BULLETIN OF MARINE SCIENCE, 39(2): 498-505, 1986 LARVAL INVERTEBRATE WORKSHOP

EXTRA-EMBRYONIC NUTRITION IN THE PROSOBRANCH GASTROPOD CINEREA (SAY, 1822)

Brian R. Rivest

ABSTRACT The drill Urosalpinx cinerea deposits egg capsules containing eggs that are surrounded by a viscous albumen. The capsules do not contain nurse eggs, but during development the embryos appear to ingest the capsular albumen. In capsules collected from , the number of embryos per capsule varied widely, as did capsule volume. Hatching size among these capsules also varied, and was strongly correlated with the mean volume of albumen available per embryo. The data suggest that in U. cinerea, albumen consumption by embryos increases their overall size at hatching and that differences among capsules in the volume of albumen available to the embryos result in a wide range of hatching sizes.

Numerous benthic marine invertebrates deposit their eggs in capsules in which they include materials consumed by their embryos during development. Nurse eggs are produced by some polychaetes (Wilson, 1928; Thorson, 1946; Blake, 1969; Rasmussen, 1973), nemerteans (Schmidt, 1932), and gastropod molluscs (Fioroni, 1966; Spight, 1976; Rivest, 1983). Variation in nurse egg consumption can influence the developmental stage at which hatching occurs (Hadfield et al., 1972; Rasmussen, 1973), and has been well documented to affect hatching size in gastropods (Thorson, 1935; 1940; Knudsen, 1950; Pearse and Thorson, 1967; Gallardo, 1979; Rivest, 1983). Encapsulated gastropod eggs, regardless of the presence or absence of nurse eggs, appear to be embedded in a viscous albumen which may contain amino acids, proteins and polysaccharides (Morrill, 1964; Bayne, 1968; De Mahieu et al., 1974; Raven, 1975). In contrast to nurse eggs, the nutritive value of this albumen is less well understood (Pechenik et al., 1984). This paper examines the nutritive value of this albumen in the oyster drill Urosalpinx cinerea. Although some have assumed that intracapsular fluid has nutritional value (Thorson, 1946; Bayne, 1968; Webber, 1977; Spight, 1981; Rivest, 1981), several investigators have recently found no evidence that albumen increases hatching size in two of marine prosobranch gastropods (Perron, 1981a; Pechenik et al., 1984). The work presented here on U. cinerea was stimulated because the figures published by Ganaros (1958) show few small zygotes in a voluminous capsule developing into hatchlings with relatively large shells. The lack of nurse eggs or other visible source of supplemental nutrition in the capsules suggested that albumen uptake by the embryos of this species results in a larger size at hatching. Furthermore, the capsules of U. cinerea are translucent so that devel- opment can be followed without opening the capsules.

MATERIALS AND METHODS

Newly laid egg capsules of Urosalpinx cinerea were collected intertidally on barnacle covered rocks in Long Island Sound near Madison, Connecticut on 15 June 1984. They were maintained in dishes floated in an aquarium of recirculating sea water at 15°C. Each dish contained 10-20 capsules that were at approximately the same developmental stage as determined visually through the translucent capsule walls. The water in each dish was replaced daily. During the first 2 weeks of August, the capsules began to hatch. The emergence of hatchlings from one capsule in a dish was used to indicate that the remaining capsules in that dish were near hatching, and they were preserved in alcohol. This was done so that the embryos within individual capsules could be counted and measured at a time that approximated that of hatching.

498 RIVEST: ALBUMEN EFFEcrS IN UROSALPINX 499

The preserved capsules were subsequently measured and then carefully opened by cutting off the apical plug. The hatchlings (=embryos near hatching) were removed, counted, and their shell height measured with an ocular micrometer at 50 x. Capsule volumes were measured by first drying the emptied capsules by pressing them between absorbent tissues, and then filling the capsule with distilled water using a hypodermic needle on a 25-ltl glass syringe. The volume of each capsule was measured three times and the mean calculated. Individual measurements of capsule volume varied by less than 4% from the mean.

