Pacific Science (1976), Vol. 30, No.2, p. 119-130 Printed in Great Britain

Distribution, Morphometry, and Seasonal Biology of the Planktonic 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, 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 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. ously bimodal and permit most individu als to Planktonic Copepods-MuLLIN AND EVANS 123

be assigned to one species or the other with the Central South Pacific than in th e N orth reasonable certainty. Although the median Pacific. In the northern part of the North size of any particular stage varies somewhat Pacific wh ere it occurs allopatrically, N . between stations and seasons , the bimodality gracilis is larger in bod y size, and is still larger always (in our experience) permits separation in its area of allopatry in the southern South of th e species in all copepodite stages. It is Pacific (Table 1). This situation superficially interesting that the size of anyone copepo dite resembles that found in Calanus lighti and C. stage of N . robustior is in most cases very close tenuicornis, in that the body size of the smaller to the size of the next older stage of N . gracilis, species (c. tenuicornis and N . gracilis) increases prior to adulthood. in areas of cooler surface water where its larger Hutchinson (1959) suggested that a ratio of congener does not occur . Indeed, the lower at least 1.3: 1 in the sizes of food-gathering mandibular: cephalot horacic rati o of the N . structures of congeneric species permitted co­ gracilis from 40°-43 ° S parallels th e trend shown existence in separate trophic nich es. The rati o of by the " intermediate form" of Calanus tenui­ cephalothoracic lengths of the two species at cornis (Mullin 1969), a geographic isolate in the anyone stage exceeds 1.3 only in the copepo­ southern South Pacific that shows appa rent dite stage Vs and adult females. Asin the sibling morphometric convergence toward the larger pair Calanus lighti and C. tenuicornis (Mullin C. lighti. In the case of Neocalanusgracilis, how­ 1969), the width s of the mandibular blades are ever, the mandibular :cephalot horacic ratio in more similar between the two species of Neo­ the southern South Pacific does not distinguish calanus in any developmental stage than are the it from the populations in other regions since cephalothoracic lengths; th e smaller N . gracilis this ratio decreases with increasing cephalo­ in the Central North Pacific has a mandibular thoracic length th rou ghout the range of the width: cephalothoracic length ratio of 0.072­ species (Table 1). (The hypothesis that a 0.073 (median values for 26 females and 27 common regression line for all populations cop epepodite Vs), while th e larger N . robustior adequately describes the relationship of man­ has a ratio of 0.068-0.069 (medians for 28 fe­ dibular width to cephalothoracic length was males and 26 copepodite Vs). H ence, even for acceptable; P ~ 0.10.) adult females, the ratio of mandibular widths of the two species is slightly less than 1.3 :1. Seasonality in the Eastern Central N orth Pacific If the two species of N eocalanus have been In temperate waters, congeneric species under competitive pressure to diverge in the often occur or at least have maximal abun­ sizes of food particles they are capable of in­ dances in different seasons, so that a temporal gesting, this pressure is not reflected as char­ separation of niches results. In subtropical acter displacement in size of the mandibular oceans, where seasonal variation in ph ysical blades. parameters is less extreme, temporal separation For each species, the mandibular: cephalo­ of nich es might be less important. In order to thoracic ratios of the copepo dite stages II, III, examine this question, we counted of and IV were very similar to the ratios found for copepodite stage IV and older that had been copepodite stage V and for female individuals caught in net tows taken from th e surface to at of the same species. In the first copepodite least 250 m in the area 125°-160° W, 20°_ stage (copepodite I) of each species, the ratio is 35° N , and plotted th e counts with respect to considerabl y less because the outermost cusp date, irrespective of year (Figure 4). Some of the mandibular blade has not yet developed. indi viduals, especially N . gracilis, may occur A small number of measur ements (Table 1) deeper than the maximum depth of some of the indicate that N . robustior is smaller in bod y tows, but any seasonal cycle of abundance size in the Central South Pacific than in the should be reflected inasmuch as the bulk of the N orth Pacific (P < 0.01 by rank sum test that population was sampled, and inasmuch as the the medians are the same as those given in copepodite stages counted should have been Figure 3). This is not true for N .gracilis, so the retained quantitatively by the meshes of the difference in size between the species is less in different nets used . 124 PACIFIC SCIENCE, Volume 30, April 1976

