Ira Rubinoff

Behavioural, physiological, and ecological aspects of the amphibious life of the pearl blenny Entomacrodusnigricans Gill

Entomacrodus nigricans Gill is a herbivorous blennioid fish that occurs in high densities on wave-swept, upper intertidal regions of San Andres Island, Colombia, and at various locations along the Caribbean coast of Panama. The habitat of this fish requires it to endure alternating periods of intense water turbulence and total emersion, and the pearl blenny depends upon its amphibious capabilities both to maintain position and move in this environment. Orientation studies show that this species is a capable jumper and terrestrial navigator, able to move in the direction of the sea even when water is not in a direct line of sight. The eyes of this fish appear modified for aerial vision. The pearl blenny does not make long, amphibious sojourns and is much less tolerant of air exposure than are more terrestrial species such as mudskippers and rockskippers. When exposed to air by a retreating wave, the pearl blenny continues to ventilate, using water around its body, and then finally gulps air and seals its mouth and opercula. The combined effects of both temperature and humidity determine the air-exposure tolerance of the pearl blenny. Mean survival time of fish exposed to air was 7.5 min in direct sunlight, 168 min in shade, and 228 min at night. Fish exposed to water-vapor saturated air at 25°C have a mean O2 consumption rate of 306 ml · kg · h−1, which is not significantly different from rates measured for fish in water. Pearl blennies cannot survive extended exposure to humidities <80%. Amphibious behaviour allows the pearl blenny to exploit supralittoral resources and probably resulted from competition amongst intertidal fishes for space and other resources.

ON PREDICTING INSULAR VARIATION IN ENDEMISM AND SYMPATRY FOR THE DARWIN FINCHES IN THE GALAPAGOS ARCHIPELAGO

For interisland variation in number of Darwin Finches in the Galapagos Archipelago, endemism is predicted by nearest-neighbor isolation; and species abundance or sympatry is predicted by average isolation. Nearest-neighbor isolation is measured by distance from the nearest island, and average isolation is the average distance to all other islands in the archipelago. The two measures of isolation are of little predictive value when tested for the avifaunas of six other archipelagos or oceanic island groupings. In these situations, area is a better predictor of species abundance or endemism; and the role of isolation appears only when measured as distance from the major avifaunal source region (e.g., New Guinea for islands of the East-central Pacific; African mainland for islands in the Gulf of Guinea). Insular isolation in these instances is, however, a relatively small contributor to variance of species number when contrasted with the greater contributions made by insular area. That numbers of insular species and endemics are respectively predicted by average and nearest-neighbor isolation, and not by area, only in the Darwin Finches demonstrates emphatically the importance of isolation in regulating endemism and species abundance (= sympatry) in the adaptive radiation of monophyletic bird groups within archipelagos. This generalization appears valid only when the intra-archipelagic speciations are mostly between islands, and not intraisland in site of origin from parental forms. It would appear to be less valid during the postspeciation, phyletic- specialization phase of radiation. It is presumed that the natural regulations of endemism and species abundance for insular avifaunas are stochastic in process. By this hypothesis, chance elements associated with isolation are of major importance early in the adaptive radiation of monophyletic bird groups, and less important later in this radiation when deterministic and chance elements associated with ecology (area, habitat, niche) predominate.

