Vol. 59, 2020
(update: 2020.07.06; 08.05)
Fossil and Modern Clam Shrimp (Branchiopoda: Spinicaudata, Laevicaudata)
Population Density Effects on
Carapace Growth in Clam Shrimp: Implications for Palaeontological
Hethke1,* and Stephen Weeks2
für Geologische Wissenschaften, Freie Universität Berlin,
Malteserstraße 74-100, D-12249 Berlin, Germany. *Correspondence:
E-mail: firstname.lastname@example.org (Hethke)
2Department of Biology, The University of Akron,
Akron, Ohio 44325-3908, USA. E-mail: email@example.com (Weeks)
(Received 30 January 2020 / Accepted 1
Special issue (articles 32-46) communicated by Thomas A. Hegna and D.
morphological data are time-averaged and generally reflect an overlap
of different sources of carapace variability. To examine whether a
proposed relationship between size and population density in fossil
spinicaudatans is biologically meaningful, we set up rearing
experiments involving two extant species: Eulimnadia texana and Eocyzicus argillaquus. Three and
five days after hydration, clam shrimp were transferred into cups of
various population densities that ranged between 1 and 15 inds/400 ml.
Size and shape were measured 14 and 16 days after hydration,
respectively. Every second day, we recorded length and sex of E. texana, which matured faster in
lower-density cups. According to our growth model, population density
and maximal carapace length follow a logarithmic relationship. At
maturity, hermaphrodites yielded similar lengths across all population
densities (~4.7 mm at 24°C), independent of age. Hence, clam shrimp can
put off reproductive maturity as a response to decreased growth under
higher density conditions. Growth rate generally decreases at maturity,
but that effect is more pronounced in clam shrimp of high population
densities, while low-density adults keep growing. For both species,
multivariate analyses reveal that carapace size of low-density
individuals is significantly larger than carapace size of
higher-density individuals, while size values of intermediate densities
cannot be distinguished. Shape distinction is strong in hermaphrodites
of E. texana: 39.8% of the
density-dependent shape variation is associated with relative umbo
height, which is generally higher in individuals of smaller population
densities. The H/L ratio,
which is often used as a simple shape indicator, does not contribute to
the main variation in shape, but it forms one of several ratios
significant for 18.3% of the shape variability. In turn, the H/L ratio drives 30% of the shape
variation in E. argillaquus.
In addition, higher densities triggered shifts in ontogenetic growth
trajectories in one third of the individuals, which led to aberrant
morphologies. The present rearing experiment shows that some of the
morphological variability on fossil bedding planes can be explained by
population density. Also, it implies a considerable amount of
ecophenotypic variability in Spinicaudata that affects our
understanding of fossil taxonomy and palaeoecology.
Key words: Freshwater ecology,
Morphometrics, Palaeontology, Phenotypic plasticity, Spinicaudata..
Citation: Hethke M, Weeks SC. 2020.
Population density effects on carapace growth in clam shrimp:
implications for palaeontological studies. Zool Stud 59:33. doi:10.6620/ZS.2020.59-33.
Materials: Table S1
| Table S2 | Table S3 | Table S4 | Table S5 | Fig. S1 | Fig. S2 | Fig. S3 | Fig. S4 | Fig. S5 | Fig. S6