78 



R. HUYS AND J. M. GEE 



However, Gee also pointed out that the mandibular gna- 

 thobase in all Psammis species bears long, relatively fine, 

 sharply pointed and widely separated teeth compared to the 

 species of Danielssenia where these teeth are short, stout, 

 blunt and closely set. On the base of this difference he 

 suggested that both genera probably utilize different food 

 items and to a certain extent are trophically isolated. In 

 combination with the fused rami in the female P5, this 

 evidence was considered as sufficient to maintain Psammis'' 

 separate generic status. Close examination of the mouthparts 

 in P. longisetosa and P. longipes and comparison with D. 

 typica has now revealed several other characters that can be 

 used to distinguish both genera. Unique features for Psammis 

 are the specialized comb-like spines on the coxal endite of the 

 maxillule, the presence of only two spines on the praecoxal 

 endite of the maxilla, and the extremely enlarged, spinulose 

 seta on the maxillipedal basis. The presence of tubular setae 

 and modified spines with tubular extensions on the maxillule 

 and maxilla is a character that is shared by both genera 

 though the precise number is not identical. It is conceivable 

 that these specialized structures might perform a sensory role 

 (as chemo- or probably mechanoreceptors) in remote food 

 detection and/or manipulation. Both genera are predomi- 

 nantly found in the upper flocculent layer of muddy sub- 

 strates where selection of food-particles probably requires a 

 different mechanism. This could be particularly true for 

 deepwater bottoms (fjords, abyss) where either turbidity is 

 high or the proportion of suspended food-particles might fall 

 below a subsistence level. The unique specialization of the 

 mandibles, maxillules and maxillae might be viewed collec- 

 tively as the result of a different dietary discrimination 

 mechanism based on successful remote selection of food 

 particles and thus avoiding the unnecessary high energy costs 

 of rejecting unsuitable items upon initial capture. It is noted 

 here that the claviform aesthetascs found on the mouthparts 

 of certain other Paranannopidae (Gee & Huys, 1991) are not 

 homologous to the tubular setae or modified spines bearing 

 tubular extensions. 



Another unique apomorphy of Psammis is illustrated by 

 the setation pattern on the endopods of P3 and P4 (Fig. 23). 

 The ancestral condition of P3 enp-3 is shown by e.g. Archise- 

 nia and consists of 1 outer spine (a), 2 distal spines (b-c) and 

 3 inner setae (d-f). This full complement of armature ele- 

 ments is also found in Psammis but is obscured by modifica- 

 tions in the distal part of the segment. The extreme reduction 

 of the inner terminal spine (c) and the distad displacement of 

 the distal inner seta (d) are the main reasons why the setal 

 formula was erroneously cited as 221 (or 121 in P4) in 

 previous descriptions. The distal elements expressed in this 

 formula are b and d, rather than b and c. The spiniform and 

 pinnate nature of seta d in Psammis did certainly contribute 

 to this misunderstanding. The reduced condition in Bathy- 

 psammis (Fig. 23) has not evolved from the Psammis pattern 

 but resulted through the loss of 2 inner setae. It is impossible 

 to determine which seta (d, e or f) has been retained in B. 

 longifurca. 



Both species of Psammis can be differentiated by the 

 number of setae on the mandibular basis (3 in longipes, 4 in 

 longisetosa), the length of the anterior seta on the syncoxa 

 which is distinctly longer in P. longipes, the ratio of endopod 

 length to exopod length in PI to P3 being much higher in P. 

 longipes, the number of setae on the § P5 exopod (4 in 



longisetosa, 5 in longipes), and the gross difference in body 

 size (± 550 u.m in longisetosa, ± 900 nm in longipes). 



DISCUSSION 



Within the Paranannopidae, aesthetascs on the mouthparts 

 are a powerful synapomorphy for separating a number of 

 genera which have recently been created or redefined, viz. 

 Jonesiella (cf. Huys & Gee, 1992), Paradanielssenia , Microp- 

 sammis, Telopsammis and Leptotachidia (cf. Gee & Huys, 

 1991), Sentiropsis and Peltisenia (Huys & Gee, in press). The 

 absence of such sensory appendages in Archisenia excludes it 

 from this lineage and allies it with the more primitive 

 danielsseniid genera, namely Fladenia, Danielssenia, Psam- 

 mis and Bathypsammis. However, the phylogenetic relation- 

 ships amongst these more primitive danielsseniid genera are 

 somewhat unclear at the moment particularly with respect to 

 the position occupied by Archisenia. The problem is that this 

 genus shows a mosaic of primitive plesiomorphic characters 

 (6-segmented female antennule; setal formula of legs P2-P4 

 with 7.8.8 setae/spines on exp-3 and 5.6.5 setae on enp-3; P5 

 with 5 setae on baseoendopod and exopod), but at the same 

 time a number of unique autapomorphies in the sexual 

 dimorphism on PI basis, P2 enp-1 and P3 enp-2. 



Within this group of genera it is clear that Fladenia is the 

 most primitive genus because it retains both vestiges of sexual 

 dimorphism involving a difference in the number of elements 

 (in this case setae) on the endopod of P3 and P4 (Gee & 

 Huys, 1990) and a primitive setal formula particularly in the 

 exopods of P3 and P4. It is also clear that Danielssenia, 

 Psammis and Bathypsammis are linked by a 4-segmented 

 female antennule, a reduced number of setae on P4 enp-3 and 

 probably by having only 2 setae on the P6 in the male (though 

 the latter character cannot be scored for Bathypsammis since 

 the male is unknown). Since it has no vestige of P3 and P4 

 setal sexual dimorphism and does not show the apomorphies 

 of the Danielssenia lineage, it is likely that Archisenia 

 diverged from the main evolutionary line after Fladenia and 

 probably before the D anielssenia-gr ouying. 



Within the Danielssenia-Psammis-Bathypsammis lineage, 

 Danielssenia is considered the most advanced genus on 

 account of the loss of a seta on exp-1 of the antenna, the basis 

 of the mandible, exp-1 of P2-P4 and enp-2 of P2. Unique 

 apomorphies for this genus are the typically ventrally 

 deflected rostrum, the blunt teeth on the mandibular gna- 

 thobase, and the dorsal, incised, hyaline frill on the 

 P5-bearing somite. Another diagnostic character for Daniels- 

 senia is illustrated by the shape of the seminal receptacle. 

 Multi-chambered receptacles have been described for a num- 

 ber of Paranannopidae such as Leptotachidia, Telopsammis, 

 Psammis and Paranannopus (Gee & Huys, 1990, 1990) and 

 might well be the ancestral state in this family. However, in 

 none of these genera the paired anterior chambers are 

 elongate, cylindrical reservoirs extending into the posterior 

 part of the P5-bearing somite. 



Analysis of the precise relationships within the Danielsse- 

 nia grouping is hampered by the absence of male Bathypsam- 

 mis. The specialized tubular structures on the endites of the 

 maxillule and maxilla provides a robust synapomorphy to link 

 Danielssenia and Psammis. A close relationship is also indi- 

 cated by the armature of the female sixth legs bearing one 