RESULTS Zygotes or young spherical embryos of Urosalpinx cinerea removed from seven recently laid capsules were 315 j.Lm in diameter. As development progressed, the size of these embryos increased as the viscosity of the capsular fluid decreased. The embryos during this growth phase were preveligers, with no projecting velar lobes, with only a small developing shell cap, and with their bodies distended with absorbed albumen (Ganaros, 1958, fig. 2). This albumen was presumably ingested for it appeared to fillthe gut much as nurse eggsdo in embryos of Searlesia dira (Rivest, 1983). If the enlarging U. cinerea embryos were removed from their capsules and placed in sea water, they soon ruptured, deflating as the viscous albumen escaped. By the time the shell had developed to enclose the embryos in undisturbed capsules, the viscosity of the capsular fluid was visually indistin- guishable from that of sea water. It was not determined, however, if at this stage the capsular fluid still contained any proteins or polysaccharides. Embryos in 82 U. cinerea capsules survived to the hatching stage. These capsules contained as few as one to as many as 19 embryos (x = 8.12 ± 3.00). During development, there was no sign of cannibalism among the embryos. In 11 capsules there was a total of 20 abnormal embryos whose longest dimension never ex- ceeded 500 j.Lm. These embryos were ignored in the calculations because their small size suggested they had absorbed little albumen relative to their capsule- mates. The hatchlings that were removed from the capsules averaged 1.24 ± 0.17 mm in shell height (N = 667). Some were as small as 0.64 mm while others reached 2.04 mm in shell height (Fig. 1). Mean hatching size among the capsules ranged from 0.92 to 2.04 mm. Capsule height (excluding the stalk) varied over 220% (x = 4.99 ± 0.69 mm, range: 2.99 to 6.64 mm). The larger capsules were pre- sumably laid by larger females (Hancock, 1959; 1960). To determine whether the consumption of albumen influenced hatching size, a measure of the amount of albumen eaten by the embryos was needed. For my purposes, the amount of albumen consumed was estimated by measuring capsule volume and assuming that the embryos ate all the available albumen in each capsule. The capsule volume originally occupied by zygotes was ignored because it on average accounted for only 1.1 ± 0.5% of the total volume. It was also assumed that all female U. cinerea produced albumen of equal concentration and nutritional quality and thus capsule volume would be indicative of the amount of extra-embryonic material available to the encapsulated embryos. Capsule vol- ume averaged 12.1 ± 2.9 j.Ll, but some capsules were three times as large as others (range: 5.4 to 19.8 J.Lt). Although capsule volume (Y) was positively correlated with capsule height (X) (Y = 0.48 + 0.12[log Xl, P < 0.001), capsule height explained only 59% of the variance in capsule volume, presumably due to variation in capsule morphology. Female snails could ensure that embryos are provided with equal allotments of albumen by controlling oviposition so that the embryo: albumen volume ratio remains the same. Pechenik et al. (1984) found that the number of embryos per capsule was positively and significantly correlated with capsule volume in Nucella 500 BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2, 1986

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Hatchling Size (mm)

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Figure I. (upper) Shell heights of 667 Urosalpinx cinerea hatchlings from 82 capsules. Figure 2. (lower) Capsule volume (=a1bumen volume) divided by the number of embryos in each capsule in 82 Urosalpinx cinerea capsules. RIVEST: ALBUMEN EFFECTS IN UROSALPINX 501

lapillus. This relationship was also true in U. cinerea (Y = - 3.33 + 1O.69[log X], P < 0.00 I), but variation in capsule volume explained less than 15% of the variation in embryo number. The number of embryos included within a capsule was not significantly correlated with capsule height (Y = 3.60 + 0.91X, P > 0.05, r2 = 0.04). The mean volume of albumen (=capsule volume) available to embryos varied widely among capsules (Fig. 2). Some embryos had only 0.6 ~I available, whereas the most voluminous capsule had a sole occupant with 19.8 ~I of albumen. The mean allotment was 1.86 ± 2.11 ~I per embryo. If hatching size was influenced by the amount of albumen consumed by em- bryos, then one would predict that hatching size would be larger in those capsules where embryos had a larger allotment of albumen. In the 82 U. cinerea capsules examined, mean hatching shell height in each capsule was positively and signif- icantly correlated with the mean albumen allotment per embryo (Fig. 3): Y = 1.13 + 0.80[log X] (P < 0.001; r2 = 0.78). Thus, capsules with more albumen per embryo produced larger hatchlings.

