Contr, Mo..?? M arin e B iologies! L ab o rato ry H singer D anm ark

O PH ELIA, 12: 117-127 (December 1973) Insfifuü! voer Z e e ^ r c ^ i p e lijk onderzoek kclh Lï i'.:... ; í ;:3ardi î i.n ses. us o... i í.!. í.n 69 841 B re d e n e - B elgium - 7 el. 59 / 8 37 15 SEASONAL AND DIEL VARIATION IN THE RHYTHMICITY OF IDOTEA BALTICA (PALLAS) AND IDOTEA GRANULOSA RATHKE. V lg G O H R L Y C K M arine Biological Laboratory, D K -3 Helsingor, D enm ark A B S T R A C T T he swim m ing activity of Idotea baltica and Idotea granulosa was investigated by m eans of photocells. T he activity patterns o f both species showed great seasonal variations. Both species show ed no sw im m ing activity in w inter (Idotea baltica from N ovem ber-m arch, and Idotea granulosa from Septem ber-m arch). The rest o f the year both species had one activity peak between m idnight and sunrise. Idotea baltica was nocturnal in early spring, in sum m er the activity was high all 24 hours, besides the nocturnal m axim um an increased activity was evident in the afternoon till two hours after SS. A t the end o f Septem ber and the beginning o f October I. baltica was nocturnal. T he same variation in the activity pattern was found for I. granulosa. U nder artificial light/dark conditions both species were nocturnal. W hen the light/dark change was right-angled this was the dom inating Zeitgeber. I N T R O D U C T I O N T he ecology o f Idotea baltica (Pallas) and Idotea granulosa R a th k e has been investigated by a. o. Syw ula (1964), N aylor (1955), M uiis (1967), an d Jansson (1971). A ccording to these au th o rs I. baltica is in D en m ark found in localities w ith continuous renew al o f w ater an d I. granulosa in m ore exposed localities w ith stro n g cu rren ts or waves. I. baltica an d I. granulosa are the m ost com m on species o f the genus Idotea along the co ast n o rth o f H elsingor. T hey can here be fo und living together a t the sam e locality an d it could thus be interesting to see if I. baltica an d I. granulosa had different p attern s o f activity w ith special reference to seasonal variations, thus avoiding com petition w ith each o th er regard in g food an d space. Contr, Mo..?? M arin e B iologies! L ab o rato ry H singer D anm ark

SS SR SS SR SS SR 24 24 24 i APR AUG NOVII FE 2 245-1 + 3 MAY AUG II NOV MAR 2 1 (148) (118) MAY AU DEC I MAR II (73) - 1 + 3 MAY SEP DEC II 'mar ) (23) 13-1 + 3 JUN I SEP II 15) (19) A U p - * DEC II ^APR +3 JUN SEP III JAN I APR - 239 (15) 1-1 O ' + 3 JUN OKT I JAN II APR 441 (8) - 1 +3-1 - (183) OKT JAN III MAY I - 1 + 3 JU - +3 - OKT FEB MAY II (286) ( 6 ) NOV FEB II MAY III ( 2 2 ) (49) V - 1 F ig. 1. Diel activity pattern o f Idotea baltica in natural light conditions, given as percentage deviation from the m ean value of the registration period. Each group averages 1 days. N um bers within brackets show the m ean num ber o f counts/24 hours.

+3 2 SR SS SR SS SR SS 24 APR AUG NOV ( 16) i FEB (3) I8 24 - +3 2 MAY (16) AUG (35 NOV MAR I (4) -1 +3/ MAY II AUG DEC MAR - +3 MAY III (14) - O Á SEP DEC MAR +3Ô 2 JUN (4) SEP DEC () APR (23) - /. + 3 O - JUN V7 (19) SEP JAN APR II +3 JUN III ( 2 ) OKT I JAN APR III (14) O -J +3' JUL OKT JAN III MAY I O -1 +3' JUL II (26) OKT FEB MAY II ( 12) O - N/ +3Ö' Ájuli (27) NOV FEB MAY (24) 1 O? ^ - 1 Fig. 2. Diel activity p attern of Idotea granulosa in n atural light conditions, given as percentage deviation from the m ean value of the registration period. Each group averages 1 days. N um bers in brackets show the m ean num ber o f counts/24 hours.

