Donald A. Thomson

Resilience of a rocky intertidal fish community in a physically unstable environment

Abstract A 7-year census of intertidal fishes has been made by repeated defaunation of tide pools in the northern Gulf of California. The intertidal fish community showed long-term resilience and hence stability, under a rigorous, unstable physical environment. Although the majority of fishes have tropical affinities (76 %), warm temperate species (24 %) constitute 33 % of total numbers and 69 % of total biomass of the entire intertidal fish community. Short-term seasonal fluctuations in species diversity and population numbers of temperate fishes were in better synchrony with the annual light regime and sea temperature cycles than those of tropical species. A winterkill due to abnormally low sea temperatures in January, 1971, exerted faunal selection by killing several tropical fishes. Warm temperate species tolerated these low temperatures both experimentally and during the 1971 winterkill. Low sea temperatures were judged to be more limiting to diversity than high sea temperatures. The seasonal tidal pattern in the northern Gulf of California moderates summer heating but accentuates winter cooling thus favoring winterkills. We conclude that this intertidal fish community, dominated by r -selected species, is more physically controlled than biologically accommodated and the temperate species play an important role in regulating the stability of this fish assemblage.

Catastrophic decline of a top carnivore in the gulf of california rocky intertidal zone.

The predatory sun star, Heliaster kubiniji, once the commonest rocky intertidal asteroid of the Gulf of California, has been rare throughout this region since summer 1978 when a devastating disease outbreak occurred. This unprecedented phenomenon and several other exceptional ecological events in marine communities of the northeastern Pacific appear to be linked to large-scale climatic changes that occurred during 1977 and 1978. Implications of the decline in Heliaster kubiniji are discussed.

Responses of Young Gulf Grunion, Leuresthes sardina, to Gradients of Temperature, Light, Turbulence and Oxygen

. 1955. Field experiments on the biology and genetic relationships of the California species of Triturus. Ibid. 129:129-148. . 1961a. Experiments of homing behavior and speciation in Taricha, p. 415-459. In: Vertebrate Speciation. W. F. Blair, ed. Univ. Texas Press, Austin. . 1961b. Second-generation hybrids of the species of Taricha. Proc. Nat. Acad. Sci. 47:1461-1486. 1964. Fertility of Taricha specieshybrids and viability of their offspring. Ibid. 51:156-161. . 1966. Of Scientists and Salamanders. W. H. Freeman and Co., San Francisco. WHITE, M. J. D. 1946. The spermatogenesis of hybrids between Triturus cristatus and T. marmoratus (Urodela). J. Exp. Zool. 102:179205. YANG, S. Y., L. L. WHEELER AND I. R. BOCK. 1972. Isozyme variations and phylogenetic relationships in the Drosophila bipectinata species complex. Studies in Genetics VII. Univ. of Texas. Publ. no. 7213:213-227.

Feeding patterns in eastern tropical Pacific blennioid fishes (Teleostei: Tripterygiidae, Labrisomidae, Chaenopsidae, Blenniidae)

SummaryIn terms of species number (47) and numerical abundance, blennioids are the most important primary resident rocky reef fishes in the Gulf of California, Mexico. We present the feeding patterns of the 34 most abundant species of blennioid fishes, 8 of which are Gulf endemics. A total of 2,144 specimens were sampled at 51 anaesthetic stations in 9 areas throughout the Gulf. Four feeding guilds were distinguished: 1) The majority (29 of 34 species) are microcarnivores exhibiting a number of different feeding strategies (ambush and stalking predators, active foragers, pickers, etc.). The more important prey categories were mobile invertebrates, and to some extent also sedentary fauna. Algae were of no importance for most of the latter species. 2) Hypsoblennius brevipinnis and H. gentilis are two omnivorous species, browsing mainly on sessile items including 52% and 13% (Vol.) algae in their diets. 3) Entomacrodus chiostictus and Ophioblennius steindachneri are herbivores, grazing on fine algae. 4) Plagiotremus azaleus specializes in cropping mucus and scales from the body surface of other fishes.Crustaceans account for 58.6% of the total volume of prey items in the 34 species investigated. Benthic amphipods were most important and made up 26% of the total volume of all prey items.Cluster analysis of percentage volumetric data using Squared Euclidian Distance and Horns Index of Overlap produced distinct subgroups which coarsely reflected taxonomic grouping.The species are separated either by their geographic ranges, habitat and microhabitat preferences, feeding, or a combination thereof. Only rarely do sympatric species significantly overlap in diet.Trophic diversity as measured by the Shannon-index provides a tool for distinguishing: 1) specialists (6 species) from 2) low diversity feeders (18 species) and 3) high diversity generalists (10 species). Two different types of specialists can be distinguished: those which feed on the same items as the generalists but utilize only a very restricted prey spectrum (Stathmonotus sinuscalifornici and the chaenopsids Chaenopsis alepidota and Emblemaria hypacanthus). A second group of specialists (Entomacrodus chiostictus and Ophioblennius steindachneri as well as Plagiotremus azaleus) feed on items not utilized by any of the generalists.There is some evidence that high diversity generalists are numerically more abundant than the other trophic groups.In the labrisomids and blenniids a phylogenetic trend from microcarnivory towards feeding on sessile items appears to be expressed.

