Ph.D. - Zoology (Ecology, Evolution and Conservation Biology)

Permanent URI for this collectionhttps://hdl.handle.net/10125/2176

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Phylogeography And Evolution Of Mesophotic Coral Ecosystems
(University of Hawaii at Manoa, 2020) Copus, Joshua M.; Bowen, Brian R.; Zoology
In this dissertation, I begin to address the phylogeography and evolution of the unique reef fish fauna of the mesophotic zone, generally regarded as beginning at 30 - 150 m depth. Chapter 1 is an introduction to the topic and background for subsequent chapters. As the field of mesophotic research matures, we can move beyond descriptive studies and begin to construct hypotheses for the origins and evolution of mesophotic reef fishes. Accordingly, Chapter 2 describes the habitat persistence hypothesis (HPH), which postulates that mesophotic reef fish communities are older and more stable than their shallow water counterparts. This hypothesis is based on two fundamental observations; 1) Mesophotic communities are extend below the surface waters that are subject to glacial sea level changes on a scale of 106 years, and 2) The shallow water biodiversity gradient, extending from the Coral Triangle (between Philippines, Indonesia, and New Guinea) to Rapa Nui in the eastern Pacific, may not apply to mesophotic communities. This chapter is a formal description of the HPH, and outlines a set of testable hypotheses to evaluate the validity of the HPH. Chapter 3 is a description of the mesophotic fish species Neoniphon pencei. Chapter 4 is a description of the mesophotic fish species Luzonichthys seaver, and Chapter 5 is a description of the mesophotic fish species Prognathodes geminus. These data will provide some of the first descriptions of MCE fishes, and provide a beginning point for testing phylogenetic hypotheses about the origins of mesophotic fishes. Chaper 6 provides a brief conclusion.
Genomics Of The Globally Distributed Echinoid Genus Tripneustes
(University of Hawaii at Manoa, 2018-05) Laruson, Aki Jarl; Zoology (Ecology, Evolution, and Conservation Biology)
Understanding genomic divergence can be a key to understanding population dynamics. As global climate change continues it becomes especially important to understand how and why populations form and dissipate, and how they may be better protected. To this effect, the globally distributed sea urchin genus Tripneustes has been highlighted as an ideal group for studying patterns of genomic divergence as the global distribution is split into two physically separated species (T. ventricosus in the Atlantic and T. gratilla the Pacific), and cryptic divergence in the absence of hard physical barriers has been suspected within each ocean. Molecular signatures of population divergence can be affected and skewed by a number of different biological realities. In the case of lower fitness of a heterozygous individual (underdominance), the degree as well as network shape of the connectivity between populations can determine wether rare alleles persist between populations, muddying population divergence signals, or are driven to fixation at one end of the population range while going extinct in the other, giving a signal of parapatric speciation. In order to address questions regarding the more nuanced molecular differences and broader evolutionary trajectories within the genus Tripneustes a draft transcriptome for the species T. gratilla was generated. In addition to showing an expansion in tumor suppressor genes when compared to the genome enabled sea urchin Strongylocentrotus purpuratus, and sex-specific gene expression differences in Sex determining Region Yassociated High Mobility Group box (SOX) genes, the transcriptome allowed for easy recovery of the full mitochondrial genome. Following isolation and sequence confirmation, the mitochondrial genome of T. gratilla was next compared to all previously published sea urchins mitochondrial genomes. A phylogenetic comparison validates the morphologically proposed superfamily Odontophora, with an estimated genesis of the group during the Eocene-Oligocene epoch transition. Estimates of selection via proportional non-synonymous to synonymous site substitution ratios suggest that purifying selection is the primary force acting on echinoid mitochondria.
