Morphology of the first three zoeal stages of the deep-sea caridean shrimp Heterocarpus fascirostratus Yang, Chan & Kumar, 2018 (Crustacea, Decapoda, Pandalidae)

Abstract The larvae of the deep-sea pandalid shrimp Heterocarpus fascirostratus Yang, Chan & Kumar, 2018 were successfully hatched and cultured to the third zoeal stage. The larvae reached the third zoeal stage nine days after hatching at a water temperature of 21 ± 1 °C. Although members of Heterocarpus A. Milne-Edwards, 1881 have rather diverse body forms and are often separated into many species groups, the early zoeal morphology of H. fascirostratus follows the general developmental pattern of the species in Heterocarpus. The main differences amongst these larvae are body size, spines on the anteroventral margin of the carapace, and the endopod setation of the third maxilliped.

Heterocarpus fascirostratus Yang, Chan & Kumar, 2018 is the last species described in the genus and belongs to a different species group from those species of the same genus with known larval morphology. Heterocarpus fascirostratus only has pleonite III bearing an overhanging spine. Heterocarpus ensifer, H. hayashii and H. sibogae have posterior spines on both pleonites III and IV. Heterocarpus abulbus completely lacks a spine on the pleon. In the present work, an ovigerous female of H. fascirostratus was collected off the South China Sea and its hatching larvae were cultured for the first time until the third zoeal stage.

Materials and methods
An ovigerous female of H. fascirostratus was captured by the research vessel 'Ocean Researcher I' (station CP 4128) with a French beam trawl at depths of 420-444 m off Dongsha Island (Pratas,Taiwan,20°44.857'N,116°08.010'E) in the South China Sea. The ovigerous female was maintained in sea water (35 ± 1‰ salinity) at 14 ± 1 °C in the laboratory. After hatching, ~ 200 actively swimming larvae each were transferred to two beakers (5 L) with aerated seawater, temperature at 21 ± 1 °C and natural photoperiod. The larvae were fed daily with Artemia nauplii and rotifers from Zoea I to III and with water changed every day. Specimens of each zoeal stage were collected immediately after the larvae molted and subsequently preserved in a 70% ethylene glycol solution. About five larvae were sampled each day to check for moults. At least two larvae from each stage were dissected, mounted on glass slides and examined under a stereo microscope (OLYMPUS SZX12) using fine entomological needles. Appendages were drawn using a camera lucida installed on a compound microscope (Olympus BX50). About 80% of the larvae developed to Zoea II, but only two larvae moulted to Zoea III and the rearing terminated then.
The descriptions and figures were arranged according to the standards proposed by Kamanli et al. (2018). Morphological terminology follows Yang and Ko (2004), Landeira et al. (2010), Clark and Cuesta (2015), Kamanli et al. (2018) and Almeida et al. (2021). Zoea I is completely described in detail; however, only appendage changes were described in the subsequent stages. Abbreviations of larval measurements are as follows: carapace length (CL), from the postorbital margin to the posteromedian end of the carapace; body length (BL), from the postorbital margin of the carapace to the posterior end of the telson; and total length (TL), from the tip of the rostrum to the tip of the telson. These are all given as mean values followed by the range (in parentheses). The parental female and larvae are deposited in the National Taiwan Ocean University (NTOU M02078).
Antennule (Fig. 2D): Peduncle unsegmented, bearing a pair of terminal plumose setae and one long plumose seta; outer flagellum with one simple seta and five long aesthetascs.
Antennule (Fig. 3D): Peduncle 2-segmented, with the proximal segment bearing a small process and six plumose setae, arranged like 1+5; distal segment bearing nine setae (two simple and seven plumose); outer flagellum with one long terminal aesthetasc, two long plumose, one small simple seta.

Discussion
Previous studies together with the present work have shown the five species of Heterocarpus with known larval morphologies to belong to three different species groups: H. abulbus lacking spines on the pleon, H. ensifer / H. hayashii / H. sibogae bearing posterior spines on the pleonites III and IV, and H. fascirostratus only has pleonite III bearing an overhanging spine. Nevertheless, the general appearances of the early zoeal stages are rather similar amongst these five species. Thus, the early zoeal morphology of H. fascirostratus also has the common characters of pandalid larvae, such as the eye peduncle narrowed at the base, carapace with two dorsal protuberances and the anteroventral margin bearing spines, antennule with the peduncle strongly concave and exopod bearing spatulate seta, the antenna with a segmented exopod, and the rostrum elongated in earlier stages (see Thatje and Bacardit 2000;Landeira et al. 2010;Jiang et al. 2014). On the other hand, the first zoea of H. fascirostratus differs from the other four congeneric species in body size, spinulation of the carapace, and setation of third maxilliped (see Table 1). Moreover, the third maxilliped  endopod has the third segment slightly swollen in H. fascirostratus but rather slender in the other four species. Although Iwata et al. (1986) reported on the ZI-ZV of H. sibogae, their description and illustrations are not detailed enough according to current standards for making comparisons (e.g., the presence of anteroventral spines on the carapace in ZI was not described; see Jiang et al. 2014). Therefore, the ZII and ZIII of H. fascirostratus can only be compared with those of H. abulbus and H. ensifer. They also differ in the number of spines on the anteroventral margin of carapace (none in H. fascirostratus, one in H. abulbus, two in H. ensifer), setation on the endopod of the third maxilliped (ZII and ZIII 2,1,0,3,4 and 2,1,2,4,5 setae, respectively for H. fascirostratus, vs. 2,1,0,2,4 and 2,1,1,2,5 respectively for the other two species), and the shape of the third maxilliped endopod (somewhat swollen in H. fascirostratus, vs. slender in the other two species). Moreover, in ZII the first segment of the endopod of the second maxilliped bears three setae in H. fascirostratus and H. ensifer, but four setae in H. abulbus. Table 2 summarizes the characteristics of the three zoeal stages of H. fascirostratus. The major characters of each zoeal stage are: • ZI-eyes sessile, three pairs of maxillipeds, slender rostrum slightly curved, antennule peduncle unsegmented and bearing one small tubercle, pleon with five somites.