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Distribution, Morphometry, and Seasonal Biology of the Planktonic Copepods Neocalanus Robustior and Neocalanus Gracilis

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Distribution, Morphometry, and Seasonal Biology of the Planktonic Copepods Neocalanus Robustior and Neocalanus Gracilis Pacific Science (1976), Vol. 30, No.2, p. 119-130 Printed in Great Britain Distribution, Morphometry, and Seasonal Biology of the Planktonic Copepods Neocalanus robustior and Neocalanus gracilis in the Pacific Ocean I MICHAEL M. MULLIN2 and PAMELA M. EVANS3 EXAMINATION of the comparative ecology of first pair of swimming legs. Bradford and closely related species of oceanic, subtropical Jillett (1974) have proposed that Calanuscrista­ zooplankton may give some insight into the tus, C. plumchrus, and C. tonsus be included in division of resources between potential com­ th e genus Neocalanus, although these species petitors in an environment of relativ ely high have relatively shorter antennae and lack the physical stability and hom ogen eity, where hook on the first pair of legs. The female N. finely divided, dispersed particulate matter robustior is distinguished from N. gracilis by th e constitutes the potential food for many species. former's ventrally swollen genital segment and The pre sent study is similar to that of Mullin larger bod y size; in the males, the endopo dite (1969) in that it discu sses the geographical and of the left fifth swimming leg of N . robustior is vertical dist ributions, sizes of body and mouth­ considerably reduced, while that of N . gracilis parts, and breeding seasons of two congeneric, is only sligh tly so (cf. Brodsky 1967b), and particle-grazing, calanoid copepods that are N . robustior is again the larger. Copepodite sympatric in tropical and subtropical waters. stages of the two species may be distinguished In the previou s study, Calanus tenuicornis was by size (see below). Our specimens of both found to have a broader geographical range species have a dorsal projection at the posterior than its larger congener, C. lighti, and to live border of the head, but this character is more in deeper water , at least during the autumn; pronounced in N . robustior. no eviden ce was found for character displace­ Both species have been reported from the ment in size of mouthparts or for seasonal Pacific (Grice 1962, and references th erein), separation of reproduction in the Central N orth Atlantic (references in Vervoort 1946), and Pacific. Indian (Sewell 1947, and references therein) T he present study conce rns another pair of oceans. They are among the 20 numerically sibling species, Neocalanus robustior(Geisbrecht) most important species of large cop epods in and N . gracilis (Dana), that po ssess elongate the Central Gyre of the North Pacific (Morris first antennae arid that differ from each other 1970; McGowan, personal communication). mainly in size. These species have often been placed in th e genus Calanus (sensu lato) (e.g., Zoogeographic Distributionin the Pacific recently, Bowman 1955, Brodsky 1967a) but are distinguished from other members of that Plankton collections at the Scripps Ins titu­ gen us by a stron g, hooked spine on the anterior tion of Oceanography were sub sampled with a surface of th e second basipodal segm ent of the plankton splitter, and the copepodite stages III, IV, V, and VI (adults) of the two species I Research was supported by the United States were counted in aliquo ts. The samples were Atom ic Energy Commission, contract no. AT(11- 1) taken during the following expeditions: Shell­ GEN 10, P.A. 20. Several other sources pro vided ship time. Contribution from the Scripps Ins titution of back, May, June, and August 1952; Capricorn, Oceanography. Manuscript received 4 May 1975. D ecemb er-February 1952-1953; T ransp ac, 2 University of California at San Diego, In stitute of Sept ember-November 1953; N orp ac, July­ Marine Resources, Post Office Box 1529, La Jolla, September 1955 ; Troll, March- April 1955; California 92093. E quapac, August- September 1956; D own­ 3 University of California at San Diego, Institute of Marine Resources, Post Office Box 1529, La Jolla, wind, October-February 1957-1958; Step I, California 92093. N ovember 1960; Tethys, June-August 1960; 119 120 PACIFIC SCIEN CE, Volume 30, Ap ril 1976 No C adults Ne ocalon us robustior 1m' , ill - o Samplu from < 250 ml 'e,. 