Pterygoplichthys up close

Research in the German Lab: Ecological and nutritional physiology


Our primary research goal is to understand how organisms are specialized to use specific resources and the consequences of specialization to ecosystem fluxes.  Our research integrates isotopic, molecular, biochemical, and physiological approaches to gain insight into the nutritional physiology of a range of taxa from microbes to vertebrates.  By understanding the resource acquisition strategies of a range of organisms within a given ecosystem, we can better understand fluxes within that system.  Our longterm goal to use this information to make more informed management decisions.

Current Projects:

Evolution of elevated carbohydrase activities in herbivores
Digestive adaptations for herbivory
Digestion in seagrass-eating, juvenile bonnethead sharks
Resource acquisition strategies of detritivores and decomposers
Abalone susceptibility to withering syndrome
  

San Simeon
Field collection site:
San Simeon, CA
picture by M.H. Horn

Peru_wood
Field collection site:
Rio Maranon, Peru
picture by A.S. Flecker

Evolution of elevated carbohydase activities in herbivores
It is now well-documented that herbivorous vertebrates have elevated carbohydrase activities (amylase and a-glucosidase in particular) in their digestive tracts in comparison to their carnivorous brethren, even when consuming a diet practically devoid of soluble carbohydrates (German et al. 2004) and when analyzed in a phylogenetic context (Horn et al. 2006; German et al. 2010; Kohl et al. 2011).  Genetic analyses suggest that increased amylase activities are explained by increased amylase gene copy number in mammals (Perry et al. 2007; Axelsson et al. 2013), but our own studies of prickleback fishes (Family Stichaeidae, at right) suggest otherwise. Convergently evolved stichaeid herbivores have wildly different haploid amylase gene copy number (6 in C. violaceus vs 4 in X. mucosus), yet both have the phenotype of elevated amylase activities. We are, thus, sequencing the genomes and transcriptomes of these convergent herbivores (plus omnivorous and carnivorous pricklebacks) to better understand the evolution of dietary specialization.  A parallel project using experimental evolution with Danio rerio is also underway.

Ap
A. purpurescens (carnivore) picture by M.H. Horn


Cv
C. violaceus (herbivore)
picture by M.H. Horn

Digestive adaptations for herbivory
In 1971, Nevo transplanted five breeding pairs of the insectivorous lizard Podarcis sicula from Pod Kopiste to Pod Mrcaru in the Croatian Adriatic.  36 years later, Herrel et al. (2008, PNAS) revealed that the transplanted lizards on Pod Mrcaru had become largely herbivorous.  The transplanted P. sicula were larger, had greater bite force, and perhaps most interesting, had developed valves in their large intestines typically only observed in highly derived herbivorous lizards (e.g., Iguanids, Agamids).  So, in just ~30 generations, P. sicula had developed morphological adaptations for consumption of a plant diet that we previously thought only evolved over longer time scales.  In collaboration with Anthony Herrel and Zoran Tadic, we received a NSF grant to investigate the digestive physiology of P. sicula from the two islets, and this forms the foundation of Beck Wehrle's dissertation research.  See Beck's page for more detail.       


P. sicula
P. sicula (from Pod Mrcara)
picture by B.A. Wehrle













Pod Kopiste
Pod Kopiste, with few plants
picture by D.P. German









Digestion in seagrass-eating, juvenile bonnethead sharks
Herbivorous sharks? Bonnethead sharks (Sphyrna tiburo) appear to consume a fair amount of seagrass as juveniles (up to 62% index of relative importance in some young-of-the-year; Bethea et al. 2007).  In collaboration with Yannis Papastamatiou (Florida International University) we are just beginning to evaluate whether these sharks have the capability to assimilate nutrients from seagrass.  It does appear that the sharks have the ability to digest components of exoskeletons (chitin) and seagrass (cellulose) via the aid of microbially-derived enzymes in their hindguts (Jhaveri et al. 2015).  This is now Samantha Leigh's dissertation research, so see her page for more detail on this exciting project.

bonnethead
Sphyrna tiburo
(image from marinebio.org)


Shark gut
S. tiburo with its gut.


Resource acquisition strategies of detritivores and decomposers
Detritivores and decomposers perform important ecosystem services in all ecosystems, yet, we understand very little about the resource acquisition strategies of either group and how this effects carbon and nutrient cycling on local and even global scales.  In aquatic systems, it appears that detritivorous fishes play an important role in organic matter processing and may facilitate the decomposition process (e.g., Taylor et al. 2006).  We argue this is because of the their preference for soluble nutrients, which causes them to have high intake of a dilute diet.  Conversely, decomposers efficiently digest compounds (e.g., cellulose) that the detritivores cannot digest.  These two strategies may work in concert to facilitate decomposition and nutrient cycling in aquatic systems.  By starting at the molecular level and scaling up to the organismal (for the fish) and community levels (for the microbes), we hope to elucidate the interactive effects of these organisms in decomposition and nutrient cycling.

Pd
Pterygoplichthys disjunctivus (detritivore)
Efficiently digests soluble compounds


Decomp_ratio
Microbial decomposition rate declines with concentration for soluble compounds, like starch.

Abalone susceptibility to withering syndrome
Abalone are susceptible to a bacterial disease called "withering syndrome" (WS). WS is caused by a Rickettsiales-like organism (RLO) that infects the digestive tract of abalone and leads to starvation and death. The RLO has been documented in most abalone species, yet the disease state thus far has been restricted to a relatively small proportion of abalone species at environmentally relevant temperatures.  Temperature increases and temperature variability have been linked to disease expression, and thus, the goals of this work are to 1) generate a robust phylogeny for California abalone and map temperature tolerance on that tree to better understand why some species are more susceptible to WS (via temperature stress) than others; 2) determine the physiological processes in the host digestive tract that link RLO infection to WS. This is the dissertation work of Alyssa Braciszewski, so see her page for more detail.    

abalone
Haliotis fulgens, green abalone
picture by A.R. Braciszewski