TABLE 1

CEPHALOTHORACIC L ENGTH AND MANDIBULAR WIDTH: CEPHALOTHORACICLE NGTH, BYARE A, OF N eocalanus robustior AND N . gracilis

MANDIBULAR WIDTH: CEPHALOTHORACIC LENGTH (m m) CEPHALOTHORACIC LENGTH

Neocalanus robustior Ne ocalanusgracilis Neocalanus robustior Neocalanus gracilis

COPEPODID COPEPODID COPEPODID COPEPODID AREA FEMALE V FEMALE V FEMALE V FEMALE V

12°- 16° S, 112°-135° W Median 3.21 2.48 2.50 2.00 0.069 0.069 0.D75 0.072 N 13 18 29 20 34°- 39° N, 125°-173° W Median * * 2.81 2.24 * * 0.069 0.067 N * * 12 15 * * 40°- 43° S, 132°-165° W Median * * 3.26 2.53 * * 0.064 0.064 N * * 24 22 * *

N OTE: 2-4 stations were pooled in each zoogeographic area. Compare with Figure 3 an d text concerning speci­ mens from the Central North P acific. * No o ccurren ces in samples examined.

The samples from the eastern Central North Clelland 1957: 67-69). If the species tend to be Pacific that were used to prepare Figures 1 and abundant in different seasons, years, or even 2 were taken in August; Figure 4 (which in­ locations, then a negative correlation might cludes data from these samples) supports the exist between their abundances ; in fact, the impression that N. robustior is the more abun­ correlation is significantly positive (P = 0.05). dant species in the fall and winter. Even during This statistical test is particularly sensitive to these seasons, however, N . gracilis is the more extremes, i.e., very high or very low abun­ common in a few samples. The population of dances. Therefore the observed correlation N. gracilis may reach its maximum somewhat could either be real (both species are abundant earlier in the year than does that of N. robustior, or rare at the same time s and places) or spurious but too few years were sampled in the same (e.g., if the volume of water filtered by a partic­ seasons to allow us to determine if this is a ular tow was grossly over- or underestimated, recurring pattern. D eevey (1971) reported the apparent abundances ofboth species will be that N. gracilis occurs year-round off Bermuda spuriously low or high). At the very least, how­ but in insufficient numbers for description of a ever, this result is inconsistent with spatial or seasonal cycle. temporal distinctness of the two species within The median abundances for all samples were the eastern Central North Pacific. 88 N . robustior and 40 N . gracilis per m2 ; these Even though the late copepodite stages do medians partly reflect the uneven distribution not appear to be separated in the Central North of samples throughout the year. Since year-to­ Pacific either areally or temporarily, the spec­ year and location-to-Iocation variability is ies might be long-lived and breed in different suppressed in Figure 4, all of the 111 pairs of seasons so that the younger developmental values for abundances of the two species were stages would occur at different times. Males of plotted as a scatter diagram and examined for both species have reduced mandibular blades degree of association by a corner test (Tate and and, because of starvation, probably do not Planktonic Copepods-MuLLIN AND EVANS 125

655 500

400 11 robustio r

3 0 0

200

N 10 0 E

Q) 0 o, J F M A s o N o 0::: W 400 rn N. gr aci lis 2 =:J Z 300

200

10 0

0 J F D

FIGUR E 4. Abundances of Neocalanus robustior and N . gracilis, copep odite IV to adult, in the eastern Central North Pacific.

