Dakin and Colefax (1940) have recognised at least three distinct Indo-Pacific species of Jaxea, viz., a New Zealand species (Jaxea novaezealandiae) of which the adult and larval stages are now fully described, a "Sydney" species from New South Wales of which only the trachelifer larval stages are known, and Jaxea sp. from Samoa, which is known only from its first larval stage. Two species are known from the northern hemisphere. These are J. nocturna from the Mediterranean and North Atlantic with both the adult and larvae described and a distinct species from the Adriatic Sea known only from a single sixth stage larva.

The larvae of all five "species" appear to be distinct though closely interrelated. Characters by which these larvae may be separated in the first zoeal stage are given in the key below. The separation of stage one larvae of the five species attributed to the genus Jaxea is difficult, as published descriptions show the first stage larvae to be closely similar. However, specific differences become more apparent in later larval stages. For example, in the unnamed Australian species and the New Zealand species, J. novaezealandiae, stage one larvae are almost identical, but the later stage larvae of these two species are quite distinct, especially in the form and armature of the telson.

The significance of observations on the presence or absence of the reduced second telson seta (thalassinid hair) is not clear. Though not recorded by Dakin and Colefax (1940) for the Sydney larvae, it is known to occur in J. novaezealandiae, the Samoan species (Gurney, 1938) and in J. nocturna (Gurney, 1942). The armature of the telson in stage six of the Adriatic species (Kurian, 1956) suggests that it may also occur in the early larval stages of that species.

In J. novaezealandiae it is, however, important to discuss the derivation of the posterior telson process which loses its articulation with the telson in stage three, and enlarges greatly in subsequent larval stages forming the inner ramus of the bifurcated posterior telson cornu. This seta can be seen beneath the cuticle of second stage larvae about to moult, and appears to be the fourth seta. In stage three larvae the third seta is reduced to a small stout spine within the bifurcated cornu. The thalassinid hair of stage two is apparently lost in the third and subsequent larval stages. Enlargement of the fourth seta occurs in the Euphausiacea, Penaeidea, later stage Thalassinidea (Upogebia, Callianassa etc., see fig. 4) and Anomura, but is most pronounced in the Brachyura in which it forms the greater part of the fork of the telson (Gurney, 1942). This character has therefore been paralleled in Jaxea novaezealandiae.

However, among the Laomediidae of which later stage larvae are known (with the exception of J. novaezealandiae) no such enlargement of the fourth seta is immediately obvious (fig. 4 I-N), as the lateral cornua of the telson are of one process as in the Brachyura. The lateral cornua of the laomediid telson are considered by many authors to be modified first setae (Caroli, 1924; Gurney, 1942; Kurian, 1956; Dakin and Colefax, 1940), and the two or three small spines usually found on the posterolateral margins in late stage laomediid larvae (fig. 4 H-N) are thought to be accessory developments. These spines are admittedly accessory developments in J. novaezealandiae (fig. 4H), and also in J. nocturna (fig. 4K) where they are medial to the lateral cornu. However, in other laomediid larvae (fig. 4I, J, L-N) these posterolateral spines may be the first, second and third telson setae in a more advanced stage of reduction than that shown by the Callianassidae and Axiidae in which the fourth seta enlarges, and the first, second and third setae are reduced to conspicuous lateral sub-terminal telson spines in later larval stages (fig. 4 A-G).

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In the Laomediidae, therefore, the uniramous telson cornua of the third and subsequent larval stages may in fact be the true fourth setae, in which case the persistence of comparatively well-developed telson setae lateral to the fourth in Jaxea novaezealandiae (fig. 4H) is a rather more primitive condition.

In possessing at least six and occasionally a seventh stage (six a) in the larval life history, Jaxea novaezealandiae passes through a much longer series of ecdyses than do the majority of the Reptantia where the usual number of larval stages is three, rarely four or five. Gurney (1924) comments on this fact, but was unable to establish whether or not the entire series was passed through by a single larva. From laboratory rearing this now appears likely, at least for the first six stages.

Key to First Stage Larvae of the Genus Jaxea

Stage one larvae of four species are known, but a fifth species from the Adriatic Sea is known only from a stage six larva (Kurian, 1956). I have therefore deduced key stage one characters of this species from Kurian's description (see footnote p. 21) in order that the five known larval species may be separated in the first zoeal stage. A complete series of larval stages has been described for the Australian species (Dakin and Colefax, 1940), for Jaxea nocturna Nardo (Claus, 1884; Brook, 1889; Cano, 1891; Bouvier, 1914; Caroli, 1924; Tattersall, 1938; Gurney, 1942; Kurian, 1956) and for J. novaezealandiae, but the Samoan species is known only from a stage one larva (Gurney, 1938).

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1 (8)	Total length greater than 3.0mm.
2 (5)	Lateral pleural process of 1st abdominal somite absent or present as a small blunt structure.
3 (4)	Rostrum present, small, not extending beyond anterior margin of eye; basis of 2nd antenna with two ventral spines; 3rd maxilliped a long uniramous rod; lateral pleural process of 1st abdominal somite absent; 7th telson seta (5th long seta) having outer margin with fine hairs along its proximal half, and small spines along its distal half	Jaxea nocturna Nardo (Caroli, 1924, etc.)
4 (3)	Rostrum absent; basis of 2nd antenna with one ventral spine; 3rd maxilliped a bud-like rudiment; lateral pleural process of 1st abdominal somite present, but small and blunt; 7th telson seta having outer margin without fine hairs, but with small spines along entire length	Jaxea sp.—Samoa (Gurney, 1938: 333–334, fig. 35)
5 (2)	Lateral pleural process of 1st abdominal somite well developed, sharp and procurved.
6 (7)	Rostrum short, extending to anterior margin of eye or just beyond; thalassinid hair absent; all 5 inner pairs of posterior telson setae having spines along both inner and outer margins of basal third, but with fine hairs distally; posterior border of telson with fine hairs medially, and between 2nd and 3rd long plumose setae only	Jaxea sp.—Sydney Harbour, Australia (Dakin and Colefax, 1940: 179–182, figs. 268, 269)
7 (6)	Rostrum longer, extending beyond eye by about half length of eye; thalassinid hair present, conspicuous and plumose; 6th, 5th and occasionally 4th telson setae having large basal spines along inner and outer margins extending along outer proximal half of 6th seta; no hairs occurring together with large spines; otherwise minute spines and fine hairs decreasing in size distally along entire inner and outer margins of 7th, 6th, 5th, 4th and 3rd telson setae; posterior border of telson with fine hairs medially and between all five pairs of long plumose setae	Jaxea novaezealandiae (fig. 3A, B, H)
8 (1)	Total length probably less than 3.0mm*	Jaxea sp.—Adriatic Sea (Kurian, 1956: 75, figs. 144–146)