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Studies on Thalassinid Crustacea (Decapoda, Macrura Reptantia) with a Description of a New Jaxea from New Zealand and an Account of its Larval Development

The First Post-Larval Stage

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The First Post-Larval Stage

The first post-larval stage of Jaxea novaezealandiae (fig. 5), total length 7.50mm, was obtained by moult from larval stages six (three times) and six a (once) in the laboratory. Post-larvae were not found in the plankton and it therefore seems likely that metamorphosis takes place on the sea floor. This has been suggested by Tattersall (1938) for J. nocturna. The unusual metamorphosis of J. nocturna involving a considerable reduction in size from that of the final trachelifer larval stage has been described by both Tattersall and by Caroli (1924). In J. nocturna, 12mm to 15mm trachelifer larvae moult to a first post-larval stage measuring only 6.5mm in total length. In J. novaezealandiae, stage six or six a larvae measuring between 14mm and 17mm in length moult to a 7.5mm post-larva (fig. 5). These specimens are rather opaque, with small red chromatophores scattered over the carapace, around the basal segments of the cephalic appendages, the joints of the pereiopods, abdominal segments and the telson. There was no evidence of a definite pattern in the arrangement of the chromatophores.

First stage post-larvae survived in finger bowls in the laboratory for up to three weeks, but did not feed.

As the first post-larval stage of J. novaezealandiae is closely similar to that of J. nocturna fully described by Tattersall (1938) and Caroli (1924), we do not therefore propose to describe this New Zealand form in great detail. Differences between the adult and post-larva of J. novazealandiae are similar to those noted by Tattersall for J. nocturna. The more important differences are listed below.

1.The post-larva of J. novaezealandiae carries a more conspicuous armature of hairs and small spines than does the adult. The five or six ventral pleural teeth found on the post-larval abdominal segments, and the small spines on the carpus and prodopus of the post-larval chelipeds, do not persist through as teeth to the adult condition (there are minute serrations on the 3rd to 6th adult pleura). The stronger ventral spines and dorsal distal spines on the merus of the chelipeds, and the lateral serrations on the rostrum are all retained by the adult. However, adult structures such as the sub-orbital carapace spines, and teeth on the dactyli of the 3rd, 4th and 5th pereiopods, and on the dorsal surface of the telson are not developed in the first post-larval stage.
2.The post-larval rostrum is relatively larger than that of the adult, and the "neck" is quite well defined in both dorsal (fig. 5B) and lateral (fig. 5A) views. All traces of cervical elongation of the carapace are lost in the adult. The linea thalassinica is incomplete and weakly developed in the post-larva but relatively strong in the adult (fig. 1).
3.The eyes are very much larger than in the adult. In the post-larva the eyes protrude beyond the margin of the carapace (fig. 5B) but in the adult the eyes are very tiny and completely concealed beneath the rostrum (fig. 1). This great reduction in the size of the eyes and the adult's virtually blind condition is presumably related to the supposed deep-burrowing habit of J. novaezealandiae.
4.The antennal scale (scaphocerite) of the post-larva is relatively larger than that of the adult. Also the basal segment of the post-larval antennal endopod is considerably shorter than in the adult where it is greatly elongated.
5.The chelipeds of the adult are more robust and sculptured than those of the post-larva. The propodus of the 2nd pereiopod is not flattened as in the adult (fig. 1). page 23
Fig. 5.—Jaxea novaezealandiae n.sp., first post-larval stage. A, lateral view; B, cephalothorax with appendages, dorsal view; C, abdominal segments, uropods and telson, dorsal view.

Fig. 5.—Jaxea novaezealandiae n.sp., first post-larval stage. A, lateral view; B, cephalothorax with appendages, dorsal view; C, abdominal segments, uropods and telson, dorsal view.

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6.The post-larval tail-fan lacks the ridges and sculpturing characterising that of the adult (fig. 2G). The telson is twice as long as broad in the post-larva (fig. 5C) but only slightly longer than broad in the adult. Transverse sutures are incomplete medially on the post-larval uropods (fig. 5C).

The differences discussed above are not great, and indicate the nature and extent of changes undergone during successive ecdyses from the first post-larval stage to the adult condition. The essential morphological features of the post-larva are so similar to those of the adult that there can be no doubt concerning their belonging to the same species, especially when larval evidence for this deduction is considered in support.

There are now four laomediid crustacean species from which the first post-larval stage is known. These are Jaxea novaezealandiae; J. nocturna described by Caroli (1924), Tattersall (1938) and figured by Gurney (1942); Naushonia crangonoides described by Thompson (1903); and "Naushonia portoricensis?" described by Gurney and Lebour (1939). Gurney and Lebour obtained the first and second post-larval stages of their "N. portoricensis?". Post-larvae of Jaxea nocturna and of J. novaezealandiae are more readily separated than are their respective larval stages. The main differences between the first post-larval stages of these two species of Jaxea and the characters by which Jaxea post-larvae may be distinguished from post-larvae of the genus Naushonia are given in the key below.

Key to Known First Stage Post-Larvae of the Family Laomediidae
1 (4) Ischium of 3rd maxilliped with from 9 to 14 strong blunt medial spines; 1st pereiopods chelate; pereiopods one to four without vestigial exopods; 1st abdominal pleuron extended ventrally as a long slender process; telson with three or more lateral teeth genus Jaxea Nardo
2 (3) Total length 6.5mm; "neck" not well defined, shape of carapace similar to that of adult; linea thalassinica well defined; suborbital carapace spines present; anterolateral margin of carapace with several small spines; 3rd pereiopod without podobranch; basal segment of uropodal exopod serrated; telson rather rectangular with lateral margins toothed, terminating posterolaterally in a strong tubercle Jaxea nocturna Nardo (Caroli, 1924; Tattersall, 1938; Gurney. 1942)
3 (2) Total length 7.5mm; "neck" well defined, shape of carapace dissimilar to that of adult; linea thalassinica poorly defined, especially anteriorly; suborbital carapace spines absent; anterolateral margin of carapace without spines; 3rd pereiopod with podobranch; basal segment of uropodal exopod not serrated but with biplumose hairs; telson rounded posteriorly with lateral margins having only three or four proximal teeth, no posterolateral tubercle Jaxea novaezealandiae
4 (1) Ischium of 3rd maxilliped serrated or smooth, but without large spines; 1st pereiopods subchelate; pereiopods one to four with page 25 vestigial exopods each with apical setae; 1st abdominal pleuron not extended ventrally; telson with one lateral tooth genus Naushonia Kingsley
5 (6) Rostrum with two lateral teeth and with a small apical process; linea thalassinica very faint; eye with a small anterior tubercle; 3rd maxilliped with ischium smooth; 1st maxilliped without arthrobranch; 1st pereiopod slender, not reaching beyond level of eye, propodus with one large inner tooth, dactyl broad Naushonia portoricensis? (Rath-bun) (Gurney and Lebour, 1939)
6 (5) Rostrum with many lateral serrations but without apical process; linea thalassinica strongly developed; eye without anterior tubercle; 3rd maxilliped with ischium serrated distally; 1st maxilliped with arthrobranch; 1st pereiopod robust, extending well beyond level of eye, propodus with two large inner teeth, dactyl slender and falcate Naushonia crangonoides Kingsley (Thompson, 1903)