DISCUSSION In two previous investigations on the importance of albumen in prosobranch molluscs, albumen did not appear to be a significant source of nutrition to en- capsulated embryos (Perron, 1981a; Pechenik et aI., 1984). Pechenik et ai. (1984) found that in the dog whelk Nucella lapillus, the dry tissue weight of shelled embryos was less than that of preshelled embryos, contrary to what would be expected if a large amount of albumen had been consumed during development. However, it is possible that albumen uptake occurred during stages younger than the preshelled embryos they first examined. The investigation of albumen uptake in N. lapillus is complicated because embryos feed on nurse eggs (Pelseneer, 1911). The youngest embryos that Pechenik et al. (1984) examined had already consumed their nurse eggs. At this stage the albumen may have already been internalized because the intracapsu1ar fluid had become nonviscous. During early development in N. lapillus, it would be difficult to separate what component of the increase in biomass is due to nurse egg consumption and what might be due to albumen uptake. Perron (1981 a) more definitively demonstrated the lack of nutritional signifi- cance of albumen in Conus pennaceus. He found that in this species, albumen accounted for only 10-14% of the total caloric content of an egg capsule, with the ova accounting for 48% (Perron, 1981b; 1982). Thus, the albumen: ova caloric ratio is about 1:4. This ratio is the lowest of the four Conus species Perron (1982) examined. Even with this relatively large supply of albumen calories, C. pennaceus embryos reared in the absence of albumen outside of their capsules grew to the same size as controls (Perron, 1981a). Unfortunately, early embryos of some other species of prosobranchs do not survive long once they are removed from their capsules (Pechenik et aI., 1984; Rivest, unpubI.). Such is the case in Urosalpinx cinerea. so an indirect test of albumen's nutritional significance in this species must be used. This test involves comparing data on U. cinerea with that available for other species. Here, two types of comparisons can be made. One examines whether albumen increases the overall hatching size in U. cinerea, and the second examines how albumen might increase the variation in hatching size. One can assume that albumen does not significantly increase hatching size in most species of prosobranch gastropods for two reasons: I) An influence of al- 502 BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2, 1986

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Log (capsule volume per embryo)

Figure 3. Mean shell height as a function of the log of the capsule volume per embryo for 82 Urosalpinx; cinerea capsules.

