BULLETIN OF MARINE SCIENCE, 36(1): 104-114, 1985

SEASONALITY AND DURATION OF THE DEVELOPMENTAL STAGES OF HETEROSQUILLA TRICARINATA (CLAUS, 1871) (CRUSTACEA: STOMATOPODA) AND THE REPLACEMENT OF THE LARVAL EYE AT METAMORPHOSIS

B. G. Williams, J. G. Greenwood and J. B. Jillett

ABSTRACT The successful laboratory rearing of the stomatopod Heterosquilla tricarinata from egg to juvenile is described. The species has a single pro pelagic stage and two pelagic stages which last 60-70 days at 15"C before metamorphosis to the postlarva. The occurrence and season- ality of the developmental stages in Otago waters is given and the unusual metamorphosis of the eye in the first postlarval stage is described.

Heterosquilla tricarinata (Claus, 1871) is an extremely abundant burrowing on the sandy intertidal fiats of the Otago Harbour (entrance-Lat. 45°47'S, Long. 170044'E) and the nearby sheltered inlets of the South Island of New Zea- land. It reaches densities in excess of 70 individuals m-2 of surface in places (Fussell, 1979). Specimens from the area were examined by Manning (1966) as part of a taxonomic study of Australian and New Zealand stomatopods but little else has been published on the species. Only one stomatopod, Gonodactylus oerstedii Hansen, 1875, has been suc- cessfully reared from egg to adult in the laboratory to date (Provenzano and Manning, 1978), but other studies based on examination of larvae from the plankton and rearing of individuals through a few molts indicate that stomatopods typically have many larval instars. A summary of the information on the number oflarval stages in several species of stomatopod is given by Michel and Manning (1972). To their list can now be added: Pterygosquilla armata (Milne Edwards, 1837) with 2 propelagic and 9 pelagic stages (Pyne, 1972), Gonodactylus oerstedii, 3 propelagic and 4 pelagic (Provenzano and Manning, 1978) and Squilla empusa Say, 1818 with an unknown number of propelagic stages but at least 9 pelagic stages (Morgan and Provenzano, 1979). The length of larval life is known accu- rately only for Gonodactylus oerstedii (Provenzano and Manning, 1978). This paper reports the small number of larval instars found in H. tricarinata and their development times based on laboratory rearing from egg to juvenile. It also describes the seasonality of the various life history stages and the unusual eye development in the species. A companion paper (Greenwood and Williams, 1984) contains descriptions of the various larval and postlarval stages.

MATERIALS AND METHODS

Laboratory Rearing Experiments, - Egg masses are readily obtainable from Marchi April through to December by digging in the sandy intertidal flats occupied by the adults. In late September and again in late December egg masses were collected and kept in aerated seawater at 12 and 15°C respectively and were examined every 2-3 days for hatching. The water was changed weekly, Once hatched the larvae were kept individually in specimen tubes of 130 ml capacity and diameter 35 mm. The seawater used was not filtered or sterilized and was of salinity 33-34.50/00. In the first successful rearing experiment the larvae were maintained at 15"C in a normal light/dark cycle. The propelagic larvae were not fed until they molted to the first pelagic stage, young Artemia nauplii then being supplied in liberal quantities. Throughout the rearing, water and food in the tubes were changed every 2-3 days at which time evidence of molting was looked for and any exuviae preserved in formalin. The duration of the propelagic stage at 12"C was also determined. In an additional set of rearing experiments, second stage

