205 The biology of spp. (Crustacea, ) 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 ? 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 > 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

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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

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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 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. onic development (unpublished observations) Breeding continues until August and some of the and growth to a small size at maturity enable it larger young of the year produce a brood at this to ripen its gonads as late as the spring, but still time. However, in September the oogonia are have the eggs hatch and the young mature and small and the females have all lost their ooste­ for some of them to breed before the next winter. gites and are in the resting stage, which continues A somewhat longer breeding season was re­ until the following April when the cycle begins ported for the Baltic region by Forsman (1944) again. .''. " and Jazdzewski (1969), but this may have been Some females (2 mm) lack oostegites and have due to the fact that their samples contained more small oogonia and therefore are classed as being than one species. Jaera albifrons is reported to in the resting stage even in the summer months. breed throughout the year by Naylore? al. (1961). Laboratory observations of individual females The occurrence of the resting stage following revealed that each female does not produce suc­ each brood is mentioned by Forsman (1944) and cessive broods. Instead the oogonia fail to en­ is demonstrated in the figures of Jazdzewski large when the brood is developing in the brood (1969). This stage is apparently obligatory and pouch and the female loses her oostegites in the therefore comparable to that of Gammarus setosus subsequent molt. Later the oogonia may enlarge (Steele 1967). The small size of the eggs is cor­ and in the subsequent molt the female regains related with a relatively high fecundity, although her oostegites and another brood of embryos is the embryos hatch in an incomplete state and do deposited in the brood pouch. not acquire the adult form until after several Individuals survive at the most for 1 year. molts. Somewhat smaller numbers of embryos 1 After breeding in the spring and summer the were reported for females of Jaera spp. in the adults disappear, so that in August the popula­ Baltic by Jazdzewski (1969). tion consists almost entirely of young . This serves as the breeding population of the Acknowledgments following spring. This study was supported by the National Males are always less abundant than females Research Council of Canada and the National in the collections and this is particularly true in Museum of Canada. Technical assistance was the spring and Summer months, suggesting that provided by Miss C. Long and photographic they die before the females. The females store assistance by Mr. R. Ficken. sperm in receptacles and do not need to mate before every brood. Thus the presence of males BATE, C. P., and J. O. WESTWOOD. 1868. A history of the is not necessary for reproduction in the summer British sessile-eyed Crustacea. Vol. 2. John Van Voorst, London. months. Precopula behavior was observed in­ BOCQUET, C. 1953. Recherches sur le polymorphisme frequently. naturel des Jaera marina (Fabr.) (Isopodes Asellotes). Arch. Zool. Exp. Gen. 90: 187-450. Fecundity BOCQUET, C, and G. PRUNUS. 1963. Recherches comple- mentaires sur le polytypisme de la super-espece Jaera The numbers of enlarged oogonia, stage A albifrons Leach = Jaera marina (Fabricius). Bull. Biol. embryos, and hatched embryos were counted Fr.Belg. 97(2): 343-353. and regression lines fitted by least squares (Fig. FORSMAN, B. 1944. Beobachtungen tiber Jaera albifrons Leach an der schwedischen Westkuste. Ark. Zool. 3). The line for the oogonia has been shifted 35A(11): 1-33. 0.5 mm to the right to account for the increase 1949. Weitere Studien iiber die Rassen von Jaera in size of the females when the brood is released. albifrons Leach. Zool. Bidrag Uppsala, 27: 449-463. HAAHTELA, I. 1965. Morphology, habitats and distribu­ The regression lines of the embryos have steeper tion of species of the Jaera albifrons group (Isopoda, slopes than those of the oogonia but this is prob­ ) in Finland. Ann. Zool. Fenn. 2: 309-314. ably due to the smaller size ranges and smaller HARVEY, C. E., and E. NAYLOR. 1968. The taxonomic status of Jaera albifrons syei Bocquet (Isopoda, Asel- numbers analyzed. lota). Crustaceana, 14(2): 185-193. STEELE AND STEELE: BIOLOGY OF JAERA SPP. 1. 211

JAZDZEWSKI, K. 1969. Ecology and biology of the Jaera NAYLOR, E., D. J. SLINN, and G. M. SPOONER. 1961. albifrons group (Isopoda Asellota) in the Bay of Puck, Observations on the British species of Jaera (Isopoda: Polish Baltic Sea. Crustaceana, 17(3): 265-281. Asellota). J. Mar. Biol. Ass. U.K. 41: 817-828. MACMURRICH, J. P. 1895. Embryology of the isopod STEELE, D. H., and V. J. STEELE. 1969. The biology of Crustacea. J. Morphol. 11: 63-146. Gammarus (Crustacea, Amphipoda) in the north­ NAYLOR, E., and I. HAAHTELA. 1966. Habitat prefer­ western Atlantic. 1. Gammarus duebeni Lillj. Can. J. ences and interspersion of species within the super- Zool. 47(2): 235-244. species Jaera albifrons Leach (Crustacea: Isopoda). STEELE, V. J. 1967. Resting stage in the reproductive J. Anim. Ecol. 35: 209-216. cycles of Gammarus. Nature (London), 214: 1034.