Porirualia megarostrum, comb. nov.

Porirualia megarostrum (Wells, Hicks & Coull, 1982) comb. nov.

Parastenhelia megarostrum Wells, Hicks & Coull, 1982

Original description. Wells et al. (1982): 159–164; Figs 5–6; Table 2.

Type locality. New Zealand, North Island, Wellington; Porirua Harbour, Pauatahanui Inlet (since renamed to Te Awarua-o-Porirua Harbour); intertidal beach some 200 m north-east of the State Highway 1 bridge; fine sand with some silt.

Differential diagnosis. This coincides with Wells et al. ’s (1982) excellent description of Parastenhelia megarostrum. See below for differences with its only congener, Po. pyriformis comb. nov.

Notes. Wells et al. (1982) remarked that, despite discrepancies in his illustrations, based on locality and collection details, it appears that Thomson’s (1883) Thalestris forficula is conspecific with Po. megarostrum comb. nov. This claim is considered highly unlikely for a variety of reasons: (a) Thomson’s female specimens are significantly larger (1 mm vs 665 μm); (b) there is no mention of the large rostrum which is one of the most conspicuous characters of Po. megarostrum comb. nov.; (c) the antennule is more slender; (d) P1 exp-2 is elongate, about 2.5 times the length of exp-1 (vs subequal to exp-1); and (e) P1 enp-1 is comparatively longer and its inner seta shorter. Thomson (1883) also stated that the central caudal setae (seta V) are “… swollen just beyond their basal articulation, and marked along the greater part of their length with annular articulations” while in Po. megarostrum comb. nov. all caudal ramus setae are unmodified. As stated above it is conceivable that T. forficula sensu Thomson (1883) is conspecific with Microthalestris antarctica comb. nov.

Porirualia megarostrum comb. nov. is so far endemic to New Zealand where it is found on fine silty intertidal sandbanks in practically all of the country’s major estuarine and harbour systems (Wells et al. 1982; Wells & Hicks in Dahms & Hicks 1996). Additional records from Porirua Harbour, including Pauatahanui Inlet, have been added by Hicks (1984, 1985, 1986, 1988, 1989, 1991, 1992), Iwasaki et al. (1993, 1999) and Blaschke et al. (2010). On the North Island it has been recorded at localities extending northward from Wellington to Kaipara Harbour on the west coast and to Doubtless Bay on the east, occurring sympatrically with Parastenhelia hornelli in Whangateau Harbour (Wells et al. 1982). The species has also been reported from the South Island, including Portobello Harbour (Otago Harbour) (Stringer et al. 2012a), Papanui Inlet (Otago Peninsula) (Leduc & Probert 2011; Leduc et al. 2009) and Okains Bay, Canterbury (Stringer et al. 2012b). Dahms & Hicks (1996) described all six nauplius stages of Po. megarostrum comb. nov. while Dahms (1993) provided details on the copepodid development of this species. Field experiments employing emergence traps indicate that adults do not swim or have only poorly developed swimming ability (Hicks 1986) while nauplii and copepodids are passively mobile, being transported by shifting sandgrains they cling on (Hicks 1988) or resuspended by tidal bottom shear currents (Hicks 1992). Nauplii have never been observed actively swimming (Dahms & Hicks 1996). Porirualia megarostrum comb. nov. is the most abundant epibenthic metazoan on Mana Bank in Pauatahanui Inlet and the dominant food source for young flatfish (Hicks 1984). Study of flat fish predation on copepods at this site indicated that although predation was high, there was little impact on overall abundance of the copepod (Hicks 1985).