1- 20 2 1- 2 0 0 • Sampl es Irom 250 - 6 0 0 m. > 2 00 F IGURE 1. Distribu tion of Ne ocalanus robustior in the Pacific Ocean. Tows were taken from the surface to the dept h indicated by open or filled circles. Contour interv als were chosen arbitrarily, based on breaks in the data on abundances. Monsoon, March-April 1960-1961 ; Ursa Ma­ Neocalanus robustior is most abu ndant in the jor, September 1964; Zetes, January 1966; and eastern Cent ral N orth Pacific (Figure 1); it Scorpio, March- May 1967. Mo st of the samples occurs only occasionally north of 35° N, and is were taken by oblique tows to various dept hs, rare or absent in the eastern tropical Pacific, th e generally with a net of 500-550 fl mesh netting Peru-Chile Current, and the western South and 1 m mouth diameter, with a flowmeter Pacific. The distribution of N . gracilis (Figure mounted in the mou th. All tows fished to at 2) is similar, both species being wa rm-water least 100 m, and tows which had fished to at cosmopolites in the term inol ogy of McGowan least 250 rn were used wherever possible so (1971), but the range of N . gracilis extend s tha t th e dept hs of maximal abundance of th e farther to the north (see also Park 1968, table 2) species were reached (see later). D etails con­ and south, and this species is relatively more cerning dates and locations of samples may be common in the western South Pacific than is found in Snyder and Fleminger (1965). N . robustior. It sho uld be noted, however, that Planktonic Copepods-s-Mur.r.ncs ANDE VANS 121 Neoeolanus 9raeilis Nairn', C ill - adult s o Sampl.. h Olll o < 2&0 "'t't,. 1- 20 • Sam pl.. fr om 21 - Z OO 2 50 - 6 00 m. > 2 0 0 F IGURE 2. Distribution of Neocalanus gracilis in the Pacific Ocean. To,;s v:ere taken from the sur face t? the depth ind icated by open or filled circles. Contour intervals were chosen arbitrarily and were based on breaks In the data on abundances. Morris (1970) reported both species as far Morphometry north as 440 N in a transect along 1600 W (riot As discussed previously (Mullin 1969, and 1600 E as reported in the title). Hei nrich (1969) references cited therein), the differences in body l e ~ s reported that both species were .much size of sympatric congeners could indicate abundant in the western South Pacific than In differences between the species in selection of the western North Pacific. sizes of particles of food, or could facili­ These conclusions may be biased, of course, tate reproductive isolation of the species, or by the season or year in which each area both. Therefore, cephalothoracic (= prosomal) was sampled, if the species have pronounc:d lengths of at least 10 individ uals of each de­ seasonal cycles or year-to-year changes In velopmental stage of Neocaianus were measured abundance. Data concerning the abundances by means of an ocular micrometer, the ani~als of the tw o species in the eastern Central having been taken from stations representlng North Pacific in various months are given particular zoogeographical areas. Since sizes below. 122 PACIFIC SCIENCE, Volume 30, April 1976 0 .96 t 60 50 40 1.9 6 ~ 30 2 .5 8 20 C I 1 CV R'1. 2 5 R ·1.31 10 1.0 2 .0 3.0 0 .96 1.11 30 'U ~ ~ Q) 2 0 C II 2 .6 6 0 =' en t 0 10 Ql E 0 en 1.0 2 .0 3 .0 'U 0 3 .35 C. ~ Ql 30 1.23 1.50 C. 0 t ~ U C III 20 2 . 4 6 9 R ' 1.2 2 R. I.3 6 0 t -... 10 Ql .0 0 E 1.0 2.0 3 .0 ::l 1.9 6 Z + 30 1.54 t 20 C IV R' 1.27 10 ot-- ,--,--....- --r-- ,---.------'lf&. o 0 .2 0 .4 0 .6 0 .8 1.0 1.2 1.4 1.6 1.8 2 .0 2 .2 2 .4 2.6 2 .8 3 . 0 3 .2 3 .4 3.6 3 .8 Cephalo th orax len gth, rnrn, FIGURE 3. Sizes ofcopepodite stages of Neocalanusgracilis and N . robustior from 280 N, 1550 W in August 1969. For each stage, the arrows and numbers abov e the histograms indicate the median cephaloth orax (pro somal) lengt h ofthe individuals assigned to each species, N . gracilisbeing the smaller in each case. The value ofR is the ratio of median lengths of the two species at that stage. No male N . gracilis was found at this particul ar station ; males of this species from other stations had a median cephalothorax length of 2.12 mrn, with a range from 1.94 to 2.31 rnm, which would result in R = 1.25 for males. of mouthparts might be more directly related Figure 3 shows the sizes of all copepodite to the sizes of food particles which can be in­ stages of both species taken from a single sta­ gested, a mandibular blade (gnathobase) was tion in the Central N orth Pacific. Copepodites removed from each animal and pressed flat of the two species may be clearly distinguished under a cover slip on a glass slide, and the by size at stage III. The size distributions of width of its molariform edge was measured by earlier stages, although overlapping, are obvi­ means ofan ocular micrometer.
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