survive adulthood nearly as long as do the V ertical Distribution females. Hence, a distinct breeding season Both species are known to occur predomin­ should be detectable as an increase in th e ratio antly in the upper few hundred meters, but of males to females. Figure 5 shows the mean findin gs have differed as to the precise vertical sex ratio for each half-month period in the distributions of the two species. Vervoort eastern Central North Pacific; alth ough there is (1946) reported N. gracilis to be found from the considerable variation, there is no clearly de­ surface to 1000 m, bein g most common at fined seasonality. The median ratios of males to middepths, and that it does not unde rtake a females are 0.17 and 0.14 for N . robustior diurnal vertical migration. N. robustior was and N . gracilis, respectively. A corner test found mainly near the surface, and Vervoort on the sex ratios in all samples indicated no disagreed with an earlier report (Rose 1933) of correlation between the ratios . Thus, there diurnal vertical migration in this species. is no evidence that the breeding activities of Vinozradov (1968) reported that N . gracilis the two species are separate in location or was ~ost abundant in the upper 200 m in the time. equatorial Pacific, but that the vertical range extends to 500-1000 m in the Pacific and even deeper in the Mediterranean. N . robustior also 126 PACIFIC SCIENCE, Volume 30, Ap ril 1976 0.5 · .. Ii: robustior 0.4 · .. 254 !§ • r-- 45 I- 0.3 r-- (f) 90 ~ ,.... 157 168 w r-- 0.2 • ~ ~ I- ...J 360 - 2~ 313 ~ 72- 9~ ~ ~

F I GURE 5. Sex ratios of Neocalanus robustior and N. gracilis in the eastern Central North Pacific. The number above each bar indicates the to tal number of adu lts counted ; absence of bar and number in dicates that no samples were taken in that half-month. apparently occurs deeper in the Mediterranean closing bongo nets (McGowan and Brown than in other seas. The N . gracilis populations 1966). Samples were taken on two transects in the Pacific Central Water Masses have been along 1550 W from Hawaii into the Alaska reported to migrate from approximately 300 m Gyre (cruises Zetes, January 1960; and Ursa in the daytime to approximately 50 m at nigh t Major, September 1964) with 505 It mesh nets, (Vinogradov 1968), the migration being over a and at single stations sampled both day and shorter distance in equatorial waters. Grice and night with 183 It mesh nets on the following Huls emann (1965) found both species occurring cruises : Climax II, September 1969; FCRG to a depth of 1000 m in the Atlantic. However, 71-2, November 1971; Cato I, June 1972; Roe (1972a) found N . robustior only in the C'Bog I,August 1972; Southtow XIII, upper 250 m day and night at a tropical At­ February 1973; Tasaday I, June 1973, lantic station; he found N . gracilis to 1000 m Tasaday XI, March 1974; and D ramamine and believed it to have a weakly developed II, May 1974. In all cases, subsamples equiv­ diurnal vertical migration. alent to at least 10m3 of water filtered were Vertical distribu tions of the two species counted. Only on Tasaday I were samples reported here are based on catches of opening- taken below 500 m. Planktonic Copepods-MuLLINAND E VANS 127

