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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
[introduction]
Larvae of the genus Jaxea are striking, long-necked, Lucifer-like zoeae to which the name of "Trachelifer" was given by Brook (1889). New Zealand trachelifer larvae, first taken in the Bay of Islands, were assigned by Gurney (1924) to Jaxea on the basis of their resemblance to larvae of J. nocturna from the northern hemisphere. Trachelifer larvae have been observed in the Wellington Harbour plankton for a long time, but the first published record from the Wellington area is that of Wear (1965) who showed that these larvae were among the most abundant and conspicuous larval decapods in the summer plankton.
The abundance of larval material has allowed me (R.G.W.) to trace the entire larval history of J. novaezealandiae up to and including the first post-larval stage. The first larval stage has not yet been hatched from ovigerous adults, so the identification of this larval series is based entirely on morphological similarities between the first post-larval stage and the adult Jaxea, and on our recognition of only one series of laomediid larvae and only one adult laomediid in the New Zealand region.
Some larval stages of this species taken from the Bay of Islands have already been described by Gurney (1924) and compared with the larvae of J. nocturna. Six essential larval stages have now been recognised in the planktonic life history of J. novaezealandiae. Five of these (stages 1, 2, 4, 5 and 6) have been described by Gurney (1924) and additional details only are given in this paper.
A further essential stage in this larval series (stage 3) not found by Gurney, and a seventh larval stage (designated here as stage 6a) of uncertain significance, which occurs only rarely, are each described in detail.
The laboratory method used to obtain successive larval stages has been outlined in a previous paper (Wear, 1964). Total length of larvae and of the first post-larval stage is here measured from the tip of the rostrum to the most posterior margin of the telson, and excludes all telson processes and setae. In descriptions of the larval stages, measurements of total length are followed by additional total length measurements italicised and in parentheses. These latter measurements were made from the tip of the rostrum to the posterior tips of the lateral telson cornua to conform with the earlier measurements of Gurney (1924), and those of Caroli (1924) on the larvae of J. nocturna. However, the difference between our measurements and those italicised is in fact the length of the lateral cornu on each side of the telson, which are modified posterior setae. This distance varies considerably with the form of the telson in successive larval stages, and therefore gives an inaccurate assessment of length increase per moult.
Nomenclature of larval limbs and limb segments is based on Borradaile (1926) and Hale (1927) with the suffix "ite" in limb terms deleted.
| 1 (4) | Total length less than 6.0mm; 1st antenna with one-segmented peduncle, no setae along inner margin; 2nd antenna having endopod with three terminal plumose setae; 1st pereiopod without natatory exopod; abdomen of five segments and a telson; uropods absent; telson with posterolateral cornu as a single spine; thalassinid hair (reduced 2nd telson seta) present; setae articulating with telson fringed with fine hairs and small spines. | |
| 2 (3) | Total length 4.0mm; protopod of 2nd antenna with one ventral spine, exopod (squama) with 10 marginal setae; 1st and
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2nd maxillipeds having exopods with four natatory setae; 3rd maxilliped as uniramous rod, without natatory exopod; telson as two slender rami, deeply cleft in posterior midline, 7 + 7 posterior setae including thalassinid hair |
Stage 1 (fig. 3A, B, H) |
| 3 (2) | Total length 4.9mm to 5.3mm; protopod of 2nd antenna with two ventral spines, exopod with 11 marginal setae; 1st and 2nd maxillipeds having exopods with six natatory setae; 3rd maxilliped biramous with exopod having six terminal natatory setae; telson triangular with shallow cleft in posterior midline, 10 + 10 posterior setae (occasionally 10 + 11 or 11 + 11) | Stage 2 (fig. 3I) |
| 4 (1) | Total length greater than 6.0mm; 1st antenna with two-segmented peduncle fringed with plumose setae along inner margin; 2nd antenna having endopod without terminal plumose setae, but with a small subterminal hair; 1st pereiopod with a natatory exopod; abdomen of six segments and a telson; uropods present; telson with posterolateral cornu bifurcate; thalassinid hair absent; setae articulating with telson fringed with small spines, but usually without fine hairs. | |
| 5 (8) | Total length less than 11.0mm; 2nd antenna with endopod not exceeding length of exopod; 1st pereiopod not chelate; pleopod buds absent or only just visible; 3rd visible telson seta (not articulating with telson) shorter than lateral process (1st seta). | |
| 6 (7) | Total length less than 8.0mm (7.4mm to 7.7mm); 2nd antenna with endopod about ¾ length of exopod; 2nd pereiopod having rod-like exopod without setae; uropod with endopod and exopod not separate from protopod, exopod without lateral tooth; telson usually with 11 + 11, 11 + 12 or 12 + 12 posterior setae (original 2nd seta or thalassinid hair absent); telson without lateral distal spines | Stage 3 (fig. 3C, D, E, J) |
| 7 (6) | Total length greater than 8.0mm (9.3mm to 10.0mm); 2nd antenna with endopod and exopod subequal in length; 2nd pereiopod having segmented exopod with four or six natatory setae; uropod with endopod and exopod separate from protopod, exopod with a prominent lateral tooth; telson usually with 13 + 13 or 13 + 14 posterior setae; telson with 4, 5, 6 or 7 lateral distal spines on each side | Stage 4 (fig. 3K) |
| 8 (5) | Total length greater than 11.0mm; 2nd antenna with endopod longer than exopod; 1st pereiopod with rudimentary chela; pleopod buds present, distinct and well developed; 3rd visible telson seta equal to or extending beyond 1st seta. | page 15 |
| 9 (10) | Total length less than 13.0mm (11.6mm to 12.5mm); 2nd antenna with endopod shorter than total length of 1st antenna; pleopod buds short, much less than half the length of abdominal somites; 3rd telson seta and lateral process (1st seta) about equal in length | Stage 5 (fig. 3L, M) |
| 10 (9) | Total length greater than 13.0mm (13.8mm to 15.2mm); 2nd antenna with endopod equal to total length of 1st antenna; pleopod buds about half the length of abdominal somites; 3rd telson seta extending well beyond lateral process | Stage 6 (fig. 3N) |