122 VIGGO H R LY C K 6 1 w 4 MAY JUL AUG SEP OKT 6 4 ü 2 NOV DEC JAN FE MAR APR F ig. 3. The num ber o f registrations per 24 hours for Idotea baltica in natural light conditions. Triangels below horizontal line indicate days o f feeding with M ytilus flesh. D ots above horizontal line indicate interruptions in the registration, i.e. no total value for all 24 hours. A C T I V I T Y P A T T E R N S I N A R T I F I C I A L L I G H T T o test th e light/dark change as an external factor controlling the diel rhythm, and the influence o f varying daylengths, th e anim als w ere kept in a dark-cham ber an d lighted by a fluorescent tu b e (5 Lux). T h e artificial day was gradually prolonged tw o hours every w eek untill L L an d then shortened tw o hours weekly untill D D. T he results are show n in Figs 5 & 6. Idotea baltica w as cau g h t an d p u t u n d er these experim ental conditions in M ay 1971 (nld was 16:8). In artificial L D I. baltica was n o ctu rn al except w hen the L -period o r the D -p erio d w as extrem ely long. (Fig. 5). W hen I. baltica h ad a light p erio d longer th a n 16 hours it show ed som e activity during daytim e a n d th e to ta l am o u n t o f activity fell. W hen the L-period

VARIATION IN RH Y TM IC ITY OF IDOTEA 123 15 C iliil n 1---------- ITT- AUG SEP OKT NOV DEC JAN 1971 1972 FE MAR APR Fig. 4. T he num ber o f registrations per 24 hours for Idotea granulosa in natural light conditions. Triangels below horizontal line indicate days of feeding with M ytilus flesh. D ots above horizontal line indicate interruptions in the regisstration, i.e. no total value for all 24 hours. was sh o rter th a n 16 hours (the activity o f I. baltica began a t the artificial SS. W h en the D -p erio d was longer th a n 8 h o u rs m ore th a n one activity p eak appeared, i.e. a typ ical bigem inus rhythm. T he activity p attern under D D w ith a p eak aro u n d 2. hrs an d 2. hrs were fo und only during th e first 3 days. A fter this n o rhythm ic activity occurred. T h e experim ent w ith I. granulosa w as sta rted in O ctober 1971 (nl D 12:12). I. granulosa show ed the sam e diel activity p a tte rn (Fig. 6) as I. baltica except th a t the activity o f I. granulosa never started rig h t after th e artificial SS, as there w as alw ays a delay before onset o f sw im m ing. T o test th e possibility o f controlling the diel rh y th m o f b o th I. baltica an d I. granulosa the follow ing experim ent w as perform ed in the dark-cham ber. The artificial d ay /n ig h t was set to be opposite to the real day/night (L D 12:12).- U n d e r these conditions b o th 7. baltica an d I. granulosa follow ed th e artificial d ay /n ig h t rh ythm, i.e. the active period occurred during th e artificial night, also w hen the light intensity a t artificial daytim e was very low (ca. 3 Lux) Figs. 7 an d 8. B oth species follow ed the artificial night, indicating th a t th e Z eitgeber w hich sta rted w orking a t the light/dark change was very sensitive, i.e. exerted a strong influence on th e activity o f the anim als.

124 VIGGO H R LY C K 12 18 24 +3 - +3 r LD 16:8 LD 23:1 18) * LD 18:6 LD 22:2 W s i / L D 4 :2 2 J LD 2:22 I 52) _ LD 2:4 I (38) $ L A I LD 2:4 r u W V ^ LD 1:23 55) ^ Jk* LD 22: LD 18:6 (185) Me _ DD (19) LD 16:8 UJ g 123ÖJ J ê FlG. 5. Diel activity pattern of Idotea baltica in artificial light/dark, given as percentage deviation from the m ean value o f the registration period; each group averages one week. N um bers in brackets show the mean num ber o f counts/24 hours. D I S C U S S I O N T he activity m easured in these experim ents was the sw im m ing activity. This m eans th a t an observation o f no activity does n o t exclude o th er activities such as feeding and preening. A s the light-beam is very n arro w it is, furtherm ore, only a p a rt o f the sw im m ing activity th a t is registered. A s I. granulosa is sm aller th a n I. baltica the probability th a t it passes though the n arro w light-beam is sm aller, a n d this m ay be one o f the reasons w hy the counts/24 h ou rs generally is low er th a n fo r I. blticaa. I. granulosa also has a longer period th a n I. baltica w ith no sw im m ing activity during the w inter. T h ere is a p ro b ab ility th a t this is due to the sam e reason as m entioned above. In n L D b o th I. baltica and I. granulosa show ed a ra th e r varying annual activity p attern. T his w as n o t found w hen the anim als w ere k ep t under

