Abstract. We examined the cnidomes (total complement of nematocysts) of medusae of the zooxanthellate and azooxanthellate jellyfishes Phyllorhiza punctata and Catostylus mosaicus (Rhizostomeae, Scyphozoa), and compared the assemblage of zooplankton captured on the oral arms of each species to determine whether differences in the types or amount of zooplankton captured were consistent with possible differences in the cnidomes. Cnidomes were described using light and scanning electron microscopy. Each species had a distinct cnidome and, in general, specimens of P. punctata appeared to have far fewer nematocysts than those of C. mosaicus. Four types of nematocysts were identified in medusae of C. mosaicus; 2 types of holotrichous isorhizae, rhopaloids, and birhopaloids. In C. mosaicus, the oral arms and bell margins possessed all of these types, but the cnidomes of the 2 regions differed in relative abundances and sizes of isorhizae and rhopaloids. Five types of nematocysts were identified in medusae of P. punctata, although not all types were found in all specimens. Round holotrichous isorhizae were found only in the bell, while oval holotrichous isorhizae, rhopaloids of 2 distinct size ranges, and birhopaloids were found in the bell and oral arms. Cnidomes of the bell and oral arms in specimens of P. punctata also differed in the relative abundance and sizes of oval isorhizae and rhopaloids. Although there were clear differences in the overall cnidomes and absolute abundances of nematocysts in each species, the oral arms (feeding appendages) of specimens of both C. mosaicus and P. punctata had similar types and relative abundances of nematocysts. Zooplankton sampled from the oral arms of each species showed that both species preyed predominantly on copepod nauplii and larvae of gastropods and bivalves. Medusae of C. mosaicus captured ∼10 × more gastropod larvae and 5 × more bivalve larvae than those of P. punctata. Specimens of P. punctata captured approximately twice as many copepod nauplii as those of C. mosaicus. Differences in the relative abundance of types of zooplankton captured by each species could not be adequately explained by differences in the cnidomes of the oral arms.

The distribution of pit organs (free neuromasts) has previously been documented for several species of pelagic sharks, but is relatively poorly known for rays and bottom-dwelling (demersal) sharks. In the present study, the complete distribution of pit organs was mapped in the demersal sharks Heterodontus portusjacksoni, Orectolobus maculatus, Hemiscyllium ocellatum, Chiloscyllium punctatum, and Asymbolus analis, and the rays Rhinobatos typus, Aptychotrema rostrata, Trygonorrhina sp. A, Raja sp. A, and Myliobatis australis. All of these species had pit organs scattered over the dorsolateral surface. The sharks also had "mandibular" pit organs (and "umbilical" pit organs in C. punctatum and A. analis) on the ventral surface, while pit organs were sparse or absent on the ventral surface of rays. All of the species examined here, except for M. australis, also had a "spiracular" group of pit organs adjacent to the eye and/or spiracle. Spiracular pit organs were also recorded for the sawshark Pristiophorus sp. A and the skate Pavoraja nitida, although the remainder of pit organs were not mapped in these species. The distribution and number of pit organs varied both within and among species. Pit organ distribution was asymmetrical in each individual examined, but no particular trend towards left or right "handedness" was observed in any species. Although rays have been thought to have fewer pit organs than sharks in general, this was not the case in the present study. All of the species examined here had few pit organs compared to the pelagic sharks previously documented, but it is not clear whether this is due to ecological or phylogenetic causes.

Resting Port Jackson sharks Heterodontus portusjacksoni with dorso-lateral pit organs ablated oriented with a mean angle of 263 to the current direction in a flume. This was significantly different (P<0·01) to controls (normal and sham operated) who had a pooled mean angle of 44 to the current. Thus the dorso-lateral pit organs of H. portusjacksoni, like the free neuromasts of some teleosts, provide sensory information for rheotaxis. 2001 The Fisheries Society of the British Isles

Elasmobranchs have hundreds of tiny sensory organs, called pit organs, scattered over the skin surface. The pit organs were noted in many early studies of the lateral line, but their exact nature has long remained a mystery. Although pit organs were known to be innervated by the lateral line nerves, and light micrographs suggested that they were free neuromasts, speculation that they may be external taste buds or chemoreceptors has persisted until recently. Electron micrographs have now revealed that the pit organs are indeed free neuromasts. Their functional and behavioural role(s), however, are yet to be investigated.