Table 2 summarizes results from Southtow DISCUSSION XIII, Tasaday I, and D ramamine II; data from other cruises are omitted for brevity but may be Almost by definition, sympatric species oc­ obtained from the senior author upon request. cupy different ecological niches ; of less cer­ On all cruises, copepodite stages other than tainty is the proposition that they must do so to copepodite I and the males of N . gracilis were avoid competitive exclusion and extinction of almost always found in greatest concentrations either. The latter statement may not be " prov­ in the upper 100 m. Ne ocalanus robustior was en" simpl y by th e observation that ecological essentially restricted to this vertical range on differences can usually be found between sym­ several occasions, while N . gracilis usually oc­ patrie species, since th e conclusion that these curred belo w 100 m as well as above this differences are sufficient to reduce competition depth. Both species could sometimes be found to an acceptable level is invalid unl ess the as deep as 850 m, as shown by the deep tows extent and nature of the presumed competition taken on Tasaday I (Table 2). The first copepo­ have been demonstrated. dite stages of both species usually were most The pelagic environment seems to be rela­ abundant at greater depths than were the older tively monotonous with respect to convention­ stages, possibly due to sinking of eggs pri or to ally measured properties, although it may be hatching and failur e of the nauplii to return to rather more diverse from the zooplankter's th e surface waters. This ontogenetic difference point of view. In any case, the plants are all in vertical dist ribution was found for N .gracilis very small (with the obvious exception of Sar­ in th e Adriatic Sea by Shmeleva and Zaika gassutn). This means th at th ere is no physical (1973). The greatest abundance of N . gracilis cover or refu ge for a prey species and that, in was usually found at a greater depth than that of contrast to terrestrial environments whe re an N. robustior during the day, but this was not th e individual plant may serve as both food and case at night. At least part of the N . gracilis substratum for many herbivorous insects dur­ population app eared to migra te vertically, since ing much of their lives, the planktonic herbi­ individuals were captured in the most shallow vores generally have no persistent spatial tows at only 2 of 13 daytime stations but at all association with a particular species of food but 11 stations sampled at night. Some copepodites mus t feed by filtration on a variety of particles. of N . robustior were found in the most shallow In th e two species of Ne ocaianus, as in Calanas tows at 11 of the 13 daytime stations, contrary lighti and C. tenuicomis (Mullin 1969), the sizes to the impression given by Table 2. of the mandibular blades ofany developmental The general impression created by all collec­ stage are more similar than the sizes of the tions is that although the vertical distributions bodies. If there had been stron g evolutionary of the two species overlap considerably, the pressure for divergence in size of feeding center of distribution of N .gracilis is somewhat appendages between sympatric competitors, deeper than that of N. robustior during the day. one would expect the mandibular blades to be This is not un expected, since th e zoogeographic at least as dissimilar as the bodies, or more so. range of the former species extends into colder In fact, failure to diverge could mean th at the water than does that of the latte r (Figures 1 and congeners do not compete for a common, 2), and there is at least an analogy to the hori­ limiting sou rce of food; that divergence in zon tal and vertical distributions of Calanus selection of food is based on some property of tenuicornis and C. lighti (Mullin 1969). At night, the food particl es other than their size or on a the depths of greatest abundance of the two morphological featu re other than mandibular species are much more likely to coincide, al­ width; or it could simpl y be a consequence of th ough the vertical range of N . gracilis is usu­ continuous breeding and th e considerable in­ ally the greater. crease in bodily size which accompa nies juven­ ile development. Con tinuous breeding means that all developmental stages of bo th congeners are present in the water at any tim e. Hence, a slight increase in mandibular size of the larger TABLE 2

V ERTICAL DISTRIBUTIONS OF COPEPODlD STAGES OF N eocalanus rohustior AND N. gracilis AT APPROXIMATELY280 N, 1550 w

