CONTRIBUTIONS TO THE BIOLOGY OF THE SEA URCHIN TRIPNEUSTES VENTRICOSUS'
B. F. McPHERSON lnstitute of Marine Science, University of Miami
ABSTRACT The growth of Tripneustes ventricosus (Lamarck) was studied using three methods; tagging, penning, and size-frequency analysis. Young urchins (smaller than 30 mm) were found mainly during summer. Growth was rapid during the first year, with the urchins reaching a mean size of about 75 mm by the following summer. There was no indication of growth in the adult population during the summer. Gonads developed during the fall at a test diameter of about 35 to 45 mm. There was a significant departure from the 1: 1 sex ratio in urchins larger than 80 mm. Gonad volume of the population indicated that there were two periods of gonad development during the year, one in winter and one in summer.
INTRODUCTION The white-spined sea urchin, Tripneustes venticosus (Lamarck, 1816), is one of the largest regular echinoids of the shallow, tropical Atlantic. It occurs at Bermuda and from Florida throughout the West Indian region to Brazil. It is also found at Ascension Island and along the west coast of Africa from the Gulf of Guinea to Swakopmund (Mortensen, 1943: 496). In Florida, it has been reported as far north as Riviera Beach (Robert Work, personal communication). In Florida, T. ventricosus was found in areas of "sea grass" (Thalassia testudinum), rocks, rubble, and coral reef. Sometimes it was found living on the open surface of rocks or rubble, and sometimes it was found under rock ledges and beneath hollow coral boulders. Moore et al. (] 963b) re- ported that during the winter it was frequently found in open patches among grass and that during the spring and summer, as the light became stronger, it tended to move from the open areas into cover of dense grass. In several recent papers on the ecology of Tripneustes ventricosus, authors have used the name Tripneustes esculentus (Leske) (Lewis, 1958; Moore et al., 1963b). Mortensen (1943) showed that A. Agassiz, and later Clark (1933), committed an error when they considered the West Indian Tripneustes to be identical with Leske's Cidaris esculenta. He point- ed out that the correct name for this West Indian urchin is Tripneustes ventricosus. Lewis (1958), working in Barbados, B.W.!., has given the most com- plete description of the life history of Tripneustes ventricosus published to date. He studied growth by caging urchins and by using size-frequency
'Contribution No. 590 from The Marine Laboratory, Institute of Marine Science, Univer- sity of Miami. This report constituted in part a thesis submitted in partial fulfillment of requirements for the degree of Master of Science at the University of Miami. 1965] McPherson: Biology of Tripneustes 229 analysis of the population. The spawning season was estimated from changes in the gonad volume and the gonad condition and from quantita- tive plankton tows. Moore et al, (1963b), working at Miami, Florida, compared the gonad growth and spawning of Tripneustes over a period of years. Test growth in relation to temperature and size was described. Some preliminary feed- ing experiments were made, and food intake was estimated tentatively. MATERIALS AND METHODS The measurement used in this study was maximum diameter of the test. It was made on a sliding block, calibrated to the nearest millimeter. The error in the method was checked by measuring 20 urchins to the nearest millimeter, three times each on the same line through the test. A mean for the three measurements was calculated. Analysis of variance showed no significant difference at the 5 per cent level in the three means. The circu- larity of urchins was checked by measuring 20 individuals, at three posi- tions, at 60° intervals around their greatest circumference. Analysis of vari- ance showed no significant difference at the 5 per cent level in the three means. Most urchins in this study were taken by diving in water less than two meters deep off Virginia Key, Florida. Some urchins were taken by diving at Boca Raton, Florida, and some small individuals were taken by dredg- ing in depths of three or four meters off Key Biscayne, Florida. The sea temperature data used in this study were taken from the daily readings of the Miami Marine Research and Test Station, Inc., on Miami Beach, Florida. These temperature data can be used only as an approxi- mation of the temperatures where the urchins were studied. Three methods were used to estimate the growth of the test: (1) hold- ing animals in pens and cages in the sea; (2) tagging animals; (3) using size-frequency analysis on the population. Ten pens were constructed below extreme low water on the beach of the Marine Laboratory of the University of Miami. Each pen covered ap- proximately 1.5 square meters of sea bottom. Rocks were put in each pen to simulate a natural environment of Tripneustes. Algae growing on the rocks and grass that washed into the pens provided food for the urchins. Individual wire cages were constructed for small urchins. These cages were cylindrical in