A Single Trait Drives Incipient Ecological Speciation in Sympatric Color Morphs of the Arceye Hawkfish
(University of Hawaii at Manoa, 2016-05) Whitney, Jonathan
Coral reef fishes represent the most diverse assemblage of vertebrates on the planet, yet our understanding of the mechanisms driving this diversity remains limited. There is growing recognition that ecological adaptation shaped by natural selection may be a major driver of diversification on coral reefs. However, few examples of ecological speciation in nature currently exist. I integrate research on ecology, behavior and genetics to outline a novel case of incipient ecological speciation in sympatric color morphs of the arceye Hawkfish (Paracirrhites arcatus). First, I demonstrate that color morphs are exploiting different niches along a steep ecological gradient, likely driven by disruptive selection favoring color patterns that are better camouflaged in contrasting microhabitats. Second, mate preference experiments show that females prefer individuals of their own morph, indicating color morphs are mating assortatively. Third, I provide genetic evidence that these premating barriers have resulted in at least partial reproductive isolation between ecologically differentiated sympatric color morphs. Taken together, these results suggest that reproductive isolation between morphs may be arising as a by-product of divergent selection on ecological differences and enhanced by the isolating effects of assortative mating. I conclude that color alone is driving incipient divergence in this species, despite high gene flow and no geographic isolation. I argue that the characteristics of this system could be quite common and thus widely applicable to thousands of reef organisms. This dissertation emphasizes the role natural selection plays in initiating speciation and should help bring us one step closer to understanding the processes driving high biodiversity in tropical seas.
Invasion Ecology and Control Feasibility of the Jackson Chameleon (Trioceros jacksonii xantholophus) in Hawai‘i
(University of Hawaii at Manoa, 2016-05) Van Kleeck, Melissa
Biological invasions provide opportunities to investigate micro-evolutionary change and the relative roles of key factors that lead to differentiation. Following introduction, release from native selective pressures and exposure to novel evolutionary forces can reduce morphological and physiological constraints and impose new ones, resulting in rapid micro-evolutionary changes. Studying ecosystem and geographic variation in invasive taxa on an ecological time scale can help provide conservation relevant information for management. I investigated ecological and evolutionary questions involving adaptation and ecomorphological variation and the roles of habitat characteristics on form, function and behavior of an invasive lizard in Hawaii. In Chapters 1 and 2 I evaluate substantial variation in head-size and horn-length among and within islands, and between native and introduced ranges. Larger heads are associated with low precipitation, and consumption and higher availability of hard prey, while longer horns are exhibited by chameleons in their native range and are associated with fight success and higher precipitation in the introduced range. Results may suggest rapid ecomorphological adaption to introduced microhabitats and release from natural selective pressures influencing sexually dimorphic characters in the native range. Chapter 3 examined predatory behavior in chameleons in response to prey size and type. Instead of the well-documented lingual prehension mechanism in Chamelaeonidae, a novel feeding mechanism is described, in association with both prey/predator size and prey type. Finally, for Chapter 4, I present the first proposed control strategy for T.j. Xantholophus in Hawaii. We tested a method based on an approach using acetaminophen deployed by resource managers to control the invasive brown tree snake on Guam. We determined minimum dosage necessary to cause >95% mortality in <48 hours. Additionally, I propose methods of field delivery, including use of introduced snail shells as a vessel for ingestion. Although there is a massive and increasing body of invasion ecology research, this study provides new insights into trait adaptation of invasive predators and how this information can help develop management strategies. In addition, this study presents a framework by which biological invasions can be used to examine evolutionary questions on an ecological rather than evolutionary time-scale.
