Pachynidae, Lowry & Stoddart, 2012

Lowry, J. K. & Stoddart, H. E., 2012, The Pachynidae fam. nov. (Crustacea: Amphipoda: Lysianassoidea) 3246, Zootaxa 3246 (1), pp. 1-69 : 5-6

Pachynidae fam. nov.

Pachynidae De Broyer et al., 2007: 157 (nomen nudum).

Diagnostic description. Head not extending greatly below insertion of antenna 2. Epistome and upper lip fused. Antennae without calceoli. Antenna 1 accessory flagellum present or absent. Antenna 2 peduncular article 3 without lateral hook. Mouthpart bundle subquadrate. Mandible incisor small, smooth; left lacinia mobilis a small smooth robust seta or absent; accessory setal row with 4 or less stout robust setae or modified into a lamina dentata or absent; molar a setose or non-setose flap or absent. Maxilla 1 inner plate small with vestigial apical setae or absent; outer plate setal-teeth in a 7/4 or modified 7/4 arrangement, setal-teeth apical or displaced down medial margin, variously reduced in number; palp 2-articulate with distal robust setae, vestigial or absent. Maxilliped inner plate reduced or absent, without apical nodular robust setae, oblique setal row absent; outer plate small (not exceeding length of palp article 2) to large (exceeding length of palp), with few or no distal slender setae, without distal robust setae, with a few vestigial medial robust setae; palp 3- or 4-articulate, dactylus (when present) well developed to vestigial. Gnathopod 1 subchelate or chelate; propodus massive, carpus extremely compressed, hidden by propodus. Coxae 1–3 large; coxa 4 posteroventral lobe usually well developed, occasionally reduced. Pereopods simple. Uropod 2 inner ramus not constricted. Uropod 3 biramous or uniramous, outer ramus 2-articulate. Telson entire.

Type genus. Pachynus Bulycheva, 1955 .

Included genera. The Pachynidae includes 12 genera: Acheronia Lowry, 1984 ; Coriolisa Lowry & Stoddart, 1994 ; Drummondia Lowry, 1984 ; Ekelofia Lowry, 1984 ; Figorella J.L. Barnard, 1962 ; Pachychelium Stephensen, 1925 ; Pachynus Bulycheva, 1955 ; Prachynella J.L. Barnard, 1964 ; Renella gen. nov.; Sheardella Lowry, 1984 ; Smaraldia gen. nov.; and Ultimachelium gen. nov.

Remarks. Among lysianassoid amphipods the pachynids have: maxilla 1 outer plate with a 7/4 setal-tooth arrangement and inner plate with apical or no setae; no mandibular molar (vestigial in a few species); coxae 1 to 3 all about the same size; uropod 3 biramous (two species uniramous); and the telson entire. Opisids, sophrosynids and kergueleniids share most of these characters, but the setal-teeth in the latter two groups are so reduced that their affinities are not understood. Pachynids have one synapomorphy, the extremely compressed carpus of gnathopod 1, which distinguishes them from all other lysianassoids.

Sexual dimorphism. Adult males have a well developed callynophore on antenna 1, but do not develop calceoli on either antenna, brush setae on the peduncle of antenna 2, an elongate flagellum of antenna 2, setose brushes on the merus/carpus of pereopods 3 and 4 or plumose setae on the rami of uropod 3. It is unlikely that pachynids have a swimming-male reproductive strategy.

Biogeography. The geographical distribution of pachynids is basically along the coastlines of the Southern Ocean and the Pacific Ocean rim. There are no records from the North Polar Seas, the Indian Ocean or (except at the margins) the North or South Atlantic Oceans and a single record from the Mediterranean Sea.

Habitat and life-style. All but two of the 26 species for which we have some record of substrate have been taken from sediments –from silt and mud through to coarse shell, but most often from sand. There are a few records associated with algae or seagrasses, but here too the animals were most likely in sediments collected around the bases of the seagrass (as recorded for Sheardella tangaroa ) or the algae (as recorded for Pachychelium barnardi ). Two specimens (one each of Pachychelium tropicalis and Pachynus denticulatum ) have been taken in baited traps. In both cases the trap was sitting on the sea bottom; in one case it definitely, and in the other probably, collected sediment. So, we know that pachynids live predominantly, maybe exclusively, on or in sea-floor sediments but we know nothing of what they do there.

Many lysianassoids (such as the hirondelleids, scopelocheirids, and many of the uristid and tryphosine genera such as Anonyx , Ichnopus and Tryphosella ) are known to be scavengers – they are readily taken in baited traps, often in great numbers. Most such scavengers have a set of morphological characters that facilitate this life style: they have a strong callynophore on antenna 1, in both males and females, which probably assists chemical detection of the food source; they have well-developed plumose setae on elongate third uropods, which assist in swimming; the incisor is a well-developed, rounded cutting tool; the mandibular molar is a well-developed strongly setose structure, often tongue-like, which pushes cut meat into the gut; and the setal teeth on maxilla 1 outer plate are usually long, strongly cuspidate and arranged in a 7/4 crown pattern. Pachynids do not have this suite of characters.

In pachynids the callynophore is well developed in males, but rarely present, and never well-developed, in females; the third uropods are small and never develop plumose setae; the incisor, although often well-developed, is not the strongly rounded cutting tool of a scavenger; the molar is rarely present and in the few species where it does occur it is not tongue-like; and the maxilla 1 setal teeth are frequently poorly or not at all cuspidate and never have a crown-like arrangement. The two records of pachynids from baited traps are most likely accidental captures. It would seem that pachynids are not scavengers.

Some lysianassoids (such as aristiids and conicostomatines) are predominantly associates of other invertebrates such as sponges, ascidians and coelenterates. The evidence for this comes not so much from their morphology but from records of collection in or on their hosts. On the rare occasions when pachynids have been recorded as taken with other invertebrates, the sample has also contained sediment and there has been no definite association between the pachynid and the other organisms. Although it cannot be ruled out entirely, it is unlikely that pachynids are close associates of other invertebrates.

Based on habitat and mouthpart morphology pachynids may process sediment, presumably digesting soft-bodied micro-organisms. The large outer plates of the maxillipeds could be used as scoops to bring sediments into the mouth.

The function of the massively developed first gnathopods is not obvious. Although the gnathopods are powerful, the position and structure of the dactylus and the unarmed palm do not appear to be adapted for grasping. Because this form of gnathopod occurs in both females and males it is unlikely that it is used for mate-guarding or in amplexus.

Distribution. Davis Strait, Mediterranean Sea, South Atlantic Ocean, North and South Pacific Oceans and Southern Ocean; from shallow continental shelf to abyssal depths.