shape, about 25 em high and 15 em in diameter. They were anchored to the bottom adjacent to the pens. A bottomless wire cage, approximately a meter in diameter and half a meter high, was also constructed for holding small urchins. This cage was anchored to the bottom of the grass flats off Virginia Key. Growth was determined from tagged individuals living in their natural environment. The tag consisted of a plastic disk fastened by a stainless steel wire 0.014 mm in diameter. The wire was bent and pushed through 230 Bulletin of Marine Science [15 (1) the test of the urchin in two places on the aboral surface, about two or three centimeters above the ambitus. Spring action of the wire held the tag in place. Between November 1962 and June 1963, 222 urchins were tagged and released in the field, and 131 were tagged and held in pens. Statistical comparisons were made on the growth of some groups of individually identifiable urchins. Two methods were used: (1) regression analysis; and (2) comparison of means. In each group the initial diameter was plotted against the growth increment. In some cases, regression lines were calculated and drawn, and regression analysis was used to test the difference between the two lines of different groups. In other cases, when the regression was not significantly different from zero, a comparison of means was used. The level of significance was set at 5 per cent. Monthly size frequencies of the Virginia Key population were made from November 1962 through March 1964. Between 90 and 175 urchins were collected, measured and returned to the sea each month. The collec- tions were made over the same area. In July 1963, the young of Tripneustes were found off Key Biscayne by dredging in grass beds. A series of hauls in that area was made from July through October. In October 1963, a population of Tripneustes was found at Boca Raton, Florida. A sample was taken in October and November to compare the sizes of these animals with those taken at Virginia Key and Key Biscayne. In November the additional measurement of test height was made for some of the urchins in the samples from Boca Raton and Virginia Key to com- pare test shape in the two areas. The means of all the samples were calculated and used to estimate growth. The means of some of the samples were determined by the prob- ability paper method described by Harding (1949). Three aspects of the reproduction of Tripneustes were studied. These were the size at first sexual maturity, the sex composition of the popula- tion, and the spawning season. The size at first sexual maturity was determined from 277 urchins taken from Virginia Key during late summer and early fall, and from 30 urchins taken at Boca Raton during October. These urchins ranged in size from 10 mm to 93 mm. The sexual condition of these urchins was defined by the presence or absence of visible gonad development and the presence or absence of ripe genital products. If gonads were visibly present, a gonad smear, taken from the region of the gonoduct, was examined microscopically for mature sex cells. To verify ripeness, the ova from a few of the smallest urchins with mature gametes were fertilized. Fertilization was considered success- ful if any cleavage was observed. The gonad samples from the years 1960, 1961 (Moore et ai., 1963b) 1965] McPherson: Biology of Tripneustes 231 and 1962-63 were used to study sex ratio. The sex of only four individuals was unidentifiable; these individuals were not included. The total of 1,415 urchins was divided into three size groups based on diameter of the test. For each group the sex ratio was determined, and the significance of its departure from 1: 1 was checked by the chi-square test. Changes in the average gonad volume of the animals in the population were used to estimate the reproductive cycle. Gonad volume was expressed . 10.gonad volume . relatIvely as I ; followmg a method used by Moore (1934). test vo ume The gonad volume was determined by water displacement in a graduated cylinder. The test volume (including spines) was determined for 94 urchins by water displacement. The diameter cubed was plotted against the test volume and a straight line regression calculated. In routine work the test diameter was measured and the test volume was calculated from this regression line. Approximately 40 specimens of T. ventricosus that were larger than 5'0 mm were collected for each gonad sample. Samples were taken monthly from October 1962 to September 1963. During several months a scarcity of urchins caused a reduction in this sample size. Sex and condition of the gonads were determined by a microscopic examination of a gonad smear and by gross observation. Estimations of the number of ripe eggs present in 13 different urchins of known gonad volume were made, following the dilution method described by Shapiro (1935). A known volume of gonad was diluted and mixed with 1,000 ml of sea water. A count was made of the number of ripe eggs in a known volume of this sea water, and the total number of eggs in the urchin was calculated.
