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The Biology of Jaera Spp. (Crustacea, Isopoda) in the Northwestern Atlantic

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The Biology of Jaera Spp. (Crustacea, Isopoda) in the Northwestern Atlantic 205 The biology of Jaera spp. (Crustacea, Isopoda) in the northwestern Atlantic. 1. Jaera ischiosetosa12 D. H. STEELE AND V. J. STEELE Department of Biology, Memorial University of Newfoundland, St. John's, Newfoundland Received August 13,1971 STEELE, D. H., and V. J. STEELE. 1972. The biology of Jaera spp. (Crustacea, Isopoda) in the north­ western Atlantic. 1. Jaera ischiosetosa. Can. J. Zool. 50: 205-211. Oogonia of female Jaera ischiosetosa begin to enlarge in April and the first hatched young appear in June. Reproduction continues until August. From September to March the females lack oostegites and are in a resting stage. Females do not produce continuous broods during the season, but lose their oostegites after every brood. Individuals survive for 1 year at the most. In August the population consists almost entirely of young of the year, some of which may breed in the same year. Egg size is small and the fecundity relatively high. STEELE, D. H., et V. J. STEELE. 1972. The biology of Jaera spp. (Crustacea, Isopoda) in the northwestern Atlantic. 1. Jaera ischiosetosa. Can. J. Zool. 50: 205-211. Chez les femelles de Jaera ischiosetosa, les oogonies commencent a croitre en avril, et en juin les oeufs se mettent a eclore. La saison de la reproduction se poursuit jusqu'en aoflt. De septembre a mars, les femelles sont depourvues d'oostegites et traversent une penode de repos. Elles ne produisent pas leurs petits de facon continue durant la saison, mais perdent leurs oostegites apres chaque portee. La duree maximale de survie est d'un an. En aout, la population se compose presqu'entierement de jeunes de l'annee; certains d'entre eux se reproduisent meme avant la fin de l'et6. Les oeufs sont petits et le taux de feconditd relativement elevS. Introduction Jaera syei is considered by some investigators to Some aspects of the biology of a population of be only a subspecies of J. albifrons (Harvey and Jaera ischiosetosa from the northwestern Atlantic Naylor 1968). All six have been reported from were studied from 1968 to 1971 as part of a eastern North America by Bocquet and Prunus larger comparative study of the species of this (1963). genus. Materials and Methods Isopods of the genus Jaera are abundant under The population of Jaera ischiosetosa studied inhabits rocks and algae in the intertidal zone in the the area inside the cobble bar at the mouth of the brook northwestern Atlantic. They have usually been in the south arm at Holyrood, Conception Bay. It was identified as either Jaera marina or Jaera albi- collected together with Gammarus duebeni and the re­ frons and considered to be identical by Bate and marks pertaining to the collections of that species (Steele and Steele 1969) also apply to Jaera ischiosetosa. The Westwood (1868) who listed them as Jaera albi- collections analyzed here were made between November frons. In the past 30 years, however, this species 1967 and October 1968, with additional ones in July 1966 was shown to be composed of a number of forms and May-June 1970. designated as races and subspecies by Forsman Only males can be distinguished with certainty as to (1944, 1949), and species of a superspecies of species, but since males of Jaera ischiosetosa only have been collected inside the bar at Holyrood, it is assumed Jaera marina by Bocquet (1953), who employed a that all of the females collected there were of this species. trinomial nomenclature. Naylor and Haahtela Specimens were measured, to the nearest 0.1 mm using (1966), however, concluded they were sympatric an eyepiece micrometer, from the anterior edge of the species and recommended the use of the binomial head to the distal end of the telson. Sex was determined system, and this practice is followed here. The i species are known as Jaera albifrons? ischiose­ TABLE 1 tosa, praehirsuta, posthirsuta, forsmani, and syei. Jaera ischiosetosa: size classes of oogonia and oocytes iStudies in Biology from Memorial University of New­ Mean size range, foundland, No. 293. Class mm Description 2Contribution No. 96 from the Marine Sciences Re­ search Laboratory. 1 (oogonia) 0.079 Colorless 3 Assuming Bate and Westwood (1868) were correct, the 2 (oocytes) 0.080-0.159 Colorless designation Jaera marina should have priority, but 3 (oocytes) 0.160-0.239 Greenish modern usage employs Jaera albifrons almost entirely. 