104 WILLIAMS ET AL.: DEVELOPMENTAL STAGES OF HETEROSQU/LLA 105 pelagic larvae were collected from the plankton and maintained at 15"C until they reached the second postlarval stage. Occurrence and Seasonality of Developmental Stages. - The occurrence and seasonality of develop- mental stages in the adult habitat were determined by qualitative sampling on the intertidal sand flats at irregular intervals over a 2-year period. A quantitative assessment of the planktonic stages was achieved by sampling the plankton at monthly intervals over a period of 14 months at 4 equidistant stations (16.5 km apart) on a transect line running offshore from the Otago Harbour entrance (lillett, 1969). Station A was over the inner shelf at a depth of 25 m, B, over the outer shelf at a depth of \00 m, C at 450 m over the continental slope and D was at a depth of 1,250-1,300 m. Samples were taken using a Clarke-Bumpus sampler hauled diagonally from near the bottom to the surface at the shelf stations and from 150 m to the surface at the two outer stations. The numerical density of H. tricarinata pelagic larvae and postlarvae in the samples was expressed as number m-3 and plotted against time and distance from shore. Further qualitative plankton samples were taken during early spring and summer in the Otago Harbour. Scanning Electron Microscopy. - Preparation of eyes for examination in the SEM followed the usual procedures, i.e., fixation, dehydration, critical point drying, and splutter coating with gold. Shrinkage of the material was the biggest problem. Best results were obtained by fixing fresh eyes in Bouins fluid for 3-4 days before washing and dehydrating in ethanol. The eyes were then placed in the critical point dryer in \00% ethanol.

RESULTS Laboratory Rearing Experiments (Fig. 1). - The propelagic stage larvae which hatch from the eggs contain large amounts of orange-red yolk which is clearly visible through the carapace. They survive extremely well under laboratory con- ditions and molt to the second larval stage (the first pelagic stage) without being fed. All of the 36 propelagic larvae reared in individual containers at 15°Cmolted successfully to the first pelagic stage, 28-29 days after hatching. The propelagic larvae maintained at l2°C developed more slowly, the duration of the stage in the 39 individuals studied ranging from 30 to 39 days (mean 33.2 days). Newly hatched propelagic larvae move very little under laboratory conditions. Feeble pleopod movements occur occasionally. Older propelagic larvae flex their ab- domens from time to time but do not leave the bottom of the containers. As the larvae age the quantity of yolk decreases and when it is exhausted they molt into the first pelagic stage. At this stage the larvae are strongly positively phototactic and they swim about the experimental vessels, catching and eating the Artemia nauplii supplied. They do not survive without food. At 15°C the duration of the first pelagic stage was between 14-17 days and 33 of the original 36 propelagic larvae successfully reached the second pelagic stage. The habits of second pelagic stage larvae are similar to those of the first pelagic stage. Forty-eight percent of the larvae which reached the second pelagic stage had molted to the first postlarval stage when the experiment was terminated 76 days after the first eggs hatched. The mean duration of the second pelagic stage was 19.5 days (range 14-24 days). First stage postlarvae are also active swimmers in laboratory vessels containing only seawater but if provided with a layer of sand, they burrow rapidly. The duration of the first postlarval stage was determined by keeping second stage pelagic larvae from the plankton in the laboratory until they molted to the second postlarval stage. In four which reached the second postlarval stage, the first post- larval stage lasted for 12, 15, 17, and 19 days. Occurrence and Seasonality of Developmental Stages. - In the intertidal areas studied here, females deposit eggs in their burrows in Marchi April. The eggs are a uniform orange-red color when newly laid and are attached to one another to form a disc-shaped egg mass, 20-30 mm in diameter which develops free within the burrow. The mean number of eggs per mass is 227 (SD = 112.5, n = 13). By Figure I. (Left) The development and survival of 36 propelagic larvae reared in the laboratory: e, propelagic stage; 0, first pelagic stage; ., second pelagic stage; 6., first postlarva. Figure 2. (Right) The seasonal distribution and abundance of H. tricarinata larvae along a transect off Otago Peninsula: Light stipple, present-I m-3; medium stipple, 1.1-10 m-3; dark stipple, 10.1-100 m-3•