GROWTH OF THE TEST Of the three methods used to estimate the growth of Tripneustes ventri- cosus, the size-frequency analysis of the population proved to be the best. Conditions in the pens were not conducive to prolonged or seasonal growth studies. Tagging proved to have a slight retarding effect on growth, and it was not feasible to use on small urchins. The untagged urchins held in the pens, of the size range available during the summer, grew about 5 mm in January-March and about 3 mm in March-May. There was no indication of growth in urchins of this size range during the summer (Fig. 1). Tagged urchins, in both field and pens, grew during the winter and spring but showed no growth during the summer (Fig. 2). Small urchins, ranging from 9 to 27 mm, that were held in cages in the sea during the summer, showed a growth rate of about 6 to 8 mm per month (Fig. 1). Generally, there was a close parallel between estimated growth from the size frequencies, and growth calculated from penned, caged, and tagged urchins (Fig. 3). 232 Bulletin of Marine Science [15(1 ) 20 ,....------.
o o 10 - -
0 -E o B E I- - N ~ 20 ~ :z 10 I- - LLJ :IE ~ooo LLJ c:: 0"---0 u e ~~ - :z ca> C~ 0 1-----:------0----1t Ii. C A Ii N· 9 - oN· 5 'A, A 10 - "" - "" , " o 1------r--1-----1r--- " .••••.=.... 00-r0 - 0)- o 25 50 75 100 IN ITIAL DIAMETER (mm) FIGURE 1. Growth of caged and penned urchins (Tripneustes ventricosus) off Virginia Key, Florida. A, 20 Jan.-20 Mar.; B, 20 Mar.-20 May; C, 4 July-l Sept. il individually caged; 0 penned. ] 965J McPherson: Biology of Tripneustes 233
o MEAN TEMPERATURE 3.0 PENS 32 .z:. c: -0 2.0 28 E ~•.. Q. 1.0 /,.../ 240 rn _-0'" E G") E 0.0 ---- 20 fg rn - (J) I.LJ ~ 0.0 20 NOV DEC JAN FEB MAR APR MAY JUN JUL FIGURE 2. Mean growth of tagged urchins (Tripneustes ventricosus). Vertical black bars are used to illustrate two standard errors of the mean corrected for sample size. The broken line connects mean temperatures for the periods from tagging to recovery. • TAG GROWTH 90 6 PEN GROWTH • CAGE GROWTH E o VIRGINIA KEY MEAN ..§. 75 • KEY BISCAYNE MEAN c:: w ~60 :E « C; 45 I- .0· . CJ') ...' w 30 '/.',. I- ,.;. , 15 *'" o AMJ JASONDJFMAMJJASO 1963 1962 -63 FIGURE 3. A comparison of the results of the methods used to estimate the growth of Tripneustes ventricosus. 234 Bulletin of Marine Science [15(1) The first size-frequency analysis was made in November 1962. The urchins at that time ranged between about 50 and 80 mm and had a mean of 62 mm. The size of the urchins increased monthly until May, when they ranged from about 60 to 100 mm and had a mean of 76 mrn. From June through November there was little change in the monthly size frequencies. The mean fluctuated slightly during this period, but in November 1963 it was 76 mm, indicating little or no growth. The maximum size and the number of large urchins (9'0 mm to 109 mm) increased during the winter of 1963-64, indicating that these urchins were probably growing again during this period (Fig. 4). A smaller size group first appeared in May, but it was August before sufficient urchins were found for the calculation of a useful mean. At that time, these urchins ranged from 11 to 49 mm and had a mean of 31 mm. The mean size of this group of urchins increased to 37 mm in September, 43 mm in October, and 50 mm in November. In December, the size frequencies of the old and young urchins merged. It was impos- sible to separate the two groups of urchins after November; however, it was evident that considerable growth of the smaller population took place during the winter. The population of small urchins dredged off Key Biscayne in July 1963 had a mean of 20 mm. The mean of the small urchins increased to 28 mm in August, 35 mm in September, and 42 mm in October (Fig. 5). A size-frequency analysis in October and November of the Boca Raton population revealed a two-peak distribution as did the Virginia Key population during the same periods. At Boca Raton the mean sizes for the two peaks were 34 mm and 60 mm in October and 