CRUISE AND DEPTH Neocalanus robustior N eoca/anu gracilis DATE TIME m II III IV V FE.'J:ALE MALE II III IV V FEMALE MALE Southto w 0904­ 25 * 0.2 0.2 0.1 0.5 1.1 * * 0.1 * * * * * XIII, 5-6 1045 50 0.1 0.3 0.4 0.2 0.3 0.6 * * 0.2 0.6 0.1 0.2 * * F eb ru ar y 1973 100 * 0.3 0.3 0.7 2.0 1.1 0.2 * 0.7 2.0 1.1 3.3 3.2 0.4 200 * * * * * * * 0.4 0.3 * * 0.2 0.1 0.1 300 * * * * * * * 0.1 * * * * 0.1 0.1 400 * * * * * * * * * * * 0.1 * 0.1 2045­ 25 * 0.6 1.3 1.6 0.5 0.1 * * 0.2 2.4 2.9 1.9 1.4 0.2 0100 50 0.6 2.2 1.9 4.5 1.5 0.3 * 0.2 0.7 0.5 0.3 0.6 * 100 * 0.9 2.7 1.1 3.7 0.7 0.2 * 0.9 2.7 5.5 0.5 5.9 0.5 200 0.3 0.2 0.3 0.3 1.1 0.3 0.1 0.3 0.9 0.4 0.9 1.1 1.3 0.1 300 * * * * * * * 0.1 * * * * 0.1 * 400 * * * * * * * * * * * * 0.2 0.2 T asad ay I, 0937­ 20 * * * * * * * * * * * * * * 19-21 June 1130 50 * 1.1 5.6 1.1 1.6 15.8 0.9 * * 0.7 0.7 * * * 1973 100 * 1.4 * * * * * * 21.0 1.9 2.1 1.2 * 0.1 200 1.3 0.4 * * * * * 4.4 0.7 * * 0.1 * * 300 1.1 * * * * * * 2.7 0.3 * * 0.1 * 0.2 400 1.1 * * * * * * 5.8 0.1 * * * * 0.1 2235­ 20 * * * * 2.4 1.1 * * * * 2.0 0.9 2.2 * 0115 60 0.2 5.8 2.2 0.4 * 1.1 * 0.4 2.7 4.4 * 1.6 0.7 * 130 0.5 1.8 0.2 * * * * 0.2 3.9 3.0 * 0.9 * 0.2 200 3.0 0.3 * * * 0.2 * 3.2 0.1 * * * 0.2 0.3 240 0.6 * * * * * * 1.8 0.1 * * * * 0.1 380 2.5 * * * * * * 1.9 0.1 * * * * 0.1 700 0.02 * * * 0.04 * * 1.13 0.02 * * * 0.18 0.04 850 * * * * 0.02 * * * * * * * 0.03 * Dramam ine 1020­ 10 * * * * * * * * * * * * * * II, 11-1 2 1330 80 * 1.8 2.1 0.7 0.2 0.4 0.1 * * * 0.1 * 0.1 0.1 May 1974 150 1.7 0.1 * * * * * 1.0 * * * * * * 240 1.0 * * * * * * 1.5 * * * 0.1 0.2 0.3 360 1.2 * * * * * * 1.8 * * * * 0.1 0.1 490 * * * * * * * * * * * * * 0.1 2135­ 10 * * 0.3 0.6 0.6 1.7 0.6 * * * * * 0.9 * 0100 55 * 0.2 4.7 4.9 2.0 0.4 1.1 * * 2.0 3.6 17.8 10.9 0.2 120 * * * 1.1 * 0.4 0.1 0.1 * * * * 0.1 0.1 210 0.2 * * * * 0.2 * 0.6 * * * 1.1 0.6 * 320 0.3 * * * * * * 0.7 * * * * 0.6 0.1 430 * * * 0.1 * * * * * * * * * 0.1

