Evidence that blue petrel, Halobaena caerulea, fledglings can detect and orient to dimethyl sulfide

Procellariiform seabirds (the petrels, albatrosses and shearwaters) are recognized for their acute sense of smell. These pelagic seabirds forage over thousands of miles of ocean to find patchily distributed prey resources. Over the past decade, much headway has been made in unravelling the variety of olfactory foraging strategies that Antarctic species employ, and it is becoming clearer that olfaction plays a key role in foraging, particularly for burrow nesting species. Now we are beginning to explore how these behaviours develop in chicks. Procellariiform chicks fledge and survive the open seas without aid or instruction from a parent, but how they are able to accomplish this task is unknown. Here we explore whether chicks leave the nest pre-tuned to olfactory cues necessary for foraging. In this study, we tested the hypothesis that blue petrel chicks (Halobaena caerulea) are able to detect and orient to a foraging cue (dimethyl sulphide, DMS) used by adults without ever having experienced this odour at sea. We first established that chicks could detect DMS at a biologically relevant concentration that they will later naturally encounter at sea (<10 pmol l-1). We then performed preference tests in a Y-maze on a group of birds 1-6 days before they fledged. Sixteen out of 20 fledglings preferred DMS (e.g. DMS+propylene glycol) to a ;control' odour (propylene glycol alone). Our results suggest that chicks can detect and may already recognize DMS as an orientation cue even before they leave the nest to forage for the first time.

Environmental rearing conditions produce forebrain differences in wild Chinook salmon Oncorhynchus tshawytscha

Recent studies suggest that hatchery-reared fish can have smaller brain-to-body size ratios than wild fish. It is unclear, however, whether these differences are due to artificial selection or instead reflect differences in rearing environment during development. Here we explore how rearing conditions influence the development of two forebrain structures, the olfactory bulb and the telencephalon, in juvenile Chinook salmon (Oncorhynchus tshawytscha) spawned from wild-caught adults. First, we compared the sizes of the olfactory bulb and telencephalon between salmon reared in a wild stream vs. a conventional hatchery. We next compared the sizes of forebrain structures between fish reared in an enriched NATURES hatchery and fish reared in a conventional hatchery. All fish were size-matched and from the same genetic cohort. We found that olfactory bulb and telencephalon volumes relative to body size were significantly larger in wild fish compared to hatchery-reared fish. However, we found no differences between fish reared in enriched and conventional hatchery treatments. Our results suggest that significant differences in the volume of the olfactory bulb and telencephalon between hatchery and wild-reared fish can occur within a single generation.

Re-evaluating NADPH-diaphorase histochemistry as an indicator of nitric oxide synthase: an examination of the olfactory system of coho salmon (Oncorhynchus kisutch)

The NADPH-diaphorase (NADPH-d) histochemical technique is commonly used to localize the nitric oxide (NO)-producing enzyme NO synthase (NOS) in neural tissues. In the olfactory tissues of vertebrates, however, NADPH-d staining can be present without the detection of NOS by other methods. We used pharmacological controls to determine whether NADPH-d staining was indicative of NOS in olfactory tissues from coho salmon (Oncorhynchus kisutch). We also compared NADPH-d staining with immunoreactivity to NOS. NADPH-d staining localized to the olfactory epithelium and to the glomerular layer of the olfactory bulb. Two NOS inhibitors, L-N-nitroarginine and L-N-methyl-L-arginine, failed to block this staining, but the less specific flavin-binder, diphenylene iodonium chloride, decreased or eliminated NADPH-d staining in all olfactory tissues. Immunoreactivity to NOS was present in short axon cells of the bulb, but was absent from the epithelium. Our findings show that NADPH-d staining was not representative of NOS in the olfactory tissues of salmon.

Olfactory foraging by Antarctic procellariiform seabirds: life at high Reynolds numbers

Antarctic procellariiform seabirds forage over vast stretches of open ocean in search of patchily distributed prey resources. These seabirds are unique in that most species have anatomically well-developed olfactory systems and are thought to have an excellent sense of smell. Results from controlled experiments performed at sea near South Georgia Island in the South Atlantic indicate that different species of procellariiforms are sensitive to a variety of scented compounds associated with their primary prey. These include krill-related odors (pyrazines and trimethylamine) as well as odors more closely associated with phytoplankton (dimethyl sulfide, DMS). Data collected in the context of global climatic regulation suggest that at least one of these odors (DMS) tends to be associated with predictable bathymetry, including upwelling zones and seamounts. Such odor features are not ephemeral but can be present for days or weeks. I suggest that procellariiforms foraging over vast distances may be able to recognize these features reflected in the olfactory landscape over the ocean. On the large scale, such features may aid seabirds in navigation or in locating profitable foraging grounds. Once in a profitable foraging area, procellariiforms may use olfactory cues on a small scale to assist them in locating prey patches.