41 mm and 63 mm in November (Fig. 5). MAXIMUM SIZE ATTAINED Mortensen (1943: 491) gives the maximum test diameter of Tripneustes ventricosus as 150 mm. The largest specimens recorded from the Miami area were 108, 112, and 116 mm from Soldier Key, and 127 mm from Ragged Keys (William Hermkind, personal communication). The largest specimen from the Virginia Key grass flats was 109 mm. RELATION OF TEST DIAMETER TO TEST HEIGHT Measurements of test diameter and test height of urchins collected at Boca Raton and Virginia Key in November 1963 were plotted in Fi~ure 6. The regression lines indicated a distinct difference in the shape of the test, the Boca Raton urchins being flatter than those from Virginia Key. The mathematical expression of the two slopes may be of value as a basis for comparing these populations with others. With H the height 1965] McPherson: Biology of Tripneustes 235 0 0 0 0 0 0 0 co lJ:) V N ..: 0 N :!: ~ ~a .Q v Go 0 lJ... j, .••.•..••••..••.. -l .•. - A c::: I ...,0 en0 !:) ....•...... ••...... J.I.... a • Ii: u Go ~ ~ 0 Q) ...... 1.__...... ••. ~ ~ ~ en z ~ '13 ._~ •.••••• aa '00•... +" 0 2:? :> 0 . --1'.& _ .•.• a e - 0•... 15. I"- ...... Go (/) en ""Q.'" :z = ::t ;::l ~ .~ c""0•... g ::c <">bIl U 2:? a N;> lJ:)0 en 2:?z ~ ~. 0 0 0 0 0 0 0 CO lJ:) V N (WW) l::l313Vol~la 1531 236 Bulletin of Marine Science [15(1) JUL AUG SEP OCT OCT NOV N = 81 13 30 30 70 46 BOCA 100 KEY BISCAYNE RATON -e e 80 - t a: ,• LU • ~ 60 • LU • ~ 2 • t c r -c • 40 -, •••• ~ • CI) t t• LU -f • ~ 20 • 0 FIGURE 5. Size frequency of Tripneustes ventricosus from Key Biscayne and Boca Raton, Florida, in 1963 in mm groups. of the test and D the diameter of the test in millimeters, the fomlUla for the Boca Raton population is: H = 6.17 + 0.51D and the formula for the Virginia Key population is: H = 8.34 + 0.55D DISCUSSION OF TEST GROWTH The 1962-63 adult population from Virginia Key, as indicated by size-frequency analysis, grew about 3 to 4 mm per month during Novem- ber and December. After this, growth decreased slightly to an average of about 2 mm per month from January to May. There was no indication of growth from May to November. Between November and March 1963-64 there were indications of growth, with some urchins reaching a size slightly over laO mm. Small urchins were found off Virginia Key in the summer. Growth, as 1965] McPherson: Biology ot Tripneustes 237 0---0 VIRGINIA KEY N = 39 50 t,---t::. BOCA RATON N ·47 -E o '~40 10 20 30 40 50 60 70 80 90 DIAMETER (mm) FIGURE6. Relation of test diameter to test height of Tripneustes ventricosus at Boca Raton and Virginia Key, Florida. indicated by size-frequency analysis, was about 6 mm per month from August to November. Growth continued through the winter, and in March 1964 these urchins had reached a size of more than 60 mm. The November distribution of sizes in 1963 differed from that of 1962 in several respects. The ] 962 distribution showed a single mode, while that of 1963 showed two modes. In 1962, only 2 per cent of the urchins were larger than 80 mm, while in 1963, 13 per cent were larger than 80 mm. Only one urchin measured less than 50 mm in November 1962, but there were 28 under 50 mm in November 1963. The August to November 1963 size frequencies showed that the Virginia Key Fopulation was composed of at least two age groups. Individuals from the older group must have been at least a year and a half old. The urchins of the younger group probably settled in late spring or early summer of 1963 and were about 6 or 7 months old by November. The data are insufficient to show whether the 1962-63 adult population was composed of more than one age group. The growth pattern during the summer and fall of 1963 would indicate, if these urchins are at all cyclic in their settlement and growth, that some of the urchins in Novem- ber 1962 were those that had settled in the summer and spring of 1962 238 Bulletin of Marine Science [15 (1) and others were those from a previous year's settlement. However, there were some