Assessing Reproductive Biology of Hawaiian Reef Fishes: The Importance of Fisher and Community Participation
(University of Hawaii at Manoa, 2015-12) Schemmel, Eva
Small-scale fisheries management is moving towards a more holistic and integrated approach, that of ecosystem–based management. This management philosophy incorporates life history, biological and physical processes, and people as integral components of the ecosystem. Although ecosystem-based management is complex, significant progress can be made by incorporating people and communities into resource monitoring and management. This research used a participatory approach and incorporated fishers and community members in research on the biology of several fish of management importance in Hawaiʻi. A fish species of ecosystem-based management importance that is often overlooked is the small pygmy goby (Eviota epiphanies). This research found that E. epiphanies exhibits fast growth and high generational turnover rates, likely contributing significantly to the bioenergetics of coral reef ecosystems. By working with fishers, reproductive biology was collected for the invasive peacock grouper (Cephalopholis argus), and the herbivorous convict tang, (Acanthurus triostegus sandvicensis). Collaborative research helped increase the number of samples collected across locations and time, improving knowledge of fish reproductive biology in Hawaiʻi across multiple scales. Findings from this research show that size at maturity and temporal shifts in spawning seasons is variable by location. These findings suggest that stewardship and management is best conducted at the local level in order to understand and respond to the variability within the ecosystem. To meet these needs, I worked with fishers, communities, NGOs, and state natural resource managers to develop community-based fishery monitoring programs to assess the reproductive biology of harvested reef fishes. These monitoring programs combined traditional Hawaiian ecological knowledge and scientific assessments to better understand local spawning seasons and optimal harvest sizes for reef fishes. By directly monitoring their resources, fishers have the information needed to track changes in their resources, and therefore the ability to respond to changing resource conditions, allowing for informed decisions on the species that are targeted and times that harvest takes place. Lastly, this research demonstrates the power of participatory approaches for collecting information need for ecosystem-based management and the social and ecological benefits of empowering fishers and communities to be monitors and stewards of their resources.
The importance of food and scale in the ecology of tropical seabirds
(University of Hawaii at Manoa, 2008) Hebshi, Aaron J.
Birds breeding in areas with large colonies experienced low fledging success and chick growth. This density-dependent pattern, on the scale of the birds' foraging area, supports the argument that reproductive rates are driven by food limitation. Shearwater populations are currently limited by breeding habitat availability due to human development and introduced predators, but as land managers protect more main-island breeding sites, food-limitation and its effects on reproductive rates may play an increasing role in population regulation. Food availability also appears to influence how Hawaiian seabirds time their breeding. Species with large foraging ranges were found to breed at a more predictable time of year presumably because they are able to buffer against small-scale variation in prey abundance.
An ecological comparison of Cephalopholis argus between native and introduced populations
(University of Hawaii at Manoa, 2008) Meyer, Amanda Leilani
The blue-spotted grouper, Cephalopholis argus, was introduced to the Hawaiian islands from Moorea, French Polynesia in the late 1950s in an attempt to establish a grouper fishery in Hawaii. C. argus was introduced from a region of high grouper diversity into an environment with little competition from other large sedentary piscivorous species, and has flourished over the last 50 years. Total length, weight, and body condition were each significantly greater in introduced populations of C. argus in Hawaii than in native populations in Moorea. Both regions showed significant positive relationships between C. argus total length and: (1) prey total length, and (2) prey body depth. There were significant regional differences in diet; in Moorea C. argus consumed significantly deeper-bodied prey than their counterparts in Hawaii. These differences are consistent with competitive release experienced by C. argus in Hawaii. Active and passive tracking confirmed that C. argus are diurnal and highly site attached. In Moorea, home range size ranged from 230 to 1389 m2, mean = 700 m2 in, and 425 to 2300 m2, mean = 1236 m2in, in Hawaii. There was a significant positive correlation between the size of C. argus and home range size, and C. argus in Hawaii held significantly larger home ranges than C. argus in Moorea. C. argus in both regions spent the majority of their time in core use areas which corresponded with areas of high rugosity and reef complexity. Differences in home range area and habitat utilization between regions may be due to the lack of competing species, especially other groupers, in Hawaii. C. argus is known in Hawaii for causing ciguatera fish poisoning (CFP). CFP is caused by Gambierdiscus toxicus, which produces ciguatoxins (CTX) that are incorporated into the fish tissues. Standard length of C. argus and CTX score were positively correlated, however, small fish could be strongly positive, and cause CFP. At all sites, the majority of C. argus tested contained CTX levels at or above those considered unsafe for consumption. C. argus of a given length in Hawaii had lower CTX scores than fish of the same size in Moorea.
