Morphological identification in skull between spotted seal and harbor seal using geometric morphometrics.

The morphology of the skull contains considerable ecological information about a species, because the skull contains sensory organs that are used to look for food, compete for mates, or to migrate. Spotted seals (Phoca largha) and harbor seals (Phoca vitulina) are similar in body size and pelage color but differ in habitat use and reproductive biology. The current study aims to clarify differences in the shapes of skulls in the spotted and harbor seals using geometric morphometrics and to discuss whether ecological differences can explain morphological differences in skulls. First, we discovered that the age at which the shape of skulls stopped changing was 7 years in both species, using the linear-threshold model. Using a total of 75 landmarks, 54 individuals (25 spotted seals, 29 harbor seals) that were older than the age at which skulls stopped changing were correctly identified at a rate of 100%. The total of 75 landmarks was narrowed down to eight key landmarks that resulted in an identification accuracy rate of 100% using random forests. Of the eight landmarks, seven were related to feeding apparatus, indicated that the harbor seal had a broader mouth and mandible than the spotted seal. Because of both species were dietary generalists and classified as pierce feeders, we suggested that the different features in the shapes of their skulls were caused not only by differences in their feeding behavior but also other differences related to reproductive behavior.

Interspecific variation of prevalence by Scaphanocephalus (Platyhelminthes: Trematoda: Heterophyidae) metacercariae in parrotfishes (Labridae: Scarini) from an Okinawan coral reef.

Metacercarial cysts of the parasite Scaphanocephalus (Platyhelminthes: Trematoda: Heterophyidae) are frequently found on the pectoral fins and skin of parrotfishes (Labridae: Scarini) inhabiting Okinawan coral reefs in southern Japan. The prevalence of metacercarial cysts in 30 parrotfish species was investigated and compared through a market survey. Although parasite prevalence differed between parrotfishes, all species examined are considered to be suitable hosts. Prevalence was high in Scarus chameleon (38.5%, n = 13), S. rubroviolaceus (33.4%, 2797), S. ghobban (26.6%, 6441), and several other species that share, in part, common feeding habits. Conversely, prevalence was low in S. prasiognathos (0.4%, 1842), Bolbometopon muricatum (0.4%, 270), and Hipposcarus longiceps (0.1%, 8512) which have different feeding habits. Despite a few exceptions, feeding ecology and other indirect behaviors are considered to affect the prevalence of metacercarial cysts in parrotfishes. Taxonomic affiliation and nocturnal mucous cocoon usage are not considered to affect parasite prevalence.

Euryphorus brachypterus (Copepoda: Caligidae) on wild pacific bluefin tuna from the Tsugaru Strait, northern Japan.

Parasitic copepods infecting large scombrid fishes have been known for a long time because their hosts are economically important. Most studies, however, have focused on their morphology or their infection status in aquaculture from pathological viewpoints, and very few quantitative surveys have been conducted under conditions in the wild. This study therefore investigated the prevalence of Euryphorus brachypterus (Caligidae) in wild Pacific bluefin tuna (PBF). Results of sampling from August to September 2014 at the western area of the Tsugaru Strait, Japan showed that 13.2% of the PBF individuals (n = 1978) were infected with this copepod. The prevalence of infections was highest in larger fish but varied among landing dates, which were classified into three clusters and in all smaller fish, the prevalence of infections was zero. This suggests that E. brachypterus mainly uses the larger PBF, which becomes sources of further infections in other seas, and that at least two host populations with different infection statuses at the strait.

Maladaptive sex ratio adjustment by a sex-changing shrimp in selective-fishing environments.

1. Selective harvesting is acknowledged as a serious concern in efforts to conserve wild animal populations. In fisheries, most studies have focused on gradual and directional changes in the life-history traits of target species. While such changes represent the ultimate response of harvested animals, it is also well known that the life history of target species plastically alters with harvesting. However, research on the adaptive significance of these types of condition-dependent changes has been limited. 2. We explored the adaptive significance of annual changes in the age at sex-change of the protandrous (male-first) hermaphroditic shrimp and examined how selective harvesting affects life-history variation, by conducting field observations across 13 years and a controlled laboratory experiment. In addition, we considered whether plastic responses by the shrimp would be favourable, negligible or negative with respect to the conservation of fishery resources. 3. The age at sex-change and the population structure of the shrimp fluctuated between years during the study period. The results of the field observations and laboratory experiment both indicated that the shrimp could plastically change the timing of sex-change in accordance with the age structure of the population. These findings provide the first concrete evidence of adult sex ratio adjustment by pandalid shrimp, a group that has been treated as a model in the sex allocation theory. 4. The sex ratio adjustment by the shrimp did not always seem to be sufficient, however, as the supplement of females is restricted by their annual somatic growth rate. In addition, adjusted sex ratios are further skewed by the unintentional female-selectivity of fishing activity prior to the breeding season, indicating that the occurrence of males that have postponed sex-change causes sex ratio adjustment to become unfavourable. 5. We conclude that the plastic responses of harvested animals in selective fishing environments must be considered in efforts to conserve wild animal resources, because such responses can become maladaptive.

Nematomorph parasites indirectly alter the food web and ecosystem function of streams through behavioural manipulation of their cricket hosts.

Nematomorph parasites manipulate crickets to enter streams where the parasites reproduce. These manipulated crickets become a substantial food subsidy for stream fishes. We used a field experiment to investigate how this subsidy affects the stream community and ecosystem function. When crickets were available, predatory fish ate fewer benthic invertebrates. The resulting release of the benthic invertebrate community from fish predation indirectly decreased the biomass of benthic algae and slightly increased leaf break-down rate. This is the first experimental demonstration that host manipulation by a parasite can reorganise a community and alter ecosystem function. Nematomorphs are common, and many other parasites have dramatic effects on host phenotypes, suggesting that similar effects of parasites on ecosystems might be widespread.

Nematomorph parasites drive energy flow through a riparian ecosystem.

Parasites are ubiquitous in natural systems and ecosystem-level effects should be proportional to the amount of biomass or energy flow altered by the parasites. Here we quantified the extent to which a manipulative parasite altered the flow of energy through a forest-stream ecosystem. In a Japanese headwater stream, camel crickets and grasshoppers (Orthoptera) were 20 times more likely to enter a stream if infected by a nematomorph parasite (Gordionus spp.), corroborating evidence that nematomorphs manipulate their hosts to seek water where the parasites emerge as free-living adults. Endangered Japanese trout (Salvelinus leucomaenis japonicus) readily ate these infected orthopterans, which due to their abundance, accounted for 60% of the annual energy intake of the trout population. Trout grew fastest in the fall, when nematomorphs were driving energy-rich orthopterans into the stream. When infected orthopterans were available, trout did not eat benthic invertebrates in proportion to their abundance, leading to the potential for cascading, indirect effects through the forest-stream ecosystem. These results provide the first quantitative evidence that a manipulative parasite can dramatically alter the flow of energy through and across ecosystems.