indications that the size frequencies of the 1962-63 adults were composed mainly of one age group. First, an examination of the size frequency, plotted on probability paper as cumulative per cent, pointed to a single mode frequency. Second, the large urchins, which represent a considerable proportion of the 1963 November sample, were conspicu- ously absent from the 1962 November sample. Size-frequency analysis indicated that urchins that settled in spring or early summer probably reached a mean size between about 60 and 80 mm by the following summer. It also indicated that most of this growth took place during the first six months to 10 months. An interesting comparison can be made between the size frequencies of the population of Virginia Key and Boca Raton in October and November. The mean diameter of the small urchins from Boca Raton was 9 mrn below the mean at Virginia Key in October and November. The mean of the large urchins from Boca Raton was 16 and 13 mm below that of the adults from Virginia Key in October and November, respec- tively. This difference in the means could be due to difference in either the growth rate or the settlement time. Lewis (1958) found that small urchins of T. ventricosus from 10 to 30 mm appeared in his samples mainly in September. He stated that growth appeared rapid from September through February and March and that it slowed from April through July. He reported that by September the urchins that were a year old had reached a size of 60 to 80 mm. There is a strong resemblance between the growth pattern from Barbados and that from Virginia Key. Several investigators have suggested a relationship between the test shape of urchins and environmental conditions. Thompson (1917: 661) considered two forces influential in shaping the test of regular sea urchins: gravity and the pull of the tube feet. He stated that the tube feet "... constitute a system of forces, with one resultant downwards, in the direction of gravity, and another outward in a radial direction; and if we look upon the shell as originally spherical, both will tend to depress the sphere into a flattened cake." Moore (1935), citing Thompson's idea, postulated that sea urchins in areas of heavy wave action would be either flatter or else thicker shelled than those in areas of relatively calm water. He found that Echinus esculentus was both flatter and thicker shelled in wave-beaten areas. The flatness of the Boca Raton urchins could possibly be explained by their rocky, surf-beaten habitat. The growth of the urchins inhabiting this area would be under a greater force or pull from the tube feet than the growth of urchins inhabiting a relatively placid area with a soft substratum, such as the grass flats off Virginia Key. 1965] McPherson: Biology of Tripneustes 239 NUMBERS 6 12 15 14 40 36 IB 16 8 10 10 20 26 30 12 4 100 ~._._.--...... r'~ .' 'v' w 80 I C> / •....« ~ 60 J u UNDEVELOPED (' MATURE GAMETES a:: w a. 40 i 20 o .~"'" o 10 20 30 40 50 60 70 80 90 100 TEST DIAMETER (mm) FIGURE 7. Sexual condition in relation to size of the urchin, Tripneustes ventricosus, from Virginia Key. SIZE AT FIRST SEXUAL MATURITY Urchins were classified into three sexual categories to describe the development of the gonads. These categories were: ( 1) undeveloped gonads; (2) immature gonads; (3) gonads with mature sex cells. Urchins that lacked visible gonad development were put in the first category. Urchins with gonads whose sex was unidentifiable or urchins with gonads that lacked mature sex cells were put in the second category. Urchins with any mature sex cells were put in the third category. Eggs were considered mature when they reached a size of 80 /1-, and did not have a visible nuclear membrane in an unstained smear. Generally, these eggs, when fertilized, showed normal cleavage. Spermatozoa were considered mature when they were active in sea water. The sexual categories of gonad development by size, for the Virginia Key population, are recorded in Figure 7. The data are restricted