Community ecology of the invasive intertidal barnacle Chthamalus Proteus in Hawaiʻi
(University of Hawaii at Manoa, 2005) Zabin, Chela Juliet
Local and regional influences on arthropod community structure and species composition on Metrosideros polymorpha in the Hawaiian Islands
(University of Hawaii at Manoa, 2004) Gruner, Daniel S.
The cephalic lateralis system of cardinalfishes (Perciformes: Apogonidae) and its application to the taxonomy and systematics of the family
(University of Hawaii at Manoa, 2004) Bergman, Laura M. Rodman
Ecology, abundance, diversity, and distribution of larval fishes and Schindleriidae (Teleostei: Gobioidei) at two sites on Oʻahu, Hawaiʻi
(University of Hawaii at Manoa, 2003-12) Wittle, Amber G.; Greenfield, David; Zoology (Ecology, Evolution, and Conservation Biology)
Light traps were deployed in Hanauma Bay (the Bay) over a 2.5-year period to determine the effect of physical factors on the catch of larval fishes and Schindleriidae. Significantly more larval fishes and Schindleria were caught in light traps moored over sand habitat than in those over rubble, coral, or mixed habitats. The currents of the outer reef of the Bay were mapped using drogue analysis, but the recorded pattern (shoreward and westerly) did not explain the catches of larval fishes or Schindleria. Other measured physical factors (e.g., wave height, wind speed, temperature, etc.) were not statistically correlated with light trap catches; only tidal range had a significant (p=0.017) relationship with Schindleria catch. The strong swimming abilities of late-stage larvae and adult Schindleria can probably overcome small-scale physical factors, specifically currents typical to the Bay. Only 119 fish larvae were caught in 81 samplings, considerably less than reported in other regions. This is probably due to an absence, in Hawaiʻi, of the most common species caught in light traps elsewhere. In Kaneʻohe Bay, paired silent and sound traps were used to determine if sound is an attractant or deterrent for larval fishes and adult Schindleria. Sound appeared to be a deterrent for larval shorefishes (p=0.13) and Schindleria (p=0.058), and an attractant for larval reef fishes (p=0.104). Shorefishes and Schindleria may use sound to avoid predation and reef fish may use sound to recruit to suitable habitat. Schindleria, with two species in Hawaiʻi, are progenetic, cryptic fishes and little is known about their ecology. Otolith and histological analyses revealed that both species have extremely high growth rates: Schindleria pietschmanni grows an average of 0.72 mm/day and Schindleria praematura grows an average of 0.60 mm/day. For both species, females were found to be larger than males (p=0.007, p=0.001) and in S. pietschmanni, temperature was negatively related to growth rate and size (p=0+). This genus is abundant in Hawaiian waters (3600+ were caught in my light traps) and may be an important part of the energy budget of a reef, due to sheer number, fast turnover, and high productivity.