bumen on hatching size is rarely claimed (Rasmussen, 1951; Giglioli, 1955; De Mahieu et al., 1974). 2) In 74 species of prosobranch gastropods that do not produce nurse eggs, shell height at hatching (Y; ~m) is less than twice the egg diameter {X; ~m): Y = 51.12 + 1.81X, r2 = 0.67, P < 0.001 (Rivest, 1983), and this is smaller than the Conus pennaceus hatchlings that hatch at 2.5 times their egg diameter whether they develop inside their capsule with albumen or excap- su1ated (Perron, 1981a; 1982). Thus, the above regression provides a yardstick with which to examine U. cinerea hatchlings. In U. cinerea, the data presented above show that the shell height at hatching was on average 3.94 times the egg diameter. Thus, U. cinerea hatchlings are more than twice as large as would be expected. There is no reason to suspect that an unusual shell allometry causes this large increase. The greatest increase in size occurs prior to shell formation when the embryos are swelling with absorbed albumen. The fact that the swollen embryos deflated if ruptured indicates that it is albumen uptake, and not genet- ically controlled morphogenesis, that is causing the increase in size. However, a loss of proteins in the capsular fluid during development and a concomitant increase of organic material in the embryos was not demonstrated as it was in the pulmonate gastropod Lymnaea palustris by Morrill (1964). An alternative hypothesis to that of the importance of albumen in influencing hatching size in U. cinerea is that it is simply capsule volume, not the amount of albumen, that helps determine hatching size. However, physical limitations on growth imposed by developing in a confined space are unlikely to be important in U. cinerea because the embryos in all the examined capsules were never densely packed, but had much room in which to move. If embryos of U. cinerea consume different quantities of albumen, they might RIVEST: ALBUMEN EFFECTS IN UROSALPlNX 503 hatch at a range of sizes larger than if albumen was equally divided among embryos or if albumen consumption had no effect on hatching size. To examine this, data on U. cinerea hatching sizes can be compared with similar data on Thais lamellosa. In T. lamellosa, albumen consumption is unlikely to have a large effect on hatching size because hatchlings emerge at only 1.69 times the egg diameter (Spight, 1976; pers. obs.). As part of an earlier study, the shell heights of 389 hatchlings from 18 T. lamellosa capsules were measured (Rivest, 1981). Shell heights averaged 0.99 mm but ranged from 0.8 to 1.1 mm, a difference of 0.3 mm (30% of the mean). In U. cinerea, the range was 0.64 to 2.04 mm, a difference of 1.40 mm (113% of the mean). Thus, albumen uptake results in not only a larger hatching size, but also a wider range of hatching sizes. Hatching size variation in U. cinerea may be caused by at least two factors. One is the variation in the average allotment of available albumen each embryo has due to the packaging of different numbers of embryos into capsules of different volumes. Few embryos in large capsules will have more albumen and hatch out larger than numerous embryos in smaller capsules. This results in an intercapsule variation in hatching size. It is unlikely that this variation was influenced greatly by differences among the capsules in the concentration or nutritional quality of albumen. If this were true, then hatching size would be poorly correlated with the volume of albumen available per embryo, and this is not so (Fig. 3). Thus the original assumption about the uniformity of albumen among capsules appears reasonable. A second cause of variation in hatching size may arise when embryos sharing a capsule acquire different amounts of albumen, resulting in an intracapsular variation in hatching size. To test if a higher than expected intracapsular variation in hatching size occurred in U. cinerea, a comparison again can be made with data on T. lamellosa hatching sizes (Rivest, 1981). The variance of the hatching sizes in each capsule can be used as a measure of how widely hatching sizes vary from the mean. Because the mean hatching size of U. cinerea (1.24 mm) was significantly different from that of T. lamellosa (0.99 mm, Mann-Whitney U = 17,621, P < 0.001), a logarithmic transformation of the data was first performed to compensate for the possibility that the variances were proportional to the means (Sokal and Rohlf, 1969). A Mann-Whitney test of the transformed data revealed that the hatching size variances in the 82 U. cinerea capsules were significantly larger than the variances in the 18 T. lamellosa capsules (U = 318, P < 0.001). Thus, U. cinerea capsu1emates emerged at a wider range of shell heights than T. lamellosa capsulemates, probably due to capsulemates succeeding in acquiring different quantities of albumen. Albumen in U. cinerea thus affects hatching size much the way nurse eggs do in other gastropod species (Rivest, 1983). Both albumen and nurse eggs can act to increase the overall size at hatching and to increase the variation in size at which siblings emerge. However, the presence of albumen or nurse eggsin capsules does not necessarily indicate an effect on hatching size. In Thais canaliculata, the few nurse eggs included in each capsule do not significantly increase hatching size or variation in hatching size (Rivest, 1981). Similarly, in some gastropod species albumen does not appear to influence hatching size (Perron, 1981a; Pechenik et al.,1984). In such species capsular albumen may be of insufficient nutritional quantity or quality to have an effect. Here, the function of albumen is unclear. Albumen does not appear to possess any bacteriostatic properties (Rivest, 1981; Pechenik et al., 1984), but it may be important in supporting embryos during early development (Costello and Henley, 1971; Cather and Crovo, 1972). How- ever, even in species in which albumen does not produce an obvious increase in 504 BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2, 1986 hatching size, the likelihood that albumen has some nutritional value is suggested by the precocious development of the so-called larval kidneys, ectodermal struc- tures specialized for the uptake of albumen (Rivest, 1980; 1981). Albumen is also utilized as a significant source of nutrition in taxa other than the Prosobranchia. Boersma (1982) discussed variation in albumen content in bird eggs. Within several species, increased egg size is due to greater albumen content, but larger eggs do not always produce larger chicks because of the con- founding effect of egg neglect by the parent birds. In pulmonate gastropods, the composition and uptake of capsular albumen is relatively well studied (review by Raven, 1975). In ascoglossan opisthobranch gastropods, Clark and Jensen (1981) found that albumen availability can influence the stage of hatching such that capsule volume may be a better indicator than egg size of the type of development. They also pointed out that since albumen consumption does not require the development of specialized feeding parts or the loss of larval structures, the evo- lution of albumen based nutrition or its loss is easier. In general, it may be that albumen production is under looser constraints than egg diameter, which may be more tightly controlled due in part to space limitations in the female reproductive system. A facet of albumen utilization in U. cinerea yet to be investigated is whether mean albumen allotments, and hence mean hatching sizes, vary among popula- tions. Interpopulation differences in mean hatching size were found in Searlesia dira (Rivest, 1983), but in this species the differences were due to different mean allotments of nurse eggs to the embryos in each population. In species where albumen uptake influences hatching size, selection pressures on hatching size might similarly act by altering albumen allocation to the embryos. This could be accomplished by a change in the nutritional quality of the .albumen, the capsule volume, or the mean number of embryos included in each capsule.

ACKNOWLEDGMENTS

T. Visel helped collect the Urosalpinx cinerea capsules. T. Abbott assisted in the maintenance and analysis of the capsules. D. Curtin prepared the figures. Several anonymous reviewers provided useful criticisms of an earlier version of this manuscript. This work was supported by a grant from the SUNY Cortland Faculty Research Program.

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DATEACCEPTED: February 14, 1986.

ADDRESS: Department of Biological Sciences. State University of New York at Cortland, Cortland. New York 13045.