Biological Bulletin最新文献

AbstractIn the high Arctic polar night, the sun remains below the horizon for a period ranging from several weeks to 6 months, providing insufficient solar radiation for photosynthesis to support the marine ecosystem. Despite this, recent studies have found evidence of active feeding, growth, and reproduction in multiple marine organisms during this period. Here, we describe the polar night diversity and activity levels of early pelagic life stages of benthic invertebrates (meroplankton), a group that has historically been largely overlooked. We assessed the taxa richness, reproductive and developmental biology, feeding, and competence to settle of meroplanktonic larvae in January 2023 in Kongsfjorden, Svalbard, using plankton tows, experiments, and DNA bar coding. Although meroplankton densities were lower than in other seasons, DNA bar coding allowed us to assess richness, which was much higher than other studies and seasons, with 48 unique taxa belonging to six phyla observed. Most were planktotrophic (feeding) larvae. Using laboratory assays and direct observations, we documented feeding in several of these planktotrophic larvae, including bryozoan cyphonautes, bivalve veligers, and an ophiuroid pluteus. We found numerous embryos, suggesting that several taxa are actively reproducing in the polar night. We observed settlement in two polychaete taxa in the laboratory, as well as metamorphosed echinoderms in our samples, showing competency to settle across phyla. Overall, we show that early life-history stages of a diverse range of benthic invertebrates are actively feeding and developing during the polar night. Further targeted research should focus on the polar night to better understand seasonal patterns in reproduction and development.

AbstractKentrogonid rhizocephalans show remarkable sexual dimorphism, with large females and much smaller (=dwarf) males as adults, although males are larger than females from the eggs to the larval stages. The kentrogonid Peltogasterella gracilis shows seasonal variation in sex ratio in that most females produce only male or female offspring, although some mixed-sex broods appear in autumn. Moreover, a link between chromosome number and the single-sex broods has been suggested, wherein females producing only male offspring are considered to have a chromosome number of 2n=30, whereas females producing only female offspring are 2n=31. Thus, there appear to be both genetic and environmental effects on variation in sex ratio. However, the mechanism of sex determination has not been fully studied in P. gracilis or any other rhizocephalans. We examined the number of chromosomes in male (larger) and female (smaller) unfertilized eggs and early-stage embryos of specimens collected in Otaru, Hokkaido, Japan. Chromosome number of unfertilized eggs was n=16 for female eggs and n=15 for male eggs, without exception. Moreover, chromosome number in most (97.4%) female embryos was 2n=31, while that of all male embryos was 2n=30. We suggest that a univalent chromosome of females acts as a sex chromosome and P. gracilis has a special type of female heterogamety (OW/OO system). This sex determination system appears to be suitable for controlling offspring sex ratio seasonally, as is known in this rhizocephalan.

AbstractThe many different muscles of sea urchins are used to control tube foot and spine movement, feeding, excretion, respiration, and gamete release. Unlike in most other animals, the delineation between skeletal and smooth muscles in sea urchins is not clear cut, with many muscles showing characteristics of both muscle types. To further our understanding of sea urchin muscle function, we sought to characterize the electrical properties of protractor muscles of the Aristotle's lantern. Aristotle's lantern comprises a complex of multiple different muscles, ligaments, and calcite ossicles and is primarily involved in feeding. Within the lantern, antagonistic muscle pairs of protractor and retractor muscles act to raise or withdraw the lantern, respectively. The protractor muscles are unstriated, similar to smooth muscle, but connected to the calcite skeleton, as with skeletal muscles. We isolated single muscle cells from the protractor muscles and measured their membrane potentials and found that they generate spontaneous currents at a frequency that ranged from 25 to 30 Hz, a functional property found in many smooth muscles. Furthermore, these currents occurred in the absence of any extraneous cellular or chemical input. Measurement of the reversal potential of the currents under control and ion-substituted conditions suggests that they may be due to the activity a nonspecific cation channel or the synchronous activity of K⁺ and Na⁺ channels.

AbstractThe setae on the antennae of caridean shrimps perform chemoreceptive functions, including the identification of potential mating partners. Variation in the number of setae on the antennal flagellum has been reported as a sexually dimorphic trait in these organisms. This difference is correlated with the distinct mating systems and reproductive behaviors of these animals. This study investigates the variation in the number of antennal flagellar setae in four species of shrimps from the genus Macrobrachium, which are grouped into two distinct mating systems. Two species with a pure search mating system (M. jelskii and M. pantanalense) and two with a neighborhoods of dominance mating system (M. acanthurus and M. olfersii) were analyzed. The setae were quantified using light microscopy, morphologically characterized through scanning electron microscopy, and then compared among the demographic groups of each species. The results show that in the pure search species, males possess a significantly higher number of setae than females (P < 0.01). In the neighborhoods of dominance species, dominant males of M. acanthurus and M. olfersii had more setae than subordinate males and females (P < 0.01). The same types of setae, denticulate and simple, were found in all species. We conclude that the variation in the number of flagellar setae is correlated with the mating system of each species and their specific reproductive behaviors, suggesting a morphological adaptation to optimize chemoreception in different reproductive events. These findings contribute to the understanding of reproductive strategies and sensory adaptations in crustaceans.