Exploring novel taxonomic character sets in the Mollusca: the Cribrarula cribraria complex (Gastropoda: Cypraeidae) as a case study
(University of Hawaii at Manoa, 2003-05) Moretzsohn, Fabio; Kay, E Alison; Zoology
The goal of this dissertation is to explore novel and non-traditional taxonomic characters that may be useful for mollusks, and combine them with radular and shell characters to carry out a taxonomic review of the genus Cribrarula (Cypraeidae). Shells in this genus have conspicuous dorsal spots, but like odler cowries, lack sculpture, spines, and other shell characters commonly used in gastropod taxonomy. For these reasons, cowrie shells are considered uninformative. The study of dorsal spots (DS) and related characters suggest that at least in this complex, the dorsal spots may represent a record of the mantle papillae. If the hypothesis is correct, then DS may provide information on the soft parts that previously was only available from the study of live or preserved specimens. Each species in the complex has a species-specific range of DS, marginal spots, and allied characters, thus suggesting that they may be useful in distinguishing species in the complex. The odontophore cartilage provides support for the radula and attachment for the muscles responsible for feeding. Although intimately connected to the radula and known since the 1800's, the taxonomic value of the odontophore has been overlooked. A study of odontophore variation in the family Cypraeidae proposes the structure as a novel taxonomic character, potentially applicable to most mollusks. The Cribrarula cribraria Linnaeus, 1758 complex is reviewed, and twelve species and six subspecies are recognized on the basis of multivariate analyses of shell characters, the radula, odontophore, and geographic distribution. The shell, radula, odontophore and distributional maps are illustrated for each taxon. The nominal species, cribraria, ranges from East Africa to the Central Pacific, and several populations are distinctive enough to be recognized as subspecies. The other eleven species are restricted to narrower ranges along the periphery of the distribution of cribraria. During the review of Cribrarula, a new species from New South Wales was described as C. gravida Moretzsohn, 2002.
An empirical evaluation of the design and function of a small marine reserve (Waikīkī Marine Life Conservation District)
(University of Hawaii at Manoa, 2003-05) Meyer, Carl G.; Holland, Kim; Zoology
The effectiveness of a small (0.34km2) "no fishing" marine reserve (Waikiki Marine Life Conservation District) was empirically evaluated by quantifying three components of reserve design and function: (1) spatial adequacy for containing daily movements of target species; (2) abundance and size of reef fishes in the reserve and adjacent fished areas; and (3) distribution and impact of fishing activities at the reserve site. Reef fish movements were quantified using a combination of sonic tracking and conventional identification tags. Tracked fishes were typically site-attached to well-defined home ranges and had predictable patterns of behavior including a crepuscular 'commute' between separate day and night habitats. Surgeonfishes (Naso unicornis) had relatively small home ranges that were strongly associated with high rugosity habitat contained within reserve boundaries. Goatfishes (Mulloidichthys jlavolineatus) and jacks (Caranx melampygus and C. sexfasciatus) had home ranges that were relatively large, and extended from the reserve into adjacent fished areas. Mobile species (goatfish and jacks) used a combined area of 1.013 km2, indicating that the existing reserve is too small to fully protect these species. Abundance & size of both target and non-target species was greater in reserve than in adjacent fished areas, suggesting that fishing is not the only factor determining patterns of fish abundance and size at Waikiki. Habitat complexity is generally greater in the reserve than in adjacent fished areas and appears to be an important factor determining patterns of fish abundance and size at Waikiki. Spear and shoreline pole & line fishing were the dominant fishing activities at Waikiki. Spear fishing had a higher catch per unit effort (kg/man h) and caught larger fish than pole & line fishing. Fishing activities were patchily distributed and clustered around public shoreline access points close to free parking. Some fishing did occur inside the reserve but at significantly lower levels than in adjacent fished areas. Little fishing occurred in the areas immediately adjacent to the northern and seaward reserve boundaries, suggesting that a fishing impact buffer zone exists around the reserve. This phenomenon may partly explain why mobile target species remain abundant inside the reserve despite daily excursions into adjacent, unprotected areas.