Sensitization of olfactory guanylyl cyclase to a specific imprinted odorant in coho salmon

The role of cGMP in olfactory signaling is not fully understood, but it is believed to play a modulatory role in intracellular signaling in vertebrate olfactory receptor neurons (ORNs). Here, we present evidence that cGMP in ORNs may play an important role in recognition of biologically relevant odors and olfactory learning. Specifically, we investigated the cellular mechanisms underlying olfactory imprinting in salmon. Salmon learn odors associated with their natal site as juveniles and later use these odors to guide their homing migration. This imprinting is believed to involve sensitization of the peripheral olfactory system to specific homestream odorants. We imprinted juvenile salmon to the odorant beta-phenylethyl alcohol (PEA) and examined the sensitivity of olfactory adenylyl and guanylyl cyclases to PEA during development. Stimulation of guanylyl cyclase activity by PEA was significantly greater in olfactory cilia isolated from PEA-imprinted salmon compared with PEA-naive fish only at the time of the homing migration, 2 years after PEA exposure. These results suggest that sensitization of olfactory guanylyl cyclase may play an important role in olfactory imprinting by salmon.

GnRH-like immunoreactivity in the peripheral olfactory system and forebrain of Atlantic salmon (Salmo salar): a reassessment at multiple life history stages

Neurons with gonadotropin releasing hormone-like immunoreactivity (GnRH-ir) were identified within the peripheral olfactory system of Atlantic salmon (Salmo salar) at multiple life history stages. Within the forebrain, GnRH-ir somata were found in the preoptic area and in the caudomedial olfactory bulb in a position comparable to the ganglion of the nervus terminalis of other teleosts. Somata positive for GnRH were also found throughout the rostro-caudal extent of the olfactory nerve, and clustered within the medial component of the olfactory nerve as it arises from the olfactory epithelium. Results from tract tracing experiments with horseradish peroxidase indicate that at least some cells in this cluster project to the retina, suggesting that they too are part of a terminal nerve ganglion as anatomically defined in other vertebrates. We suggest that the presence of a distinct cluster of terminal nerve ganglion cells in the immediate vicinity of the olfactory epithelium may point to a peripheral site of neuromodulatory control in the olfactory system in salmon and perhaps in other teleosts as well.

Evidence for a peripheral olfactory memory in imprinted salmon

The remarkable homing ability of salmon relies on olfactory cues, but its cellular basis is unknown. To test the role of peripheral olfactory receptors in odorant memory retention, we imprinted coho salmon (Oncorhynchus kisutch) to micromolar concentrations of phenyl ethyl alcohol during parr-smolt transformation. The following year, we measured phenyl ethyl alcohol responses in the peripheral receptor cells using patch clamp. Cells from imprinted fish showed increased sensitivity to phenyl ethyl alcohol compared either to cells from naive fish or to sensitivity to another behaviorally important odorant (L-serine). Field experiments verified an increased behavioral preference for phenyl ethyl alcohol by imprinted salmon as adults. Thus, some component of the imprinted olfactory homestream memory appears to be retained peripherally.

An electrophysiological characterization of ciliated olfactory receptor cells of the coho salmon Oncorhynchus kisutch

Electrical properties of ciliated olfactory receptor cells isolated from coho salmon (Oncorhynchus kisutch) were studied using the whole-cell mode of the patch-clamp recording technique. 1. Voltage-dependent currents could be separated into two inward and three outward conductances, including a Na+ current, Ca2+ current and three K+ currents. 2. The components of the outward current varied with the life stage of the salmon from which cells had been isolated. In cells isolated from juvenile fish (parr), a Ca(2+)-dependent K+ current dominated the outward current, whereas in cells isolated from older fish (i.e. fish that had undergone smoltification), a transient K+ current became prominent. 3. Differences in response characteristics of outward currents to internal dialysis with cyclic GMP (but not cyclic AMP) were also correlated to the life stage of salmon. Under conditions in which the Ca(2+)-activated current was blocked, relaxation of the outward current was slowed by dialysis with cyclic GMP only in cells isolated from smolts and sea-run fish, but not in those isolated from mature spawners. 4. From these results, we suggest that hormone modulation of olfactory receptor cell development or differentiation may play a role in establishing these differences.

Do fish sniff? A new mechanism of olfactory sampling in pleuronectid flounders

Although olfaction is known to be a highly developed sense in a variety of fishes, little information is available about behavioral mechanisms by which fishes sample the olfactory environment. This study was undertaken to assess the role of spontaneous jaw protrusion (‘coughing’) as a potential mechanism for olfactory sampling in pleuronectid flounders. Investigations were carried out using a combination of physiological, morphological and behavioral techniques. Physiological results show (1) that typical respirations are coupled to measurable olfactory sac pressure fluctuations and (2) that during a coughing event, water is rapidly sucked into the olfactory sac. Morphological results indicate (3) a direct linkage system between the protrusion apparatus and the olfactory or associated accessory sacs, and (4) that coughing is associated with a rapid expansion or stretching of these sacs. Lastly, behavioral studies demonstrate (5) that coughing rates increase significantly over background activity when flounders are presented with attractive food odorants. From these results, I propose that coughing in pleuronectid flounders represents a behavior truly analogous to sniffing in certain air-breathing organisms.