June small black eye spots are clearly visible in the developing eggs which have a mean diameter of 1.62 mm (SD = 0.054, n = 52) at this time. By September most eggs contain embryos with well developed eyes and clear abdominal seg- mentation and have a mean diameter of 1.78 mm (SD = 0.056, n = 53). In the intertidal areas the first eggs hatch in late September and propelagic larvae can be found in the adult burrows from this time until late November/early December. They are never taken in the plankton. First stage pelagic larvae, presumably newly molted individuals, are occasionally found with the propelagic larvae. First and second pelagic stage larvae and first postlarvae are common in the plankton and the latter can also be found in burrows on the intertidal sand flats from late September onwards into summer. Figure 2 shows the spatial and seasonal abundance of the planktonic stages of H. tricarinata in the study area. They occur from mid winter to late spring and do not extend seawards beyond station B. The maximum density recorded was 19.5 m-3 in October at Station A. A plankton sample taken in the Otago Harbour in mid September contained first and second pelagic stages and first postlarvae in the ratio of 12.25:7.25: 1 respectively. Development of the Eye. - In the course of development the eye undergoes a series of remarkable changes which result in atrophy offunctional elements of the larval eye and their replacement by a new set in the postlarva. In first stage pelagic larvae the eye has an iridescent lime-green pigment and a regular array of om- matidia with hexagonal facets (Fig. 3A). The medial surface of the eyestalk just proximal to the cornea has a more irregularly sculptured surface than does the rest of the stalk (region I in Fig. 3A). By the second pelagic stage this area has developed further into a crescent-shaped swelling (Figs. 3B and 4A). The surface WILLIAMS ET AL.: DEVELOPMENTAL STAGES OF HETEROSQU/LLA 107

Figure 3. Scanning electron micrographs of pelagic stage larvae: A, 1st stage, left eye from dorsal side (length of eyestalk 0.91 mm); B, 2nd stage, left eye from dorsal side (length of eyestalk 1.10 mm); C and 0, detail of B; I, presumptive adult eye.

sculpturing of this swelling is still somewhat irregular but the facets of the pre- sumptive adult eye are now discernible (region I in Fig. 3C). The enlargement of this area is accompanied by some rearrangement and repackaging of the original ommatidia and disrupts the regular hexagonal pattern particularly in the proximal region of the cornea (Fig. 3D). The developing adult ommatidia remain without pigment in the second pelagic stage and also in the newly molted first postlarval stage, although in the latter the pattern of the facets on the surface is more obvious. In the 4-5-day-old first postlarvae spots of dark brown pigment associated with the developing adult ommatidia can be seen for the first time (Fig. 4B). The 108 BULLETIN OF MARINE SCIENCE, VOL. 36, NO. I, 1985

A

Figure 4. Light micrographs of eyes: A, 2nd pelagic stage; B, 4-5-day-old 1st postlarval stage; C, 10- 12-day-old 1st postlarval stage.

presumptive adult eye continues to enlarge and develop throughout the first post- larval stage and in a 8-10-day-old first postlarva the developing adult eye with its dark pigment is only slightly smaller than the original eye with its green pigment. At this stage the separation of the adult eye into two parts by an equatorial band of larger facets is obvious (Fig. SA region I), a common feature in the eyes of adult stomatopods. As development continues the larval ommatidia degenerate progressively (Fig. SB region II and Fig. 4C). After the molt to the second postlarval stage the only remaining sign of the larval facets is an area of stalk slightly sunken and with an irregular surface texture just proximal to the adult cornea in the lateral position (Fig. SC region II). This disappears completely at the next molt (Fig. SD) when the eye assumes the final adult form, with the cornea divided into two parts by a band of larger facets, six facets wide.

DISCUSSION The rearing of H. tricarinata larvae in the laboratory proved to be surprisingly easy. The single propelagic stage does not require food and the two pelagic stages thrive well on Artemia nauplii. The use of Artemia nauplii to rear other sto- matopod larvae has met with varying success. They were eventually used suc- cessfully to rear the larvae of Gonodactylus oerstedii (Provenzano and Manning, 1978), earlier failure (Manning and Provenzano, 1963) being attributed to pro- vision of insufficient quantities of nauplii. Like Provenzano and Manning (1978) we did not quantify the amount of nauplii given to the larvae. Morgan and Provenzano (1979) however, when rearing the larvae of Squilla empusa, supplied approx. 700 nauplii/larva/day, older stomatopod larvae being given decapod lar- vae and adult Artemia. Although this proved sufficient to sustain S. empusa larvae through several molts, they did not complete their pelagic development. Pyne (1972) also used Artemia nauplii when rearing the larvae of Pterygosquilla armata WILLIAMS ET AL.: DEVELOPMENTAL STAGES OF HETEROSQUlLLA 109