AbstractThe chiton Chaetopleura isabellei is a common species in the intertidal shores of the southwestern Atlantic Ocean. This study investigates the age, individual growth, and mortality of C. isabellei on an intertidal boulder shore in San Antonio Bay (40°45' S, 064°56' W), Patagonia, Argentina. Growth rings on the valves were validated as annual, forming during the warm season, a pattern unusual among chitons. The age structure showed a stable population, with most individuals between 2 and 7 years and a maximum observed longevity of 11 years. The growth pattern, analyzed through both polynomial models and the von Bertalanffy growth model, indicated rapid growth during early years, followed by slower growth in later stages. Mortality was best described by the Siler mortality model, indicating variable mortality rates across the lifespan, with higher risks during early and late life stages. These findings provide insights into the life history of C. isabellei, emphasizing the influence of environmental factors on growth and mortality in this species.

AbstractIn ponds of the northeastern United States, benthic macroinvertebrates can dominate the local biomass. Isopods, predatory leeches, and fingernail clams can attain dense populations and suffer heavy predation pressure by fish. We predicted that pond benthic macroinvertebrates would recognize the proximity of predatory fish and avoid or vacate that area as an inducible behavioral defense. We deployed cages with and without predatory fish (sunfish and golden shiners) in a naturally fishless pond in October and November of 2020 and 2021. After at least 2 days, we collected leaf packs from directly under the cages and compared the number of invertebrates residing within. Surprisingly, the population densities of the dominant taxa (isopods, leeches, and clams) suggested that they did not avoid fish. Leeches and isopods may even reside in higher numbers near live sunfish, perhaps because feces from the fish augment the locally available food and nutrient levels. Our present field results support earlier laboratory findings: benthic macroinvertebrates in ponds may not respond to fish cues. Bottom-up control may dominate in ponds, providing important implications for conservation of these threatened ecosystems.

AbstractThe Atlantic white shrimp, Penaeus setiferus, is a commercially important species that is abundant along the United States' east coast and Gulf of Mexico. Like other similar organisms, this species is vulnerable to black gill disease, where gills become heavily melanized as part of an immune response associated with gill irritants or parasitic infection. The melanization blackens the gills, making the disease obvious. Black gill is thought to be stimulated by high temperature, high salinity, and low oxygen. In this study, we investigated whether the presence of black gill influences the ability of shrimp to take up oxygen across the gills. Shrimp were made to exercise on an underwater treadmill while measurements of oxygen uptake were made. Measurements were made in well-oxygenated water (100% air saturation) and moderate (50% air saturation) and severe (30% air saturation) hypoxia. In quiescent animals, there was no difference in oxygen uptake between control shrimp with no black gill and those with obvious black gill infections. Oxygen uptake increased by as much as twofold when shrimp were active on the treadmill. In both control and black gill groups, oxygen uptake declined in hypoxia, but the decline was greater in black gill shrimp, suggesting an impairment to taking up oxygen. Thus, black gill significantly impairs the ability of shrimp to take up oxygen under hypoxic conditions when shrimp are active. These results provide a mechanistic basis for potential negative impacts of shrimp populations suffering with outbreaks of black gill.

AbstractMany marine invertebrates possess biphasic life histories, during which larvae develop in the plankton and adults inhabit the benthos. The transition between phases entails the settlement of larvae onto substrata, completion of metamorphosis, and survival as vulnerable early juveniles. The perimetamorphic period, encompassing settlement and the interval immediately following settlement, is a key determinant of adult abundance and distribution. However, because settling larvae and early juveniles are difficult to observe in the field, the ecology of this period remains poorly understood. We performed experiments to elucidate the settlement preferences of Asterias forbesi and Asterias rubens, keystone predators on the east coast of North America, on substrata common to their intertidal habitats. Larval Asterias exhibit clear selectivity in settlement, with shells of the blue mussel, Mytilus edulis, most preferred. The algae Chondrus crispus and crustose coralline algae also induced high rates of settlement, while little settlement was observed on rocks with biofilm and no settlement occurred in controls. When inductive cues were subsequently added to controls, high frequencies of settlement occurred immediately, confirming the competency of larvae to settle and their ability to delay metamorphosis in the absence of appropriate cues. Our results demonstrate that Asterias larvae have specific settlement preferences and that settlement can be postponed in this species if no suitable substrate is available.

AbstractRibbed mussels are typically found in Spartina alterniflora salt marshes and can form dense aggregations along low marsh shorelines and tidal creeks. The presence of ribbed mussels within marshes is well documented, and many studies have examined their importance in these ecosystems. However, it is not known whether mussels settle preferentially in S. alterniflora habitat and what factors influence observed aggregations of mussels. Knowing this is important for current efforts in salt marsh restoration. Therefore, we conducted experiments with competent larvae to test whether chemical or physical cues of S. alterniflora or adult conspecifics stimulate settlement and metamorphosis. More larvae settled and metamorphosed in the presence of a S. alterniflora leaf, although not on the leaf, and in water conditioned with S. alterniflora than in seawater controls. The presence of chemical or physical cues from conspecifics had no effect on settlement and metamorphosis relative to controls. Larvae settle and metamorphose in response to chemical cues from S. alterniflora, resulting in ribbed mussels being found predominantly in S. alterniflora habitat. However, because there was no response to conspecifics, other factors are likely responsible for their aggregated distribution. Further work is required to determine the factors resulting in mussel aggregation, including movement of juvenile mussels and environmental filtering.