Publisher Summary This chapter focuses on habitat selection by aquatic invertebrates. It explains why animals are found in certain habitats and not in others, and it is restricted to a consideration of habitat selection by marine and freshwater invertebrates as revealed by experimental analysis. Habitat selection is essentially the relationship between behavior and environment, and it largely determines the local distribution of animal species. On the larger scale of geographical distribution, it is as yet uncertain how important habitat selection is, although it almost certainly plays a significant role. The chapter discusses the physical and chemical environment of intertidal animals, marine animals, freshwater animals, and interstitial animals. The reactions of animals to their biological environment and the part these play in habitat selection are very varied. Animals are continuously assessing the suitability of their environment and moving from place to place so that they can take best advantage of the range of conditions available to them. The behavioral mechanisms by which animals select their habitats, light reactions, gravity responses, gregariousness, and so on, have been covered in the chapter. Habitat preferences may remain fixed throughout the life span of an animal, or they may alter depending upon its physiological state, its age, its previous experience and learning, or its past and present environment. The marked variation in behavior shown by individuals of some species has been discussed along with the possible ways in which animals may colonize new habitats and to the part habitat selection might play in the origin of new species.

Habitat Selection by Aquatic Invertebrates

The magnitude and extent of jellyfish blooms are influenced not only by the biology and behavior of the animal, but also by the geographic setting and physical environment. Hydrography alone is often thought to cause or favor gelatinous zooplankton aggregations, however, it is clear that interactions between biology of the animal and physics of the water are very important sources of population variations, especially at local scales. We summarize the role of physical processes and phenomena that promote aggregations of gelatinous zooplankton. We have identified and discussed a suite of physical gradients that can be perceived by gelatinous zooplankton. These include light, gravity, temperature, salinity, pressure and turbulence. A recurring theme is accumulation of jellyfish around physical discontinuities such as fronts (shelf-break, upwelling, tidal and estuarine) and pycnoclines (thermoclines and haloclines). Interestingly, there are few data to suggest that large-scale, quasi-stationary features, such as the largest oceanic fronts, serve to physically aggregate gelatinous animals at a similar scale. Rather, examples of local aggregations appear to dominate the literature. We also discuss various jellyfish behaviors that are theorized to promote aggregation, feeding and reproduction in relation to physical discontinuities.

A physical context for gelatinous zooplankton aggregations: a review

The lateral line system is the only sensory system unique to aquatic vertebrates, being restricted to fish and some aquatic amphibians. It contains at least two, and probably three, different receptor classes: ciliated mechanoreceptors, electroreceptors, and a largely unknown group of receptors called the vesicles of Savi, found only in certain elasmobranch fishes (Garman 1888; Coggi 1891; Norris 1932). While all these organs are innervated by branches of the lateral line nerve and may have common embryological origins (Coggi 1891; Norris 1932; Northcutt 1985), it is quite clear that ciliated mechanoreceptors and electroreceptors respond to different stimuli and that their CNS projections are segregated at least at the level of the brain stem (McCormick 1983; Northcutt 1985). Thus, the concept of a single lateral line system has given way to one of at least two different sensory systems, with emerging evidence of anatomical functional variability within each of these.

Diversity of Lateral Line Systems: Evolutionary and Functional Considerations

Electroreceptive neurons in the posterior branch of the anterior lateral line nerve of three species of electric fish (Gymnotoidei):Sternopygus macrums, Eigenmannia virescens, andApteronotus albifrons, show speciesspecific differences in the filtering of electrical stimuli. All of the tuberous electroreceptor fibers of an individual are tuned to the same frequency: that of the electric organ discharge (EOD) of the species, more specifically, to that of the individual. The fibers inSternopygus are tuned to 50–150 Hz; those inEigenmannia to 250–500 Hz, and those inApteronotus to 800–1,200 Hz (Figs. 3, 5, 8). Two classes of organs inSternopygus andEigenmannia, P and T units, respond to sinusoidal stimuli at the unit's best frequency (BF) with a phase-locked partially-adapting (P), or tonic (sustained) (T) discharge. T-units are more sharply tuned and are more sensitive than P-units. Only one class of organs,P or partially adapting units, have been found inApteronotus and phase-locking is less evident than it is in other species.

Stimulus Filtering and Electroreception: Tuberous Electroreceptors in Three Species of Gymnotoid Fish

Publisher Summary The non-living organic matter in sea water is derived from a variety of plant sources, including phytoplankton, attached aquatic vegetation in shallow waters, macroscopic pelagic algae, and waterborne and windborne materials of terrestrial origin. The relative importance of these various sources cannot be stated precisely, but there is little doubt that phytoplankton is the most important source. Phytoplankton production, as determined by C14 fixation, falls within the range of 50-15Og of carbon per m2 in a year in the major ocean basins; however, some regions, notably the high arctic, are less productive than this and higher levels are attained in some coastal and estuarine waters and probably in oceanic areas of intense upwelling.