The behavior and broadband acoustic signaling of Hawaiʻian spinner dolphins (Stenella longirostris)
(University of Hawaii at Manoa, 2003-05) Lammers, Marc O.; Au, Whitlow W. L.; Zoology
This dissertation explores the design of dolphin social acoustic signals by addressing the question: how have the signals used by these animals been adapted to the physical, sensory and social environments they inhabit? To obtain answers to this question, the behavior and acoustic signals of spinner dolphins (Stenella longirostris) and spotted dolphins (Stenella frontalis) were examined. A rhythmic pattern of behavior is described for a population of spinner dolphins resident off the island of O'ahu. The population is tied to both trophic and physiographical resources associated with the island's coastline, where groups fuse and fragment in predicable patterns over the course of a daily cycle of activity and rest. Acoustic signals are thought to playa critical role in the coordination of animals within and between groups. A broadband analysis of the whistles and burst pulses of these two species reveals that they span a much broader range of frequencies than typically discussed in the literature. A considerable amount of energy is found in the higher frequency harmonics of whistles, as well as in the ultrasonic bands of burst pulses. Many burst pulses in fact have energy exclusively at ultrasonic frequencies and are thus inaudible to human hearing. Most if not all of the frequency hearing sensitivity typical for dolphins appears to be exploited in the design of their social acoustic signals. Evidence is also presented on the directionality of whistles together with a discussion on how the directional transmission of harmonics may provide important cues to listening animals about the orientation and direction of movement of a signaler. The "mixed-directionality" of dolphin whistles is proposed as a signal design feature that may be very important for the coordination of individuals within groups. Finally, a discussion is presented on how these findings fit together to provide a more complete picture of the nature of dolphin social acoustic signals. Their functional design is considered along with the features that make them well suited for echolocation and communication underwater. A roadmap is provided for future investigators who wish to further explore the design of dolphin social acoustic signals.
Pharmacological and molecular investigations of mechanisms of metamorphosis in the marine gastropod Phestilla sibogae
(University of Hawaii at Manoa, 2003-05) Carmen, Kimberly A. del; Hadfield, Michael G; Zoology
Metamorphosis is the critical link between the larval and adult forms in the biphasic life cycle of many diverse metazoans. Metamorphosis in the marine gastropod Phestilla sibogae is rapid (taking less than 24 hours) and results in a change in habitat and feeding mode from larval facultative planktotrophs to benthic, carnivorous adults feeding on the coral P. compressa. During metamorphosis larvae of P. sibogae lose several structures such as the larval shell and operculum, the velum, larval kidney cells, larval retractor muscles, nephrocysts, and foot glands. In addition, morphogenic changes such as synthesis of the buccal mass and epidermal cell movement also occur during metamorphosis. The goal of this dissertation was to elucidate mechanisms that regulate metamorphosis in P. sibogae. A survey of agents with putative affects on transcription, translation, and phosphorylation indicated that further investigation into the possible role of transcription and translation in the progression of metamorphosis, and of phosphorylation in both the induction and subsequent metamorphic response is warranted. Pharmacological assays using transcription inhibitors to determine the importance of gene expression before, during, and after metamorphosis in the marine gastropod Phestilla sibogae indicated that no increase in the net rate of transcription occurs during metamorphosis. However, changes that occur within the first 12 hours, such as epidermal cells movement, degradation of larval musculature, and synthesis of the buccal mass, plus completion of metamorphosis, and early juvenile development may depend on de novo transcription. Additional approaches were necessary to measure quantitative changes in specific transcripts during metamorphosis, and therefore cDNA libraries were constructed and then a subtractive screen was performed to narrow the pool of possibly interesting candidate genes with a role in metamorphosis. Measurements by Real Time RT-peR indicated that the relative abundance of a putative laminin receptor in cDNAs isolated from a single metamorphosing larva was 8-fold greater than in an individual competent larva. Together these studies add to the knowledge of what role transcription plays during metamorphosis in P. sibogae.