Figure 5. Scanning electron micrographs: A, 8-IO-day-old 1st postlarval stage, left eye, dorsal view (length of eyestalk 1.14 mm); B, 1O-12-day-old 1st postlarval stage, right eye, dorsal view; C, 2nd postlarval stage, left eye, dorso-Iateral view; D, 3rd postlarval stage, left eye, lateral view; I, developing adult eye; II, degenerating larval eye.

but failed to rear them past the first pelagic stage. He was able to rear seventh, eighth and ninth pelagic stages collected from the plankton through as many as six ecdyses although he gave no detail of the type of food supplied to them. The larvae of H. tricarinata survived well under the conditions used here, 39% of the pro pelagic larvae reaching the first postlarval stage. In the only other stomatopod to be reared from egg to metamorphosis in the laboratory, G. oerstedii (Provenzano and Manning, 1978), the survival rate to the first postlarval stage was II%. Our success in rearing H. tricarinata is no doubt due for the most part to the 110 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.1, 1985

large size (Table 1) and comparatively advanced stage of development of the first pelagic larvae. Greenwood and Williams (1984) noted that the first pelagic stage larvae of H. tricarinata are at a stage equivalent to the fifth or sixth pelagic stage of other stomatopods. They might therefore be expected to be more successful at capturing Artemia nauplii than the smaller first pelagic stages of other species. Not surprisingly the pro pelagic larvae of H. tricarinata are also bigger than the similar stage of other stomatopods that have been the subjects oflaboratory rearing programs (Table 1). The majority of stoma to pod species are found in tropical littoral and sublittoral habitats (Dingle and Caldwell, 1978). H. tricarinata however occurs in large numbers in the Otago region of New Zealand where air temperatures can be a few degrees below freezing in winter and rarely above the mid-twenties in summer. Water temperatures of the shallow sublittoral habitats in the region vary from 8- 9°C to 14-1 5°C. Its life history differs from that of other stomatopods that have been described in that it possesses only 3 larval instars, 2 of which are pelagic, compared to between 7 and 11 instars in other species. At 15°C the larval stages take a total of 60-70 days after hatching before reaching the postlarval phase with approximately half of that period being spent at the propelagic stage and passed in the relative safety of the adult burrow. In Gonodactylus oerstedii (Provenzano and Manning, 1978), the duration of the larval stages (3 propelagic and 4 pelagic) was 35 days at 25°C with approximately one third of this period occupied by the propelagic instars. The reduction ofthe number of instars in H. tricarinata there- fore has not led to a shorter larval life than in the warmer water species G. oerstedii. When compared to another New Zealand stomatopod P. armata (Pyne, 1972) however the length of the larval period in H. tricarinata is seen to be dramatically reduced. Pyne estimates 258-260 days from hatching to late in the final pelagic stage at 9-14SC, with only 9-12 of these days occupied by the two pro pelagic stages. Another temperate species Squil/a empusa which has nine pelagic stages takes 6 weeks to molt seven times when reared at 20°C or 25°C (Morgan and Provenzano, 1979). The reduction in the total number of pelagic instars means a much shorter period is spent in the plankton than might otherwise occur in the relatively cold waters of the region, while the increase in the proportion of the larval period spent in the pro pelagic phase also serves to reduce the length of the planktonic phase. In terms of its numerical abundance, the species is certainly an extremely suc- cessful one in the Otago area. The short pelagic phase in its life cycle presumably contributes to this by reducing predation of the larvae and decreasing the numbers lost through offshore displacement. Working with tropical and subtropical stomatopod species, Reaka (1979) con- cluded that the number of eggs per brood and egg size increase with the mean body size of females. Although H. tricarinata is a stomatopod of much higher latitudes than those studied by Reaka, it is of interest to compare our data with hers. The largest specimens collected in our study area were 75-80 mm in total length (Fussell, 1979) and this species therefore has much larger and many fewer eggs than would have been predicted using the relationships given by Reaka. She did, however, also conclude that for a given body size egg number is lower at higher (subtropical) latitudes and our data support such a trend. The eggs of H. tricarinata are considerably larger than those of any of the stomatopod species listed by Reaka (1979) and Manning (1963). Although the former author was unable to establish any relation between egg size and latitude in stomatopods, egg size is known to increase with latitude in some other marine invertebrates. The brooding of an egg mass in which the eggs are loosely attached together, WILLIAMS ET AL.: DEVELOPMENTAL STAGES OF HETEROSQUILLA III

