Dicranodromia doederleini Ortmann, 1892

Figures 1, 2, 3

Dicranodromia doederleini Ortmann, 1892: 549, pl. 26, fig. 4; Guinot 1995: 202, figs 2C, 11a, c, d, 12A-C; Ikeda 1998: 54-55, pl. 1 figs 1-6; Ng et al. 2008: 39 (for complete synonymy, see Guinot 1995: 202).

Material examined.

Japan: 1 ♀ with 1 megalops (15.9 × 20.2 mm), Sagami Bay, from aquarium trade, 8 Apr. 2015 (ZRC 2017.1214, COI sequence: OK351331) ; 1 ovigerous ♀ (14.5 × 19.2 mm), 35°32.51'N, 139°54.74'E, Futttsu, Kanaya, Chiba Prefecture, 200-250 m, 19 Sep. 2007 (ZRC 2021.0469, COI sequence: OK351333) ; 1♂ (10.3 × 8.5 mm), station 29, 34°40.21'N, 139°18.62'E, SW of Izu-Ohshima Island, Izu Islands, 289-307 m, TRV Shin’yo-maru, 2002 research cruise, coll. T. Komai, 24 Oct. 2002 (CBM-ZC 16572, COI sequence: OK351332) .

Remarks.

This species is well known (for synonymy and records, see Guinot 1995; Ikeda 1998) but may be a species complex, and specimens from outside the type locality in Japan all need to be rechecked (see Guinot 1995; Ng and McLay 2005).

One female specimen (ZRC 2017.1214) was imported to Singapore via the aquarium trade in early April 2015. On 8 April, the specimen was obtained by Paul YC Ng and observed to have between 10-20 large eggs under the pleon with the eyes just visible. It was kept in a cold-water aquarium (ca. 15-20°C) with other crustacean and fish species. On 18 April, he noted that several eggs had been released into the aquarium (Fig. 1D) which appeared ready to hatch, and that some of the egg membranes had ruptured revealing what appeared to be a dead first zoeal stage (Fig. 1E). One specimen, however, was apparently a freshly hatched and dead megalopa (Fig. 1F). He observed the first free-crawling megalopa on the female specimen on 24 April (Fig. 1B, G). Unfortunately, no larvae except one megalopa was preserved (PYC Ng, personal communication).

The observations above on the eggs and megalopa of D. doederleini provide some clarity on the larval development in the genus. While it is known the eggs are large and the development is abbreviated, it is not sure of the eggs hatch into an advanced zoeal stage or directly into megalopa. Caustier (1895) was the first to report on the first zoea of Dicranodromia ovata A. Milne-Edwards, 1880, from the Atlantic but he based this on unhatched embryos and unfortunately, the description was brief, and no figures were provided. Martin (1991) found a specimen of D. felderi Martin, 1990 from the western Atlantic, which had well-developed eggs and removed some embryos. On the basis of these, he described what he regarded was the first zoea. Guinot (1995: 105) reported that a specimen of D. nagaii from Japan had about 20 megalopae under the pleon and suggested the eggs hatched directly into this stage. The eggs of D. doederleini are full of yolk, and even the "first zoea" observed are of the lecitotrophic type, with yolk sacs and appendages, which are poorly or not setose (Fig. 1E). They are very similar to the condition observed or the dromiid Cryptodromia pileifera Alcock, 1900 which has only one lecitrophic first zoeal stage before the megalopa (Tan et al. 1986). In Cryptodromia pileifera, however, the zoea is still able to swim and move around in the water column although it only lasts two days before metamorphosing. For the specimen of D. doederleini that was kept in the aquarium, it would appear that if it was natural, the young would develop into an advanced zoeal stage while still inside the egg membrane, and break free only after it metamorphoses into the megalopa. The transition between the "first zoea" and megalopa, however, is clearly very short, perhaps a day or less. The condition for Dicranodromia is thus probably similar to that of eubrachyuran marine crabs, some other podotreme crabs and various enbrachyurans like the epialtids Paranaxia serpulifera ( Guérin, 1832) and P. keesingi Hosie & Hara, 2016 (Rathbun 1914, 1924; Morgan 1987; Hosie and Hara 2016), and the pilumnids Pilumnus novaezealandiae Filhol, 1885 and P. lumpinus Bennett, 1964 (cf. Wear 1967); taxa which undergo direct development.