Subsurface Buoyancy Regulation by the Sea Snake Pelamis platurus

Sea snakes (Pelamis platurus) acclimated to a 10-m tank for 24 h routinely dove with sufficient lung volumes to be near neutrally buoyant at depth. Naive snakes in the same tank dove with much more variable lung volumes, and one of these snakes had sufficient volume to be neutrally buoyant at 30 m (4 atm). With longer dive times, acclimated snakes, because of their consumption of pulmonary O₂ and the transcutaneous losses of CO₂ and N₂, lost lung volume and became negatively buoyant. Sixtyfive percent of the dives of free-swimming snakes tracked at sea are characterized by a four-phased pattern of (a) descent, (b) bounce ascent, (c) gradual ascent, and (d) final ascent. The gradual ascent phase accounts for about 82% of total submergence time on each dive and may reflect a period when free-swimming snakes gradually ascend and, by Boyles Law, compensate for buoyancy lost because of the decline in pulmonary volume. The capacity of P. platurus to dive deeply and repetitively is attributable to increased lung 02 supply, to specializations for cutaneous gas exchange, and to an intracardiac pulmonary shunt. Shunting, which is well developed in sea snakes, assures management of pulmonary O₂ reserves in a fashion that augments cutaneous respiration and establishes favorable transcutaneous diffusion gradients that facilitate the loss of N₂ (to avert Caisson disease) and the uptake of O₂. Our studies suggest that shunting also minimizes the rate of buoyancy change and thus lowers the energetic costs of swimming for a diving snake.

Diving of the sea snake Pelamis platurus in the Gulf of Panamá

The horizontal movement patterns of eleven yellow-bellied sea snakes [Pelamis platurus (Linnaeus)] were determined incidentally to acoustic tracking studies of their diving behavior in the Gulf of Panamá during 1983–1985. The average speed of the snakes was 1.9 km h-1 (range 0.3 to 7.1 km). Movement direction is influenced by phasic northsouth tidal currents and secondarily affected by combinations of factors that affect local current velocity. The velocity of diving snakes did not always agree with that observed for the surface current. Drift rate was not correlated with average or maximum dive depth, however, snakes that surfaced to breathe one or more times during a period had greater average drift rates than those that did not surface. The role of such factors as feeding, predator avoidance, thermoregulation, orientation, and the avoidance of surface turbulence are considered in a discussion of the adaptive significance of diving for P. platurus. Diving in this species may enable it to avoid surface disturbance, facilitate relative position changes within the surface drift, and contribute to various aspects of its feeding behavior.

Diving of the sea snakePelamis platurus in the Gulf of Panam: I. Dive depth and duration

AbstractFifteen yellow-bellied sea snakes,Pelamis platurus, fitted with pressure-sensitive ultrasonic transmitters, were tracked in the Gulf of Panamá during 1983–1985. Snakes spent up to 99.9% (

The sea-level canal controversy

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Species Abundance: Natural Regulation of Insular Variation

= 87%) of the tracking time under water and dived to 50 m. The maximum voluntary submergence time observed was 213 min, and of 202 complete dives logged, 19 exceeded 90 min. Dive durations of tracked snakes were typically longer than expected, based upon their estimated body-oxygen stores, and some were even longer than the reported survival times of forceably submerged snakes. Snakes, however, dived repeatedly and did not spend long periods at the surface between dives, suggesting that they did not develop an oxygen deficit during diving. Diving snakes may be able to avoid anaerobiosis by having a reduced metabolic rate, an enhanced rate of cutaneous oxygen uptake, or both. Surface conditions and subsurface temperatures influence the diving behavior ofP. platurus. Laboratory experiments in Panamá indicated that a larger number of snakes were submerged when surface water was turbulent. During February and March, the period of dry season upwelling in the Gulf of Panamá, sea snakes were found to avoid cooler, subsurface water and to make significantly shallower dives: mean maximum depth 6:8 m (n=76) in contrast to a mean maximum depth of 15.1 m (n=147) during the wet season. The dives during the dry season tended to be of shorter duration, with 44% lasting less than 15 min, compared to only 19% of the dives recorded during the wet season being completed in less than 15 min. General avoidance of subsurface temperatures cooler than 19°C was confirmed by laboratory experiments in the 10 m-deep tank at Scripps Institution of Oceanography.

THE ENVIRONMENTAL CONTROL OF INSULAR VARIATION IN BIRD SPECIES ABUNDANCE.

THE Panama Canal (Fig. 1) represents a potential pathway for the interoceanic dispersal of marine fishes. The fresh waters of Lakes Gatun and Miraflores have, however, effectively maintained the isolation of all but a few of the marine shore fishes which are found on both coasts of the isthmus, those species which are able to cross this barrier are not known to breed on the opposite coast1.