to July through October of 1963 when small urchins were available. Urchins smaller than 29 mm had no visible gonad development. From 30 to 34 mm half of the urchins had no visible gonad development and half had immature gonad development. From 35 to 39 mm, 10 per cent of the urchins had mature sex cells (4 males and no females), 35 per cent were immature, 55 per cent were undeveloped. The first females were found in the 40 to 44 mm size group. This group was composed of 61 240 Bulletin of Marine Science [15( 1) per cent urchins with mature sex cells (12 males and 10 females), 20 per cent immature urchins, and 19 per cent urchins with undeveloped gonads. The 45 to 49 mm group consisted of 61 per cent urchins with mature sex cells (7 males and 4 females), 22 per cent immature urchins, and 17 per cent urchins with undeveloped gonads. The 50 to 54 mm size group had only 13 per cent immature urchins and 87 per cent with mature sex cells (6 males and 8 females). Most individuals larger than 54 mm had some mature sex cells. Fertilizations were attempted with some of the smallest urchins that had mature sex cells. In most cases a division of the eggs was noted, indicating ripeness. The smallest ripe individuals were a 37-mm male and a 41-mm female. The sample from Boca Raton was extremely small, and did not indicate clearly the size at first sexual maturity. However, a comparison with the Virginia Key population suggested a later maturity at Boca Raton. Visible gonad development appeared in the 35-39 mm group at Boca Raton and in, the 3'0-34 mm group at Virginia Key. The first mature sex cells were in the 50-54 mm group at Boca Raton and the 35-39 mm group at Virginia Key. A few small urchins from other areas have been examined for ripeness. An examination of about 20 small specimens of Tripneustes ventricosus, collected in Bimini in June of 1961 by H. B. Moore, indicated that immature gonads were developed at sizes of about 20 to 30 mm. Lewis reported, from casual observations in Barbados, that urchins which had reached a size of 20 to 30 mm in Mayor June had full gonads containing ripe eggs and sperm. SEX RATIO To study the sex ratio, urchins from the gonad samples of the years 1960, 1961, and 1962-63 were divided into three groups. In the groups of urchins from 45 to 69 mm and 70 to 79 mm there was no significant departure from the 1: 1 male to female ratio. However, in the 80 to 100 mm group the females were significantly more abundant than males. This group was composed of 60 per cent females and 40 per cent males. Unequal sex ratios are not rare in the animal kingdom. They may be due to differences in growth or mortality between the sexes, or to her- maphrodjtism. ' Hermaphroditism is rare in echinoids. Harvey (1956), in a survey of the literature, records only 29 cases of hermaphroditism from hundreds , of thousands of urchins examined. Boolootian (1960) found a number of hermaphrodites of Strongylocentrotus purpuratus in areas that had a wide temperature range. He never found hermaphrodites of this species in warm' or cold waters exhibiting a normal temperature range. Moore et al. 1965] McPherson: Biology of l'ripneustes 241 (1963a; 1963b) reported that populations of both Lytechinus variegatus and Tripneustes ventricosus were partially hermaphroditic during 1959. They were unable to find this condition in other years and thought that it was due to the unusually cold winter of 1957-58. REPRODUCTIVE CYCLE The best indicators of ripeness proved to be the conditions observed in the microscopic examination of gonad smears, gross observation, and gonad volume. Each had its limitations and its advantages. The main purpose of the microscopic examination of the gonad smears was to determine if ripe genital products were present. Gross observation was useful in detecting urchins at either end of the maturity range; however, urchins of intermediate ripeness were not easily classified by this method. The gonads of ripe urchins were large and fluid in texture. Spawning was