Particulate Organic Matter in Sea Water

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1968.13.1.0156

Reactions of plankton organisms to water stratification

The Interrelations between Electroreceptors, Electric Organ, Lateral-line Organ and Nervous System in Mormyridae ( Teleostei, Pisces) Summary. The investigations are based upon Gnathonemus petersii, Petrocephalus simus and a number of different species of the Mormyridae (Mormyriformes), all bearing weakly electric organs and being able to perceive electric stimuli. Receptors probably are three types of mormyromasts, A, B and tuberous organs (C, Knollenorgane), whose distribution, innervation, vaseularization and electrical autoaetivity are described. They are confined to well defined areas of the epidermis and both epidermis and receptors form a functional unit, called "electroreceptor-system". As the epidermis of these regions has a particular structure, which in a similar way is developed in the Gymnotoidei (Cypriniformes) only, also weakly electric fish that are able to detect electric stimuli , the term "electroceptor-epidermis" is proposed. Its essential component are columns of very thin, flat, hexagonal cells 60 tzm in diameter, invariable in all species and body sizes. The height of the columns depends on the location, but increases with body length. The mormyromasts of any kind are not covered with hexagonal cells, instead of these an areola of small polyedrie cells in circular arrangement has developed, which probably leads the current to the receptors. The A-type mormyromasts, possessing an opening towards the surface, show an even distribution with a relatively wide space in between. The B-type, without opening, are much more numerous than both other types. They are more closemeshed in arrangement, but also evenly distributed. Both Aand B-type mormyromasts increase only slightly in number during growth of the fish. The tuberous organs (type C) lack also an open connection to the surface, but form distinct patterns. They can be classified according to the number of their sensory (giant) cells (1 to 10). The number of tuberous organs as well as those of their sensory ceils increases with growth. In the genus Petrocephalus the tuberous organs above the eye, in the neck region and near to the throat form larger aggregates first described here and called "rosettes". The single organs of a rosette usually contain a larger number (up to 40) of cells, as compared to the tuberous organs of the other mormyrids. All mormyromasts are innervated by lateral line nerves, with exception of the tip of the chin or the barb, which are connected to the N. trigeminus. The mormyro-

Die Beziehungen zwischen Elektrorezeptoren, Elektrischem Organ, Seitenlinienorganen und Nervensystem bei den Mormyridae (Teleostei, Pisces)

An schwachelektrischen Fischen der Art Gnathonemus petersii (Gunther, 1862) wird die Empfindlichkeit gegenuber von ausen einwirkenden elektrischen Feldern und gegenuber Veranderungen des vom Tier selbst aufgebauten elektrischen Feldes gemessen. Als Reaktion fur die Wahrnehmung dieser Reize dienen charakteristische Anderungen in der Pulsfrequenz des elektrischen Organs. Die Reizschwelle gegenuber von ausen einwirkenden elektrischen Feldern ist von Tier zu Tier unterschiedlich. Die empfindlichsten Tiere reagieren auf eine Feldstarke von etwa 0,7 mV/cm. Die Reizung erstreckt sich hierbei auf einen kleinen Bereich der Korperoberflache des Tieres. Am empfindlichsten ist der Bereich zwischen Auge und Kiemenoffnung. Die Verteilungsdichte der epidermalen Sinnesorgane wurde bestimmt; sie ist in den Bereichen hoher Empfindlichkeit besonders gros. Damit wurden fruhere Befunde anderer Autoren bestatigt, welche diese Sinnesorgane als die Elektrorezeptoren erkannten. Die Empfindlichkeit gegenuber Veranderungen des vom Fisch selbst aufgebauten elektrischen Feldes deckt sich in ihrem allgemeinen Verlauf mit der Empfindlichkeit gegenuber von ausen einwirkenden elektrischen Feldern. Somit kommen fur beide Arten von Reizen die gleichen Rezeptoren in Frage.

Zur Empfindlichkeit des schwachelektrischen Fisches Gnathonemus petersii (Gthr. 1862) (Mormyriformes, Teleostei) gegenüber elektrischen Feldern

In der vorliegenden Arbeit werden Untersuchungen an schwachelektrischen Fischen der Art Gnathonemus petersii (Gunther 1862) beschrieben.

Wilhelm Harder

Papers

Reactions of plankton organisms to water stratification

Die Beziehungen zwischen Elektrorezeptoren, Elektrischem Organ, Seitenlinienorganen und Nervensystem bei den Mormyridae (Teleostei, Pisces)

Zur Empfindlichkeit des schwachelektrischen Fisches Gnathonemus petersii (Gthr. 1862) (Mormyriformes, Teleostei) gegenüber elektrischen Feldern

Zur Funktion des elektrischen Organs von Gnathonemus petersii (Gthr. 1862) (Mormyriformes, Teleostei)

Nachweis aktiver (elektrischer) Ortung bei Mormyridae (Teleostei, Pisces)