Cryptic diversity and species distributions
Assessing biodiversity is essential to understand evolutionary and
ecological processes and to observe changes in the distribution of
species as a consequence of climate change and other anthropogenic
stressors. A fundamental unit of biodiversity are species, though
intraspecific genetic diversity is also considered a crucial factor
(Błażewicz, Jóźwiak, Menot, & Pabis, 2019; Laikre, 2010; Brix et al.,
2020). Delimiting species can be challenging in cases where differences
between putative species are small and intraspecific variability and
interspecific variation are not clearly demarcated (Kaiser et al.,
2021). In the case of Haploniscus bicuspis , several genetically
divergent lineages were recovered, of which some probably constitute
separate, morphologically cryptic species. Commonly observed
intraspecific COI distances within deep-sea isopods were usually below
6% (e.g., Brix, Riehl, & Leese, 2011; Brix et al., 2018a, 2018b;
Havermans et al., 2013; Brandt, Brix, Held, & Kihara, 2014; Bober,
Riehl, Henne, & Brandt, 2018), but intraspecific distances of more than
8% have been reported (e.g., Riehl, Lins, & Brandt, 2018). Conversely,
interspecific COI distances as low as 4.6% were reported (Riehl et al.,
2018), possibly suggesting that some of the larger intraspecific
distances may be due to cryptic diversity, but commonly exceed 10%
(e.g., Havermans et al., 2013; Brandt et al., 2014; Brix et al., 2018a,
2018b). In the following, we will discuss whether the five lineages
might constitute different species.
Lineages I, II and III (ICOI-IIICOI and
IRAD-IIIRAD) are consistently delimited
from lineages IV and V in COI and nuclear loci, and with a few
exceptions also in the proteomic data. However, these three lineages are
not consistently delimited from each other. There is a clear discordance
between the mitochondrial and nuclear data, with individuals being
assigned to different lineages (among I, II and III) based on the
respective analyses. Our data also strongly suggests multiple instances
of inter-lineage hybridization and gene flow most notable at station
1219 (see Figure 1) where all three lineages appear to have hybridized.
The genetic distances in COI among these lineages are also smaller than
among most other deep-sea isopods. We therefore suggest that these three
lineages constitute a single species (referred to as H. bicuspisI-III in the following).
Lineages IV (IVCOI and IVRAD) and V
(VCOI and VRAD) are consistently
differentiated from each other and from H. bicuspis I-III in COI
and nuclear loci. COI p -distances between lineage V and all other
lineages exceed commonly observed intraspecific distances within
deep-sea isopod species. For lineage IV, COI p -distances are
lower in comparison to H. bicuspis I-III than commonly observed
among deep-sea isopods. Furthermore, the p -distances are
comparable to those observed within H. bicuspis I-III. However,
in the nuclear loci, lineage IV is the most divergent. No instances of
hybridization have been inferred for lineages IV or V. Distribution
modelling suggests that their habitats are ecologically differentiated
from those of H. bicuspis I-III and at least partially
differentiated from each other. We therefore tentatively suggest that
lineage IV and V constitute two morphologically cryptic species withinH. bicuspis (termed H. bicuspis IV and H. bicuspisV in the following).
Similar patterns of cryptic diversity have been observed for other
Icelandic Isopoda, which were previously assumed to be widespread
species (Brix et al., 2014; Jennings, Brix, Bober, Svavarsson, &
Driskell, 2018). The geographic distribution of the different putativeH. bicuspis species fit well into the distribution patterns of
other benthic deep-sea isopods around Iceland (Brix et al., 2018). Our
discovery that the waters around Iceland alone may be inhabited by three
cryptic species within H. bicuspis strongly questions the
occurrence of H. bicuspis in the South Atlantic Ocean (Brökeland
and Wägele, 2004; Menzies, 1962). The presence of further cryptic
diversity in this presumably widespread species is highly likely. We
deliberately did not perform molecular clock analyses to date the
differentiation of the herein observed species, because evolutionary
rates for deep-sea crustaceans are still lacking. A recent study
suggests that the commonly applied rate of 1.4% per million years
(Knowlton & Weigt, 1998) is too conservative and rather suggest a K2P
divergence of 5-5.2% per million years for arctic marine invertebrates
(Loeza-Quintana et al., 2019). If such rates are applicable to deep-sea
benthic peracarids, such as Haploniscus , the divergence between
the three putative species started roughly half a million to a million
years ago. This is relatively young and suggests recent and putatively
ongoing speciation processes.
The distribution of the three species appears to be largely governed by
water masses and associated ecological parameters. In particular, the
GIF Ridge represents a crucial barrier separating species. Whether the
GIF Ridge is a physical barrier that hinders dispersal or just separates
water masses and thereby shapes species distributions remains
unanswered. The GIF Ridge has been discussed as an isolation barrier for
many isopod species in the North Atlantic, for instance anthuridean
isopods (Negoescu & Svavarsson, 1997), valviferan isopods (Stransky &
Svavarsson, 2006), and desmosomatid and nannoniscoid isopods (Brix &
Svavarsson, 2010). Haploniscus bicuspis sampled in this study
were found between 316 m depth at station 1136 and 2422 m depth at
station 1172, therefore they can easily cross the saddle depth of the
GIF Ridge. Previously, H. bicuspis have been collected in the
Arctic, even in shallow waters of 198 m depth (Brandt, 1997).