Dynamics of the Hawaiʻian mesopelagic boundary community and their effects on predator foraging behavior
(University of Hawaii at Manoa, 2003-05) Bird, Kelly J. Benoit; Au, Whitlow W. L.; Zoology
In the Hawaiian Islands, a distinct, resident community of micronekton is distributed over the underwater slopes of the islands. Despite the importance of the mesopelagic boundary community to both nearshore and oceanic ecosystems, it has not been well studied. This work investigated the spatial and temporal dynamics of the mesopelagic boundary community and their impact the foraging behavior of its predators. Several active acoustic (sonar) techniques were utilized, complimented by trawling and optical techniques. Target strength measurements of live animals from the boundary community facilitated the use of echo-energy integration techniques and allowed conversion of acoustic backscattering measures to estimates of caloric content. Using these measures, ship-based echosounder surveys showed that mesopelagic animals are heterogeneously distributed, in both time and space. The Hawaiian mesopelagic boundary community fits the hierarchical patch structure model with patches within patches that are part of a larger scale matrix of patches. Surveys also revealed that the boundary layer undergoes diel horizontal migrations in addition to its vertical migrations. At night organisms were found within 1 km of shore, in waters much shallower than their daytime habitat. Because of this horizontal movement, the highest densities of mesopelagic animals, reaching up to 1800 animals/m^3, were observed nearshore. A series of bottom-mounted, sonar moorings measured vertical migration rates of 0-1.7 m/min and average horizontal rates of 1.67 km/hr. High levels of biomass were observed moving rapidly, over a great distance, into shallow waters very close to shore, providing insight into the significant link the boundary community provides between nearshore and oceanic systems. To understand how temporal and spatial heterogeneity of prey affected a pelagic predator, an echosounder was used to simultaneously measure the abundance of spinner dolphins and the boundary community. Spinner dolphins followed the migration patterns of their prey both horizontally and vertically. Overlap was observed between spinner dolphins and their prey from several minutes to an entire night, at spatial scales of 20 meters to several kilometers. An understanding of the spatial and temporal dynamics of the mesopelagic boundary community permitted the accurate prediction of predator movement patterns and an increased understanding of their behavior.
The prolactin cell of a euryhaline fish, the tilapia, Oreochromis mossambicus: a model for osmoreception
(University of Hawaii at Manoa, 2002-12) Seale, André P.; Grau, E Gordon; Zoology
Prolactin (PRL) plays a central role in fresh water (FW) adaptation in teleost fish. Consistent with the freshwater-adapting actions of prolactin (PRL), the release of this hormone from the tilapia pituitary is stimulated as extracellular osmolality is reduced both in vitro and in vivo. The main objective of this research was to investigate the mechanisms involved in the mediation between an osmotic stimulus and PRL release. The present studies also provided evidence of the unique osmosensitivity of the PRL cells when compared to the other pituitary cell types such as GH cells. Evidence now suggests that growth hormone (GR) acts in the seawater (SW) adaptation in some euryhaline fish. The release and content of PRL in response to long-term (days) changes in medium osmolality were markedly different than those of GH and corticotropin (ACTH) measured from the same pituitaries. Repeated blood withdrawal and transfer from SW to FW increased circulating PRL levels, whereas GH levels were unchanged. PRL release was not only more sensitive to a decrease in extracellular osmolality but long-lasting, when compared with the response of GH cells in dispersed cells and whole pituitaries. The tilapia PRL cell provides an excellent model to investigate osmoreception. Working with this model, a technique was developed for examining changes in cell volume and PRL release from the same preparation of cells, as well as measurements of [Ca2+]i. These approaches were utilized to demonstrate that the rapid increase in PRL release in response to reductions in medium osmolality is largely dependent on extracellular calcium. Studying the relationships between cell volume, [Ca2+] and PRL release in response to varying osmolalities, depolarizing conditions and ion channel blockers, provided evidence that stretch-activated calcium-permeant ion channels are responsible for the transduction of reduced extracellular osmolality into increased PRL release. The present findings support the proposed signal transduction model for osmotic stimuli in PRL cells. A decrease in extracellular osmolality leads to an increase in cell volume. Cell swelling increases the open probability of stretch-activated ion channels, and results in the entrance of extracellular calcium into the cell and stimulation of PRL release.

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