger otoliths than non-luminous species in the same family Both beryciforms and luminous fishes live in low-light environments, where acute colour vision is probably impossible. Most fishes of the epipelagic surface waters have very small otoliths, perhaps due to background noise and/or excessive movement of heavy otoliths in rough seas. Bathypelagic species usually have small otoliths and regressed or absent swimbladders. Other habitats have species with a range of otolith sizes. While the relationship between hearing ability and otolith length is unknown, at least some groups with modified swim-bladders have larger otoliths, which may be associated with more acute hearing.

ABSTRACT. This study of more than 500 specimens recognises two cetomimid subfamilies (one newly described), nine genera (four newly described) and about 35 species (four newly described). Characters of the gill arches, head laterosensory canals, lateral line scales, cavernous tissue, anal lappets and subpectoral organ (the last three of unknown function) are used to distinguish taxa. A cladistic analysis of 39 characters utilised Rondeletia and Barbourisia as outgroups to polarise character states within the family and the beryciform families to polarise family characters. Three synapomorphies, the presence of gill rakers in some form other than elongate and flattened and the absence of pelvic fins and pleural ribs, support the monophyly of the family. The monotypic Procetichthyinae, defined by four autapomorphies, is the primitive sister group of all other cetomimids. It retains such pleisiomorphic features as a fully developed nasal organ, a fully developed eye with lens, a pseudobranch and 19 principal caudal rays. Ditropichthys is the primitive sister group of the remaining seven genera, with the next line including the related Cetichthys and Notocetichthys. The relationships of the remaining five genera are equivocal, except that Cetomimus and Gyrinomimus are sister taxa defined by lateral line scale shape. A working hypothesis of relationships is presented that places Danacetichthys and Cetostoma as sister groups of the remaining three genera. Multistate characters of gill raker-tooth plate shape and extent of the fourth gill slit support the hypothesis. Cetostoma and Rhamphocetichthys are highly derivative forms, with each monotypic genus defined by three or four autapomorphies. The genera Gyrinomimus and Cetomimus each have more than ten species, which will be reviewed in later papers. The other genera each have one or two species that are fully detailed here. All examined specimens with recognisable gonads are females, confirmed by histology of eight specimens representing four genera. Egg sizes of at least 2.0, 1.3 and 0.6 mm diameter are attained by the genera Procetichthys, Gyrinomimus and Cetostoma respectively. The vast majority of examined specimens had eggs 0.1 mm diameter or less. The few specimens with larger eggs had bimodal egg sizes. Males and individuals less than 25 mm are unknown. Maximum size is at least 390 mm in one species of Gyrinomimus, while Ditropichthys and Cetostoma apparently do not exceed 140 mm and 250 mm, respectively. Crustaceans are the primary food of whalefishes. The family is distributed in all oceans, from 52°N to nos. At the species level, two distribution patterns are apparent. The two commonest species, Cetostoma regani and Ditropichthys storeri, have cosmopolitan distributions between 50 0 N and 40 0 S. The two most frequently captured species of Gyrinomimus are restricted to the north Pacific between 39° and 52°N and circumglobally in the Southern Ocean between 32° and nos. There are too few collections of the other species to ascertain distributional limits, but ...

One of the most common visual adaptations seen in the mesopelagic zone (200–1000 m), where the amount of light diminishes exponentially with depth and where bioluminescent organisms predominate, is the enlargement of the eye and pupil area. However, it remains unclear how eye size is influenced by depth, other environmental conditions and phylogeny. In this study, we determine the factors influencing variability in eye size and assess whether this variability is explained by ecological differences in habitat and lifestyle within a family of mesopelagic fishes characterized by broad intra- and interspecific variance in depth range and luminous patterns. We focus our study on the lanternfish family (Myctophidae) and hypothesise that lanternfishes with a deeper distribution and/or a reduction of bioluminescent emissions have smaller eyes and that ecological factors rather than phylogenetic relationships will drive the evolution of the visual system. Eye diameter and standard length were measured in 237 individuals from 61 species of lanternfishes representing all the recognised tribes within the family in addition to compiling an ecological dataset including depth distribution during night and day and the location and sexual dimorphism of luminous organs. Hypotheses were tested by investigating the relationship between the relative size of the eye (corrected for body size) and variations in depth and/or patterns of luminous-organs using phylogenetic comparative analyses. Results show a great variability in relative eye size within the Myctophidae at all taxonomic levels (from subfamily to genus), suggesting that this character may have evolved several times. However, variability in eye size within the family could not be explained by any of our ecological variables (bioluminescence and depth patterns), and appears to be driven solely by phylogenetic relationships.

The oceanic bathypelagic realm (1000–4000 m) is a nutrient-poor habitat. Most fishes living there have pelagic larvae using the rich waters of the upper 200 m. Morphological and behavioural specializations necessary to occupy such contrasting environments have resulted in remarkable developmental changes and life-history strategies. We resolve a long-standing biological and taxonomic conundrum by documenting the most extreme example of ontogenetic metamorphoses and sexual dimorphism in vertebrates. Based on morphology and mitogenomic sequence data, we show that fishes currently assigned to three families with greatly differing morphologies, Mirapinnidae (tapetails), Megalomycteridae (bignose fishes) and Cetomimidae (whalefishes), are larvae, males and females, respectively, of a single family Cetomimidae. Morphological transformations involve dramatic changes in the skeleton, most spectacularly in the head, and are correlated with distinctly different feeding mechanisms. Larvae have small, upturned mouths and gorge on copepods. Females have huge gapes with long, horizontal jaws and specialized gill arches allowing them to capture larger prey. Males cease feeding, lose their stomach and oesophagus, and apparently convert the energy from the bolus of copepods found in all transforming males to a massive liver that supports them throughout adult life.

lord Howe Island, some 630 kilometres off the northern coast of New South Wales, Australia at 31.5° South latitude, is the world's southern most locality with a well developed coral reef community and associated lagoon. An extensive collection of fishes from lord Howelsland was made during a month's expedition in February 1973. A total of 208 species are newly recorded from lord Howe Island and 23 species newly recorded from the Australian mainland. The fish fauna of lord Howe is increased to 447 species in 107 families. Of the 390 species of inshore fishes, the majority (60%) are wide-ranging tropical forms; some 10% are found only at lord Howe Island, southern Australia and/or New Zealand. less than 4% of the shore fishes are endemic to the lord Howe region (including Norfolk Island). Some 32% of the inshore species are restricted to the southwestern or southern Pacific Ocean.