Table 1. Size (in mm) of propelagic and first pelagic stomatopod larvae

Stage Species Reference Total Length Carapace Length Propelagic Heterosquilla Greenwood and 5.88 (SO = 0.28) 2.08 (SO = 0.15) (first stage if tricarinata Williams (n = 8) (n = 8) more than one (1984) propelagic Pterygosquilla Pyne (1972) 2.3-3.2 0.90-0.95 stage) armata Gonodactylus Manning and 2.5-2.7 1.0-1.3 oerstedii Provenzano (1963) first pelagic H eterosquilla Greenwood and 10.26 (SO = 0.03) 2.51 (SO = 0.11) tricarinata Williams (n = 10) (n = 47) (1984) Pterygosquilla Pyne (1972) 4.2-5.6 1.15-1.40 armata Gonodactylus Manning and 3.8-4.4 2.3-2.5 oerstedii Provenzano (1963) Squilla Morgan and 2.9-3.3 0.8-1.3 empusa Provenzano (1979)

does not occur in all stomatopod species. Manning (1979) notes that on three occasions the eggs of various species of Nannosquilla have been recorded as occurring singly. For this and other reasons he suggests that the lysiosquillids comprise a fundamentally different stock from the squillids and gonodactylids. The method of egg brooding in H. tricarinata does not, however, support this suggestion. It is also of interest to note that unlike the stomatopod species studied by Reaka (1979), which have year round reproductive cycles with eggs brooded for ap- proximately 3 weeks, H. tricarinata has one brood per year. The period between the laying of the egg mass and hatching is 4-5 months and the planktonic larvae are restricted to the spring and early summer, a time when planktonic food is maximally abundant (Jillett, 1976). In spring inshore plankton samples, H. tri- carinata larvae are often the most numerically abundant plankters. Our obser- vations on the numerical abundance of H. tricarinata larvae in spring plankton are confirmed by Murdoch (pers. comm.) who recorded a maximum abundance of 14.2 larvae m-3 at an inshore station a few km south of the Otago Harbour entrance in September. The number of eggs in each egg mass in H. tricarinata however is at the lower end of the range of values given for other stomatopods by Manning (1963) and Reaka (1979). Such large numbers of H. tricarinata larvae in the spring plankton are not, however, seen throughout the New Zealand region. Wear (1965), working in Wellington Harbour, found them rarely in August, few from September into January and none at other times of the year. They are also rare in the plankton of the Hauraki Gulf, northern New Zealand (Jillett, 1971). It is difficult to comment on the significance of these differences when the distribution and relative abun- dance of the adults within the country is not known in any detail. In our study area adult H. tricarinata are very common intertidally and also occur in shallow subtidal areas (Rainer, 1969). The species has also been recorded from the sub- antarctic Auckland Islands (Hutton, 1878) and in the northernmost parts of New Zealand (Grace, 1972). Powell (1947) states that it occurs throughout the country 112 BULLETIN OF MARINE SCIENCE, VOL. 36, NO.1, 1985 but we have no data on the relative abundance in the various regions (Manning, 1966). The other stomatopod commonly recorded for New Zealand is Pterygosquilla armata. It occurs in shallow subtidal areas on open coasts in the Otago area and appears to occupy a similar habitat in the Whangateau Harbour (Grace, 1972). Unlike H. tricarinata, it has a typical stomatopod life cycle with a large number of pelagic larval stages. Late stage larvae are present throughout the year in the plankton of Wellington Harbour while first and second stage larvae are common in March and April and very common in October (Wear, 1965; Pyne, 1972). In the Otago area P. armata larvae are present for most ofthe year but the maximum density recorded is 0.31 m-3 (Murdoch, pers. comm.). Jillett (1971) did not find them in the Hauraki Gulfplankton. Again, lack of information on the distribution and relative abundance of the adults makes these observations difficult to evaluate. The occurrence of both intertidal and subtidal populations of H. tricarinata perhaps accounts for the discrepancy in the time when planktonic larvae first appear in the plankton each spring (i.e., late July/early August) and the time of the first appearance of the propelagic larvae in intertidal burrows (September). The eggs of the subtidal population could well be expected to develop faster than those in the intertidal region which experience lower temperatures when exposed to the air in winter. The first pelagic larvae to appear in plankton samples in early spring are probably those from the subtidal population, those from the intertidal populations joining the plankton later in the season. This fact also explains the large difference between the length of the planktonic phase in the life cycle (approx. 1month) and the length of the period during which the larvae are found in plankton samples (4-5 months). The external morphology of the adult eye of H. tricarinata is similar to that of Odontodactylus scyllarus (Linneus, 1758) and Gonodactylus chiragra (Fabricius, 1789) described by Horridge (1977), with an equatorial band of large facets di- viding it into two hemispheres. Double eyes, though not necessarily with an identical organization to those of these three species (Schonenberger, 1977), are characteristic of the Order. There appear, however, to be no descriptions of other species of stomatopod with development of the eye similar to that seen in H. tricarinata, despite the relatively large number of species in which details of metamorphosis have been recorded (Alikunhi, 1967; Gurney, 1946; Michel, 1969; Michel and Manning, 1972; Morgan and Provenzano, 1979; Provenzano and Manning, 1978; Townsley, 1953). The figure of the first postlarva of C. tuberculata (Borradaile, 1907) in the paper by Michel and Manning (1972, fig. 3B) shows the eye to be rather similar in appearance to that of a 6-10-day-old first postlarva of H. tricarinata, in that it shows the eye with two discrete pigment masses. This is in contrast to the single pigment mass shown in most drawings of first stage postlarval eyes in the literature (Provenzano and Manning, 1978, fig. 5a, for example). Michel and Manning however make no mention of the replacement of the larval facets during development, stating only that the cornea ofthe postlarva is "characteristically" bilobed and that this becomes clear at the molt from last larva to postlarva. In H. tricarinata, the bilobed appearance of the eye in the 4- 5-day-old postlarva is the result of the enlargement of the presumptive adult facets alongside the original larval ones, a structure very different from the double eye characteristic of adult stomatopods. The latter is first discernible in H. tricarinata on the developing adult lobe of the bilobed eye in an 8-10-day-old first postlarva. Alikunhi (1950) figures the eyes of the final pelagic larva and early first postlarva of Squilla nepa and notes that the eyestalk changes in shape at metamorphosis with the cornea becoming "set more in the fashion characteristic of the adult." WILLIAMS ET AL.: DEVELOPMENTAL STAGES OF HETEROSQU/LLA 113

