EABFI'AT SELEOTION BY AQUAmC INVERTEBRATES
331
ture of the interstitial archiannelid Protodrilus symbioticus which lies
almost exactly midway (f15'C) between its upper (+34-3'C) and its
lower (-3.7OC) lethal limits (Gray, 1965) (Fig. 6a). On the other hand
there are records of animals whose preferred level of a particular
variable is very close to the lower or upper lethal limit imposed by that
hizwera baliico
Nitocha fohciosa
Schizopera baliico
T
1
1
5
10
15
20
25
30
5
10
15
Saliniiy (%d
FIG. 6. Lethal limits and preferred range. (A) The preferred temperature of the
intertidal interstitial archiamelid Protodrilua syrnbioticw lies almost exactly midway between the species' upper and lower lethal limits. (Modified from Gray, 1965.)
(B) The preferred salinities of two brackish water interstitial harpaoticoid
copepods, Nitochra fallacioaa and Schizopera baZtica, lie at their lower lethal
limits. (Modified from Janason, 1968.)
- - - - = lethal limits: 0-0
=
preferred range.
same variable, or even, which seems entirely paradoxical, at or outside
the lethal limits. The preferred salinity of various interstitial organisms
studied by Jansson (1968) are just such a case in point, and lie very
close to or beyond the lower lethal limits (Fig. 6b), and Jansson (loc.
cit. p. 61) found it difficult to explain his results. Similarly, Lyster
(1965) gave the larvae of PhyZZodoce maculata (L.), an intertidal poly-