initiated by cutting the urchins open, or by handling them. The gonads of the ripe males were bright yellow and those of the females were bright orange. The gonads of the unripe urchins were small, non-fluid in texture, and dull colored. There was little or no spawning when these urchins were cut open. An index of ripeness based on gonad volume had the advantage over microscopic and gross observation of being quantitative. However, gonad volume did not indicate whether an urchin was unripe, ripe, or spent. It only gained significance when it was used in conjunction with micro- scopic or gross observation (to determine if ripe sexual products were present), and when it was compared with the range of gonad volumes possible in the species. A significant correlation (+ .98) was found in Tripneustes ventricosus between the gonad volume and the number of ripe eggs present. As the gonad volume increased, the number of ripe eggs increased. The mean relative gonad volumes were used in 1962-63 to estimate the reproductive cycle of Tripneustes ventricosus (Fig. 8). Moore et ai. (1963b) used this method in the years 1959, 1960, and 1961 for T. ventricosus. The standard deviations and the ranges of the relative gonad volumes for the year 1962-63 are arranged in Table 1 to illustrate individual variation in the monthly samples. The mean relative gonad volume increased from about 0.4 in October 1962 to a maximum of about 0.7 in January 1963. In February, the female index dropped sharply to 0.3. The male index decreased, though not as markedly. By the first of March the male index had dropped to 0.5. From a low in March, the mean relative gonad volume rose continu- ously through the following months to a maximum in June of 0.8 for the females and 1.0 for the males. The volume dropped through the rest of the summer to 0.4 for the females and 0.5 for the males in September. In each monthly sample there were some individuals with large gonads 242 Bulletin of Marine Science [15(1) 1.0 I'• I \ ,I \ I 1962· 63 8i .• •0 o N 0 J F M A M J J A SON FIGURE 8. Mean relative gonad volume of the population of Tripneustes ventricosus from Virginia Key, Florida, in 1962-63. and others with small gonads. This is illustrated in Table 1 by the large dispersion of the relative gonad volumes in any monthly samples. Active sperm and ripe eggs were found in every monthly sample. In fact, most urchins had either some ripe eggs or active sperm. However, urchins were generally riper, as determined by gross observation, during periods which corresponded with the maximum gonad volume in the population. DISCUSSION OF REPRODUCTIVE CYCLE There were several indications that some spawning possibly took place throughout the year. The presence of mature sex cells in most urchins sampled showed that there was at least a potential to spawn at any time. Gross observations indicated that some individuals were at different degrees of ripeness at the time of each sampling. Finally, the gonad volumes of individuals showed a wide range or distribution in most monthly samples. Mean relative gonad volume, then, probably gives only a rough esti- mation of the reproductive cycle. So far, however, it is the best index for the delineation of a reproductive period in a population of urchins. Both Lewis (1958) and Moore et ai. (1963b) found maximal gonad size was reached in late spring or early summer. In 1961, Moore et at. found a two-peaked pattern in the reproductive cycle very similar to that found in 1962-63 but very different from the pattern of two earlier years. Lewis reported that larvae of Tripneustes ventricosus appeared in the plankton in early summer and disappeared in early fall, with the greatest 1965] McPherson: Biology of Tripneustes 243 TABLE 1 RELATIVE GONAD VOLUMES OF Tripneustes ventricosus FROM VIRGINIA KEY, FLORIDA. IN 1962-63 Number of Mean Standard Range Urchins Deviation Oct. 2 11 .33 .23 .06- .75 ~ 8 .33 .23 .01- .60 Nov. 2 16 .43 .11 .35- .63 ~ 24 .51 .21 .32-1.05 Dec. 2 38 .42 .18 .12- .70 & 26 .57 .16 .28- .90 Jan. 2 23 .63 .20 .27-1.21 ~ 29 .72 .22 .23-1.10 Feb. 2 23 .33 .11 .11- .51 & 17 .64 .17 .30-1.07 Mar. 2 15 .33 .29 .22- .57 & 34 .46 .16 .24-1.01 Apr. 2 28 .44 .16 .20- .98 ~ 28 .49 .12 .25- .70 May 2 21 .58 .15 .35..