These first postlarvae were reared until the molt to the second stage but the author makes no mention of any further change in eye structure. The relationship between the eyes of the larval stages of and the adult stage appears to have received little attention in the literature. Meyer- Rochow (1975) describes the larval and adult eyes of Panulirus longipes (Milne Edwards, 1868) in which the apposition-type facets of the larval compound eye are modified during the development fTOmpuerulus to adult into facets with clear zones, an adaptation to the low light intensity habitat of the adult. Fincham (1980) also describes modification of the larval eye in Palaemonetes varians (Leach, 1814) at the time of metamorphosis. This involves a change in shape from the hexagonal-shaped facets of the larva to the square-shaped facets of the adult. It would seem that in most other crustaceans modification of the original om- matidia is sufficient to accommodate the change from a pelagic to a benthic lifestyle. The significance of the development of a completely new set of ommatidia at the onset of the benthic phase in the life history of H. tricarinata cannot be judged without some knowledge of the differences in structure and function of larval and adult ommatidia. Apart fTOmthe difference in pigmentation, we have no information on this subject at present.

ACKNOWLEDGMENTS

We are most grateful to B. Dickson, D. Sanderson and C. Munro for expert technical assistance, willingly provided. We acknowledge with thanks information on stomatopod larvae supplied by R. Murdoch from his unpublished plankton surveys.

LITERATURE CITED

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DATEACCEPTED: February 29, 1984.

ADDRESSES: (B.G. W.) and (J.B.J.) Portobello Marine Laboratory, University oJOtago, Dunedin, New Zealand; (J.G.G.) Department of Zoology, University oJQueensland, Brisbane, Australia.