VARIATION IN R HY TM ICITY OF IDOTEA 125 +3 - +3-1 12 1 8 24 +3 ' T LD 16:8 «- +3 - +3'; -1 J LD 12:12 (39) LD 14:1 (7) s i à % B LD 18:6 I i» I LD 2:4 ( 2 ) I 12 18 24 6 12 18 24 i i i i LD 22 :2 ( 1 ) LL ( 2 ) LD 23:1 (3) LD 18:6 (23) 1 LD 16:8 K K 1 J LD 14:1 (17) i H\ / LD 12.-12 (37) LD 2.-4 LD 1:1 (33) J A 6 12 18 24 a L: 8:16 (26) LD 6: (14) y 8 A Ivl / V " LD 4 :2 F ig. 6. Diel activity pattern o f Idotea granulosa in artificial light/dark, given as percentage deviation from the m ean value of the registration period. Each group averages one week. N um bers in brackets show the m ean num ber o f counts/24 hours. artificial lig h t/d ark conditions. In artificial lig h t/d ark b o th I. baltica an d I. granulosa w ere n o ctu rn al and the decreasing activity, w hen the light p erio d was less th a n 8 hours (/. baltica) or 12 hours (/. granulosa), w as m issing. M oreover the L D -experim ent w ith I. granulosa was p erform ed during the w inter period. I f the low er tem perature in w inter was an im pulse fo r n o activity one should expect n o activity. O n th e co n trary the activity level w as q uite high. T h e difference betw een the L D an d the n L D experim ents is th a t th e light/ d a rk change is right-angled an d the light intensity was co n stan t d uring the light period. In th e nl D -experim ent there is tw ilight betw een n ig h t a n d day an d the m ean light intensity is low er in w inter th a n in sum m er. A s the experim ent was p erfo rm ed in d o o r this slow change betw een light a n d d ark w as prolonged. This show th a t w hen th e lig h t/d ark change is right-angled this sudden change is the dom inating Zeitgeber.

126 VIGGO H R LY C K + 3 - - F ig. 7. Diel activity pattern o f Idotea baltica given as percentage deviation from the m ean value o f the whole registration period in artifical light/dark (LD 12:12). Light intensitity during the artificial day was 3 Lux. A ccording to Jansson and K ällan d er (1968) I. baltica starts the activity period w hen the light intensity falls below one Lux, an d the p erio d o f inactivity begins a t 4-8 Lux. T his was n o t fo u n d in this experim ent. Even w hen the aq u aria w ere lighted by ex tra light d uring daytim e, the anim als show ed the sam e activity p attern as in n L D, i.e. they show ed activity during daytim e. T h e experim ents perform ed w ith low light intensity (Figs. 7 an d 8) also show th a t the anim als are able to rea ct u p o n very low changes in light intensities. The phase setting o f the rh y th m seems m ore to depend u p o n changes in light intensity th a n on absolute values. T he w ay o f light change (ab ru p t o r slow) also seems to be o f great im portance. A sim ilar phenom enon has been discussed fo r various species by H a rk e r (1964). W hen the activity period lasts m ore th a n 6 hours, the activity p attern s for b o th I. baltica an d I. granulosa are o f bigem inus or trigem inus type dependent u pon the length o f the activity period. A t shorter activity periods only one m axim um appears. T his type o f activity p attern is also show n by th e brackish-w ater shrim p Crangon vulgaris (H agerm an, 197) and by th e freshw ater am phipod Gammarus p u le x (M üller, 1966). As a supplem ent to the la b o ra to ry studies the species have been observed by diving in daytim e. T hey never sw am up in to the free w aters b u t w ere only seen sw im m ing fro m p la n t to p la n t, an d this w as also the situ atio n in the aquaria in the labo rato ry. G enerally th e tw o species I. baltica an d I. granulosa show n o significant differences in their diel rhythm s. A s to th e variations during th e year, /. granulosa show s a longer period o f no activity in w intertim e than I. baltica. T here seems to be no difference in the activity p attern s o f the adult anim als o f these species w hich m ay hin d er com petition fo r food and space.

VARIATION IN RH Y TM IC ITY OF IDOTEA 127 6 12 18 24 + 3- F i g. 8. Diel activity pattern o f Idotea granulosa given as percentage deviation from th e m ean value o f the whole registration period in artificial light/dark (L D 12:12). L ight intensity during the artificial day was 3 Lux. - - A s h o f f, J. (Ed.) 1965. Circadian clocks. N o rth H olland Publishing Com pany, Amsterdam. 479 pp. H a g e r m a n, L. 1 9 7. Locom otory activity patterns o f Crangon vulgaris (Fabricius) (Crustacea, N atantia). Ophelia 8: 255-266. H a r k e r, J.E. 1964. T he physiology o f diurnal rhythm s. University press, Cambridge, 114 pp. J a n s s o n, B.,-. & C. K ä l l a n d e r. 1968. On the diurnal activity o f some littoral peracarid crustaceans in the Baltic Sea. J. exp. M ar. Biol. Ecol., 2: 24-36. M ü l l e r, K. 1966. D ie Tagesperiodik von Fliesswasserorganismen. Z. M o rp h.ö kol.tiere., 56: 93-142. M uus, B.J. 1967. T he fauna of Danish estuaries and lagoons. M eddr D anm.f isk. og Havunders., N.S., 5: 1-316. N a y l o r, E. 1955. T he ecological distribution o f B ritish species o f Idotea (Isopoda). J.anim. Ecol., 24: 255-269. R E F E R E N C E S S y w u l a, T. 1964. A study on the taxonom y, ecology and geographical distribution o f species o f genus Idotea Fabricius (Isopoda, Crustacea) in Polish Baltic. II, Ecological and zoogeographical p art. Bull.Soc. Sei. Lettr.Poznan, Ser. D, 4: 173-.

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