•.90 ~ 19 .63 .13 .43-1.01 June 2 14 .83 .18 .53-1.35 & 26 1.02 .22 .64-1.54 July 2 26 .76 .25 .29-1.18 & 14 .77 .23 .41-1.40 Aug. 2 22 .54 .22 .25-1.00 ~ 18 .69 .26 .27-1.40 Sept. 2 20 .39 .16 .11- .61 ~ 20 .55 .32 .02-1.25 number of metamorphosing larvae in July. He found young urchins that year in late summer and early fall. In the Miami area, the very small urchins were found mainly during the summer. The smallest urchins, from 5 to 7 mm, were found in July. ACKNOWLEDGMENTS I am indebted to Dr. H. B. Moore for his advice in the preparation of this paper. I am grateful to various students and members of the staff and faculty of the University of Miami for their help in this study and to the National Science Foundation for support of the work under Grant GB-19. SUMARIO CONTRIBUCION A LA BIOLOGfA DEL ERIZO DE MAR Tripneustes ventricosus Los ejemplares de Tripneustes ventricosus usados en este estudio fueron tornados de las aguas bajas frente a Virginia Key, Key Biscayne, y Boca Raton, Florida. Tres metodos fueron usados para estudiar el crecirniento de la concha de esta especie: (1) marcarlos (2) enjaularlos (3) medirlos. 244 Bulletin of Marine Science [15(1) EI analisis de la frecuencia de cada tamaiio fue el metodo mas uti!. Los erizos j6venes (menores de 30 mm) fueron eneontrados prineipal- mente durante el verano. El crecimiento fue rapido durante el primer ano, con los erizos alcanzando el verano siguiente un tamano promedio de 75 mm. No hubo indicaci6n de ereeimiento en la poblaci6n adulta durante el verano; sin embargo, hubo indieaei6n de que el crecimiento empez6 otra vez durante el segundo inviemo. EI tamano maximo registrado en Virginia Key, Florida, fue 109 mm. Rubo una signifieativa difereneia en la forma de la concha de los erizos de Virginia Key y los de Boca Raton. Fueron hechos estudios de las g6nadas de los erizos recolectados frente a Virginia Key. Las g6nadas se desarrollaron durante el otono en conchas con 35 a 45 mm de diametro. Rubo una diferencia significativa de la relaei6n sexual 1: 1 en erizos mayores de 80 mm (60 por ciento hembras y 40 por ciento machos). El volumen de las gonadas de la poblaci6n mostr6 dos perfodos de gran tamano durante el ano 1962-63, uno en eI inviemo y uno en el verano. En volumen maximo de las gonadas fue en Junia. Algunos huevos maduros y esperma fueron encontrados durante todo el ano. REFERENCES BOOLOOTIAN, R. A. 1960. The effect of temperature on gonadal growth of Strongylocentrotus purpuratus. Anat. Rec., 137 (178) : 342-343. HARDING, J. P. 1949. The use of probability paper for graphical analysis of polymodal frequency distributions. J. Mar. bioI. Ass. U. K., 28 (1): 141-153. HARVEY, E. B. 1956. The American Arbacia and other sea urchins. Princeton University Press, Princeton, 1-298. LEWIS, J. B. 1958. The biology of the tropical sea urchin Tripneustes esculentus Leske in Barbados, British West Indies. Canad. J. Res., 36: 607-621. MOORE, H. B. 1935. A comparison of the biology of Echinus esculentus in different habitats. Part 3. J. Mar. bioI. Ass. U. K., 21 (2): 711-720. MOORE, H. B., T. JUTARE, J. C. BAUER AND J. A. JONES 1963a. The biology of Lytechinus variegatus. Bull. Mar. Sci. Gulf & Carib., 13 (1): 23-53. MOORE, H. B., T. JUTARE, J. A. JONES, B. F. MCPHERSON, AND C. F. E. ROPER 1963b. A contribution to the biology of Tripneustes esculentus. Bull. Mar. Sci. Gulf & Carib., 13 (2) : 267-281. MORTENSEN, T. 1943. A monograph of the Echinoidea. Camarodonta. 3 (2). C. A. Reitzel, Copenhagen, 1-547. SHAPIRO, H. 1935. The validity of the centrifuge method for estimating aggregate cell volume in suspensions of the eggs of the sea urchin, A rbacia punctulata. BioI. Bull. Woods Hole, 68 (3) : 363-377. THOMPSON, D. W. 1917. On growth and form. Cambridge Univ. Press, Cambridge, 1-793.