4 (oocytes) 0.240- Light green 8 0\ o January F MAM.i ) A <3 O N December > <? 9 c?9 cf d" .9 & 9 d- $|c 9 c9l cf 9 ? 9 9 cf 9 <? 0 10 '68 I9'68 16'68 c 1468 ,2-68J 7'66 126B 27 68 I? 68 14'67 667 z o 1966-67-68 ^ r^ < c '< 4 > 2.0 r 1 '? 1.0 © © @ @ ® & — ^< © o >fl 5.0 E N E 19 th 6th l.t O 4.0 *- •i FEMALES WITH oostegites o 1970 „ - O z • FEMAIES WITHOUT " 3.0 1U 4 s _l ^ ,< 2.0 Q y J ii o O I 8 12 6 12 S i n 12 i S 12 S < o1 1.0 "PI , I.I. Ml, l I I ©i i ©, ©, 1 , . 1 1 1 1 1 1 1 1 i 1 i 1 i 1 > 1 . 1 i 1 i 1 1 1 1 1 1 1 i i i 1 i i i r FIG. 1. Size compositions of /aera ischiosetosa collections. The numbers of specimens examined are shown in circles. STEELE AND STEELE: BIOLOGY OF JAERA SPP. 1. 207 ID mmMffiSm to at 9 o V) § ar­ I— ea zo y O UJ UJ 8 a O •s «I O t— • if O to <o < z iotf o o r- 208 CANADIAN JOURNAL OF ZOOLOGY. VOL. 50, 1972 by the shape of the operculum (Haahtela 1965). The served in 5% formalin. The mean size of the females were further classified according to the presence sample was calculated to be 0.254 mm (range = or absence of oostegites and the stage of the embryos in the brood pouch. The stage of the embryos was deter­ 0.230-0.270, S.E. = 0.002). The living embryos mined according to the criteria in Fig. 4. The ovaries were of similar size (Fig. 4). were then removed and the oocytes classed by size accord­ Embryonic development has been described by ing to the scale of Table 1. Those in which the oogonia MacMurrich (1895) and Forsman (1944). The were too small to be seen with a dissecting microscope were placed in class 1. stages used in this study are illustrated and briefly described in Fig. 4. The embryos increase Results in size until hatching. Size at Maturity The newly hatched individual retains a large Because of the fact that in each sample there amount of yolk and while it flexes and straightens were always some females without oostegites, its body it is apparently incapable of locomotion. often of large size, it was impossible to determine It undergoes one molt in the brood pouch. At the onset of maturity from field collections alone. this time the yolk is largely resorbed but still However, since the smallest ovigerous females present in small amounts. Although it is now were 2.0 mm in length, it has been assumed that highly mobile and active it still lacks the last pair females of this size and larger were mature in the of legs and for this reason was termed a 'mancas- subsequent analysis. tadium' by Forsman (1944). Embryonic Development Life Cycle and Reproductive Cycle of the Female The sizes of the embryos were determined by The size compositions of the collections and measuring the dimensions (length '+ width/2) of the reproductive stages of the females are given 20 stage A embryos collected in July and pre­ in Figs. 1 and 2, respectively. From these and 1 I T" - .„_, —i r 30 F == 12.60x-27.92 . A EMBRYOS -Stage A /, / 25 # /8\ A F = 12-24 > - 25-73 EMBRYOS- Hatchedj 20 r?4 rL A /'A 15 yC F = 6.45L -289 y y OOCONI/ 10 - /& - A ... i i 3 4 BODY LENGTH - mm FIG. 3. Numbers of oogonia and embryos per female. STEELE AND STEELE: BIOLOGY OF JAERA SPP 1. 209 Stage | Mean size, mm Range S.E. No. Description 0.259 0.240-0.270 0.002 20 First cleavage stages and early develop­ ment B 0.308 0.250-0.380 0.006 23 Green yolk internal, rudiments of ap­ pendages at periph­ ery 0.344 0.323-0.380 0.005 12 Body in the form of a'C Head visible 0.340 0.314-0.380 0.003 10 Head well devel­ oped. Tail reaches front of head Tip of telson over­ laps head. Eye visible 0.584 0.561-0.591 0.005 18 Embryo hatches. It contracts but is un­ able to walk. Yolk prominent 0.780 0.759-0.825 0.004 10 Young molts and is now very mobile. A small amount of yolk is still visible. This is the 'man- castadium' of Fore­ man (1944) FIG. 4. Jaera ischiosetosa: stages and sizes of living embryos. 210 CANADIAN JOURNAL OF ZOOLOGY. VOL. 50, 1972 laboratory observations it is possible to deter­ Summary and Discussion mine the major features of the reproductive cycle. Jaera ischiosetosa is a relatively small animal The oogonia of the females begin to enlarge in with small eggs and many of its characteristics April but embryos are not found in the brood are related to these properties. Its rapid embry­ pouch until 'May and do not hatch until June.
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