Ocean temperature is highly variable around Iceland. North of the GIF
Ridge, bottom water temperature can be as low as -0.9 °C, and it only
becomes warmer as it reaches the shelf (up to about 3 °C, Jochumsen et
al., 2016). In contrast, south of the GIF Ridge, the North Atlantic
Water can reach up to 10.5 °C (Hansen & Østerhus, 2000). Another
environmental variable that may limit species distribution is the
sediment structure, which was previously found to be important in
peracarid crustacean distribution around Iceland (Stransky &
Svavarsson, 2010). Ostmann et al. (2014) found that sediment
characteristics vary in the surrounding waters of Iceland, with coarser
sand found around the Reykjanes Ridge and more silt and clay found in
the deep-sea east of the GIF Ridge. Oxygen concentration may also
influence migration and gene flow between isopods, especially in the
deep sea. Expanding oxygen minimum zones may have contributed to
allopatric speciation in the past (White, 1988).
The distribution models provide interesting hypotheses that should be
tested in future studies. These include the hypothesized distribution ofH. bicuspis IV (and partly V) along the Norwegian coast, the
occurrence of H. bicuspis I-III south of the GIF Ridge off the
coast of Greenland, and whether H. bicuspis IV, V, or both occur
in the abyssal plains south of Iceland. The former would require
dispersal along a narrow habitable corridor along the north-west of
Great Britain, whereas the expansion of H. bicuspis I-III south
of the GIF Ridge would require crossing a region with unfavorable
ecological conditions. This could have been mediated by transport via
the north to south overflow of arctic deep-water. Whether H.
bicuspis IV, V, or both occur on the abyssal plains of the North
Atlantic south of Iceland cannot be answered with the available data.
The ecological conditions appear suitable for both (possibly slightly
better for IV), and sympatric occurrences of these two species were
already observed for the Reykjanes Ridge. By examining additional
populations of H. bicuspis genetically, future studies may shed
light on these questions.
A comparison of proteomic profiles revealed differences between
specimens from the northern (I-III) and southern (IV and V) species, and
also between the two southern species IV and V. However, differentiation
between species IV and V was less distinct. Differences between species
are very small and occur mainly in a few relatively low-expressed
proteins, which show differences in presence and relative intensities.
Similarly, high similarity in proteomic patterns and only a small number
of differentially expressed proteins were observed in cryptic mosquito
species (Müller et al., 2013; Dieme et al., 2014). Like H.
bicuspis, peaks in the mosquito Anopheles gambiae Giles, 1902
species complex were shifted by only a few Daltons, potentially
reflecting few amino acid substitutions within proteins or minor
post-transcriptional modifications (Müller et al., 2013). These small
shifts in masses of specific proteins may thus infer polymorphisms of
these molecules in H. bicuspis . Differences between the putative
species were at least partly caused by a prevalence of a certain allele
in the respective putative species. Despite apparent gene flow
interruptions, differences between IV and V were less pronounced.
Proteomic profiles depend on physiological responses to the environment,
e.g., variations in proteomic profiles varied with season and habitat in
ticks (Karger, Bettin, Gethmann, & Klaus, 2019). Thus, it remains
unanswered whether the higher similarity of putative species IV and V
results from very similar environmental impacts and thus comparable
selection pressures in the past causing a similar physiological
response. We interpret the high similarity between the differentH. bicuspis species found in this study as a reflection of recent
or ongoing speciation processes, as reported in fishes (Takács et al.,
2014, Maasz et al., 2020). It is likely that young species are both
morphologically and physiologically cryptic, revealing the limit of
proteomic fingerprinting for species identification in such instances.
Haploniscus bicuspis was first described from the Norwegian Sea
(Sars, 1877) north of the GIF Ridge and a few hundred kilometers east of
our sampled area, but no genetic information is currently available for
Norwegian representatives. Given the species’ distribution around
Iceland, we suggest that H. bicuspis I-III represents the trueHaplonicus bicuspis (Sars, 1877) because the known and modelled
distribution of H. bicuspis I-III extends well into the waters
east of Iceland towards the Norwegian Sea and would correspond to the
subspecies H. b. bicuspis described by Wolf (1962). However,
Wolff differentiated the two subspecies based on the morphology of
pleopod I, and both variants were found in H. bicuspis I-III
(unfortunately no adult males were available for the other two putative
species). In juvenile males the respective section of the male pleopod I
is smooth and narrow and widens as they mature into preparatory males
(see also Wolff 1962). It is plausible that the different pleopod I
shapes assigned to the two subspecies are in fact successive
developmental stages, with the distinct corner ascribed to H. b.
bicuspis being the later stage. Even though pleopod I morphology might
not be useful to distinguish Wolff’s subspecies, H. b. tepidusmight still represent a valid species (H. tepidus ). Whether this
corresponds to H. bicuspis IV or H. bicuspis V cannot be
answered with the data currently available, as H. b. tepidus was
described from the Reykjanes Ridge.