The pit organs (free neuromasts) of sharks are part of the lateral line sensory system, but there is still confusion about their exact morphology and function(s). This is partly because of reported physiological differences between the pit organs and the lateral line canal neuromasts, and partly because the morphology of pit organs has not been adequately documented. To compare their morphology, the pit organs and canal neuromasts of the gummy shark Mustelus antarcticus (Chondrichthyes: Triakidae) were examined using transmission and scanning electron microscopy. Both pit organs and canal neuromasts had hair cells with the `staircase' arrangement of sensory hairs (stereovilli) characteristic of vertebrate mechanoreceptors. Stereovilli bundles of different sizes were distributed haphazardly throughout the pit organs and canal neuromasts. The density of hair cells was similar in the pit organs and canal neuromasts, but differences in the overall size and/or shape of the sensory epithelia might account for some of the reported differences in mechanosensitivity.

The pit organs of elasmobranchs (sharks, skates and rays) are free neuromasts of the mechanosensory lateral line system. Pit organs, however, appear to have some structural differences from the free neuromasts of bony fishes and amphibians. In this study, the morphology of pit organs was investigated by scanning electron microscopy in six shark and three ray species. In each species, pit organs contained typical lateral line hair cells with apical stereovilli of different lengths arranged in an "organ-pipe" configuration. Supporting cells also bore numerous apical microvilli taller than those observed in other vertebrate lateral line organs. Pit organs were either covered by overlapping denticles, located in open grooves bordered by denticles, or in grooves without associated denticles. The possible functional implications of these morphological features, including modification of water flow and sensory filtering properties, are discussed.

 Abstract Peach, M.B and Rouse, G.W. 2004. Phylogenetic trends in the abundance and distribution of pit organs of elasmobranchs. -Acta Zoologica (Stockholm) 85 : 233-244Pit organs (free neuromasts of the mechanosensory lateral line system) are distributed over the skin of elasmobranchs. To investigate phylogenetic trends in the distribution and abundance of pit organs, 12 relevant morphological characters were added to an existing matrix of morphological data (plus two additional end terminals), which was then re-analysed using cladistic parsimony methods (  * 4.0b10). Character transformations were traced onto the most parsimonious phylogenetic trees. The results suggest the following interpretations. First, the distinctive overlapping denticles covering the pit organs in many sharks are a derived feature; plesiomorphic elasmobranchs have pit organs in open slits, with widely spaced accessory denticles. Second, the number of pit organs on the ventral surface of rays has been reduced during evolution, and third, spiracular pit organs have changed position or have been lost on several occasions in elasmobranch evolution. The concentrated-changes test in  (version 4.05) was used to investigate the association between a pelagic lifestyle and loss of spiracular pit organs (the only character transformation that occurred more than once within pelagic taxa). Depending on the choice of tree, the association was either nonsignificant at P = 0.06 or significant at P < 0.05. Future studies, using species within more restricted elasmobranch clades, are needed to resolve this issue.

Rheotaxis (orientation to water currents) is commonly observed in fishes. Facing upstream is thought to be an element of shark behaviour during prey search and station-holding, but quantitative studies of rheotaxis by sharks in the wild are lacking. In this study, rheotaxis by the epaulette shark, Hemiscyllium ocellatum, was investigated on a coral reef flat at Heron Island, Great Barrier Reef, Australia. The orientation of 78 individuals in open areas (on sand or on top of coral heads, directly exposed to the prevailing current) was analysed with respect to current direction and velocity. H. ocellatum showed a significant (P < 0.05) preference for facing upstream (mean angle 1�, where the current origin was taken to be 0/360�) while resting on the substrate (n = 23), but showed no evidence of a preferred direction while swimming (n = 50). Observations of foraging were few (n = 5) but there was no indication of a preferred direction by these sharks. Resting H. ocellatum faced significantly more upstream at faster current velocities (P < 0.05), suggesting that rheotaxis may function in station-holding. There was no apparent relationship between rheotaxis and current velocity for swimming H. ocellatum.