N OTE: V alues rep resent number s of individuals p er 10 m" . * N o an imals foun d in subsample ex amined . Planktonic Copepods-MuLLINAND EVANS 129 congener may make it more distinct from the the upper 200 m in the eastern Central smaller at th e same developmental stage, but at North Pacific; the center of distribution of each stage except that of th e adult female the N. gracilis is genera lly deeper than is that of mandibular size of the larger congen er would N . robustior during the day. still be very similar to the next older stage of the 2. The copepodite stages of the two species smaller congener. Similarly, little advantage are distinguishable by body size, a given may be gained by a reduction in mandibular size stage of N. robustiorbeing approximately the of the smaller of two congeners as long as the size of the next older stage of N. gracilis. juvenile stages of the larger are always present. There was no evidence of character dis­ Vertical partitioning of the pelagic habitat placement in size of the mandibular blade. has often been cited as a means by which related 3. Both species probably breed throughout the species can reduce competiti on. Baker (1970) year in the eastern Central North Pacific, and found that congeneric species of nonmigratin g there was no evidence that the season al pat­ euphausiids tend to live at different depths, al­ terns of abundance of th e two species differ. though migratin g species overlap considerably in vertical distribution; Brinton's (1967) data do not indicate marked separation between con­ AC KNOWLEDGMENTS generic euphausiids. Pu gh (1974) found little We thank the officers and crew of the R.V. vertical ove rlap in the ranges of 12 of 13 A rgo, Thomas Washington, and A lpha Helix and species of the nonmigratin g siphonophore our many colleagues on these vessels who took genus L ensia. Foxton (1970) felt that the day­ part in the often frus trating study of vertical tim e depths of congeneric decapods are reason­ distribution; in this regard E . L. Venrick de­ ably distinct, and Clarke (1973) discussed the serves special acknowledg ment. D . M. Gar­ importance of different vertical distributions at diner and E. R. Brooks counted many of the night for morphologically similar myctophid samples, and J. R. Beers, A. Fleminger, T . A. fishes. Zalkina (1970) found that the centers of Clarke, and an unidentified referee improved vertical distribution in the upper 300 m of the manuscript by their comments . tropical, cyclopoid copepods were different, and partial vertical separation of congen eric species of Metridia and Pleuromamma (calanoid LITERATURE CITED copepods) is suggested by data of Roe (1972b). The depths of maximal abundance of Cala­ BAKER, A.DE C. 1970. The vertical distribu­ nus lighti and C. tenuicornis in the eastern Central tion of euphausiids near Fuerteventura, Can­ North Pacific are distinct du ring at least part ary Islands (Discovery SOND cruise, 1965). of th e year. As in the present case, the species J. Mar. BioI. Assoc. U.K. 50: 301-342. of smaller bodily size and greater geographical BOWMAN, T . E. 1955. A new of the range tends to occur deeper in th e water col­ genus Calanus from the northeastern Pacific umn. In both Calanus and Neocalanus, however, with notes on Calanus tenuicornis Dana. Pac. the overlap between congeners is considerable, Sci. 9 (4) : 413-422. and the efficacy or even the necessity of partial BRADFORD, J. M., and J . B. JILLETT. 1974. A vertical separation as a means of reducing what revision of generic definitions in the Calani­ wo uld otherwise be intolerable competition dae (Copepoda, ). Crustaceana 27: remains to be dem onstrated . 5-16. BRINTON, E . 1967. Vertical migration and avoidance capability of euphausiids in th e California Current. LimnoI. Oceanogr. 12: SUMMARY 451-483. 1. N eocalanus robustior and N . gracilis are sym­ BRODSKY, K. A. 1967a. Types of female geni­ patric in much of the Pacific; the latter has a talia and heterogeneity in the genus Calanus wider geographical range to the north and (Copepoda) [in Russian]. D ok. Aka d. Nauk south. Both species are most abundant in SSSR 176: 222-225. 130 PACIFICSCIEN CE, Volume 30, April 1976