Tidal influence on the evolution of egg size in the grunions (Leuresthes, Atherinidae)

SynopsisThe duration of natural in-beach-sand incubation of Gulf of California and California grunion embryos is dictated by the tidally-driven wave activity in their respective habitats. Analyses of these tidal regimes reveal that the incubation period can be three times longer on beaches of the California coast than on those of the northern Gulf.In response to the more irregular tides of California, Leuresthes tenuis, the less primitive of the two species, has evolved an egg volume 310% larger than the Gulf grunion, L. sardina. The. larger egg not only supplies greater yolk reserves for differentiation, growth and maintenance, but also produces a larger embryo. The more abundant yolk stores and greater embryo size helps California grunion embryos to remain viable in the sand longer than those of the Gulf grunion.

Responses of young California grunion, Leuresthes tenuis, to gradients of temperature and light

In a laboratory temperature gradient, young California grunion, Leuresthes tenuis, exhibited a final modal preferendum of 25 C, 7 C lower than the final preferendum of young L. sardina from the Gulf of California. No circadian rhythms in thermal preference were found, and the final preferendum was unaffected by acclimation temperature. Like L. sardina, young L. tenuis were strongly attracted to bright light (10,000 lux). In a simultaneous opposed gradient of temperature and light, the fish spent more time at the preferred temperature than at the preferred light intensity. The laboratory results are discussed in relation to observed field distribution and geographic range.

Temperature and salinity tolerances of larval Californian grunion, Leuresthes tenuis (Ayres): A comparison with gulf grunion, L. Sardina (Jenkins & Evermann)

Abstract Temperature and salinity tolerances were determined for larval California grunion, Leuresthes tenuis (Ayres), and compared with previous data for Gulf of California grunion, L. sardina (Jenkins & Evermann). Larvae of similar age and acclimation history showed little interspecific difference in thermal tolerance, as measured by half-hour LT50 values for 20–30 day old late postlarvae acclimated at various temperatures, and by upper and lower incipient lethal temperatures for 18°C-acclimated prolarvae. The upper incipient lethal temperature differed by 1 deg.-C (32°C for L. tenuis, 31°C for L. sardina), while the lower incipient lethal temperature for the 18°C acclimated prolarvae of both species was 7.5°C. L. tenuis larvae were much less euryhaline than L. sardina, with incipient lethal salinities of 4.2–41 %. for prolarvae and 8.6–38 %. for 20-day-old postlarvae; comparable values for L. sardina are 4–67.5 %. and 5–57.5 %. Both species show a decrease in temperature and salinity tolerance with age. The larvae of these disjunct congeners show a significant physiological divergence in euryhalinity but not in overall temperature tolerance. These tolerances are discussed in relation to the respective geographic ranges and behavioral responses of the two species.

Ontogenetic change in the response of the Gulf of California grunion, Leuresthes sardina (Jenkins & Evermann), to a salinity gradient

Abstract Larval and juvenile Gulf of California grunion, Leuresthes sardina (Jenkins & Evermann), hatched and reared in the laboratory, were subjected to salinity gradients in a Staaland salinity gradient device to determine their locomotive responses. Newly hatched prolarvae selected salinities in the range 15–24‰, while late postlarvae and juveniles preferred salinities in the range 45–54‰. Early postlarvae exhibited a bimodal preference, indicating that the behavioral change was in progress at that age. These behavioral responses may serve several functions in the physiological and behavioral ecology of the Gulf grunion.