Breeding and bionomics of the British members of the Jaera albifrons group of species (Isopoda: Asellota)

Abstract

A detailed 24 month study (1968–1970) of the breeding cycles and population structures of the four British members of the Jaera albifrons Leach group of species (Isopoda: Asellota) has been carried out in Milford Haven, Pembrokeshire. The species often show overlapping ecological distributions and they can only be identified on male secondary sexual characters. Sampling stations had therefore to be selected carefully to ensure that single species populations were studied. In this way virtually single‐species populations of Jaera forsmani Bocquet, Jaera praehirsuta Forsman, and Jaera ischiosetosa Forsman have been studied but the Jaera albifrons Leach population was somewhat mixed with J. ischiosetosa, particularly when the stream normally inhabited by the last species dried up in the summer. Only one hybrid was found (between J. albifrons and J. ischiosetosa) in a total of 6214 specimens collected. Gravid females were taken every month for each species with peaks of breeding occurring during spring and summer. Young were liberated throughout the year by J. albifrons and J. ischiosetosa, but with summer peaks. J. praehirsuta and J. forsmani had a particularly limited summer period for the release of juveniles. J. forsmani was consistently the largest species and has a restricted geographical distribution. J. praehirsuta also has a patchy distribution. The sex ratio of males to females was never 1:1 for any species. Females outnumbered males by up to 14:1, with the sex ratio varying throughout the year except in J. praehirsuta where it remained at about 1.5:1.

Female-biased sex ratios unrelated to Wolbachia infection in European species of the Jaera albifrons complex (marine isopods)

“…Interestingly, J. ischiosetosa not only showed a female bias at birth but it was also the species with the strongest bias in adults from natural populations in this study (Table 1 and Figure 1) and other works (e.g. Jones and Naylor, 1971). Here we found that the bias in adults (r=0.18) was significantly stronger than in offspring reared in the laboratory (r=0.29, binomial proportions test p=0.03), suggesting that the factor that causes the female bias at the adult stage in other species may also be acting in J. ischiosetosa.…”

“…Nearly all demographic analyses of the four species that are found on the European coasts have reported female-biased sex ratios in adults in natural populations (Cléret, 1966b;Jaźdźewski, 1969;Jones and Naylor, 1971;Naylor and Haahtela, 1967;Naylor et al, 1961;Piertney and Carvalho, 1996;Solignac, 1976) with male proportions locally as low as 0.07 (J. ischiosetosa in the UK, Jones and Naylor, 1971). To our knowledge no such estimations have been reported for J. posthirsuta.…”

The annual pattern of reproduction of Talitrus saltator (Crustacea: Amphipoda: Talitridae)

“…As in Jaera (Jones & Naylor, 1971) no statistically significant difference in the degree of mortality was recorded in T. saltator between the non-motile and motile development stages. This contrasts with the apparently disproportionate increases in the brood mortality of the motile stages of Dynamene bidentata (Holdich, 1968) and Tryphosella kergueleni and Cheirimedon fernoratus (Bregazzi, 1972).…”

Ecology and systematics of a new species of Uromunna (Crustacea: Isopoda) from Spanish eelgrass beds

“…Other peracarid species has shown seasonal fluctuation in the Zostera meadows of O Grove, with maximum densities (including adults and juveniles) in late summer, coincident with the maximum enlargement of the leaves and thus the availability of substrate and food resources (Esquete et al 2011(Esquete et al , 2013. Indeed, seasonal fluctuations in the population in response to the maximum food availability are a common pattern for Asellota species: The observed seasonality is similar to that shown by other Uromunna species (García-Guerrero and Hendrickx, opus cit) as well as other Asellota in temperate latitudes (e. g. Jones and Naylor 1971;Jones 1974). Likewise, Asellota isopods in deep sea show a seasonal pattern, with reproduction maxima displaced to the winter in response to detritus deposition (Harrison 1988).…”

“…Likewise, seasonal variation in populations parameters presumably controlled to changes in temperature and/or food availability has been reported for other species of isopods. For instance, Jones and Naylor (1971) observed seasonal variation in the mean size of adult specimens of species of the asellote genus Jaera Leach 1814 owing to death of older individuals during the winter. Likewise, Luxmoore (1981) observed that growth in winter was significantly slower than in summer in the Antarctic species Paraserolis polita (Pfeffer, 1887) and considered this to be due to reduced food availability.…”