- --. 1967b. Formation of swimming limbs the siphonophores collected during the in the genus Calantis (Copepoda) and latitu­ SOND Cruise, 1965. J. Mar. BioI. Assoc. dinal zon ality [in Russian] .D ok. Akad. D.K. 54 : 25-90. Nauk SSSR 176: 1441-1444. ROE, H. S. J. 1972a. The vertical distribution CLARKE, T . A. 1973. Some aspects of the ecol­ and diurnal migrations ofCalanoid copepods ogy of lanternfishes (Myctophidae) in the collected on the SOND cruise. II. Systematic Pacific Ocean near Hawaii. U.S. Nat. Mar. account: families up to and includ­ Fish. Serv., Fish. Bull. 71: 401-434. ing the Aetideidae. J. Mar. BioI. Assoc. DEEVEY, G. B. 1971. The annual cycle in quan­ U.K. 52 : 315-344. tity and composition of the zooplankton of ---. 1972b. The vertical distributions and the Sargasso Sea off Bermuda. I. The upper diurnal migrations of the calanoid copepods 500 m. LimnoI. Oceanogr. 16: 219-240. collected on the SOND cruise, 1965. III. 'FOXTON, P. 1970. The vertical distribution of System atic account: families Euchaetida e up pelagic decapods (Crustacea: Natania) col­ to and including the Metridiidae. J. Mar. lected on the SOND cruise 1965. II. The BioI. Assoc. U.K. 52 : 525-552. Penaeidea and general discussion. J- Mar. ROSE, M. 1933. Copepodes Pelagique. Faune de BioI. Assoc. U.K. 50: 961-1000. France, No. 26, Office Central de Faunis­ GRICE, G.D . 1962. Calanoid copepods from tique, Paris, 374 pp . equatorial waters of the Pacific Ocean. U.S. SEWELL, R. B. S. 1947. The free-swimming Fish Wild .Serv., Fish. Bull. 61: 172-246. planktonic Copepoda. Systematic account. GRICE, G. D ., and K . HULSEMANN.1965. Abun­ Scientific reports of the John Murray Ex­ dance, vertical distribution and taxonomy of pedition 1933-34, 8: 1-303. calanoid copepods at selected stations in the SHMELEVA, A. A. , and V. YE ZAIKA . 1973. northeast Atlantic. ]. Zool. 146: 213-262. Vertical distribution of copepodite stages HEINRICH, A. K . 1969. On the tropical plank­ of copepods in the Adriatic Sea [in Russi an]. ton communities in the western Pacific. J. Okeanologiya 13(5): 872-876. Cons, Cons.Int. Explor. Mer 33 : 45-52. SNYDER, H. G., and A. FLEMINGER. 1965. A HUTCHINSON, G. E. 1959. Homage to Santa catalogue of zooplankton samples in the Rosalia, or, why are there so many kinds of marine invertebrate collections of Scripps animals? Am. N at. 93: 145-157. Institution of Oceanography, S.I.O. Ref. MCGOWAN, J. A. 1971. Oceanic bio geography No. 65-14, 140 pp., 48 charts. of the Pacific. Pages 3-74 in B. M. Funnell TATE, M. W., and R. C. CLELLAND. 1957. Non­ and W. R. Riedel, eds. The micropaleontol­ parametric and shortcut statistics in the ogy of oceans. At the University Press, social, biological, and medical sciences. Cambridge. x +828 pp. Interstate Printers & Publishers, D anville, MCGOWAN, J . A. and D. M. BROWN. 1966. A Illinois. 171 pp . new opening-closing paired zooplankton net. VERVOORT, W. 1946. Biological results of the Scripps Inst. Oceanogr. Ref. 66-23, 56 pp. Snellius Expedition. XV. The bathypelagic MORRIS, B. 1970. Calanoid copepods from Copepoda Calanoida of the Snellius Expedi­ midwater trawls in the North Pacific along tion. 1. Families Calanidae, Eucalanidae, 1600 E . J . Fish.Res. Board Can. 27: 2297­ Paracalanidae, and Pseudocalanidae. Tern­ 2321. minckia 8: 1-181. MULLIN, M. M. 1969. Distribu tion, morpho­ VINOGRADOV, M. E . 1968. Vertical distribution metry, and seasonal biology of the plank­ of the oceanic zooplankton [in Russia n]. tonic copepcds, Calantis tenuicornis and C. Akad. Nauk. SSSR, Inst, Okeanol., Moscow, lighti, in the Pacific Ocean. Pac. Sci. 23 (4) : 320 pp. 438- 446. ZALKINA, A. V. 1970. Vertical distribution and PARK, TAl Sao. 1968. Calanoid copepods from diurnal migration of some Cyclopoida the central North Pacific Ocean. U.S. Fish (Copepoda) in the tropical region of the Wild . ServoFish.Bull. 66: 527- 571. Pacific Ocean. Mar. BioI. 5: 275-282. PUGH, P. R. 1974. The vertical distribution of