1.0 INTRODUCTION
Life history theory assumes that there are trade-offs between different
traits in organisms, such as growth, reproduction and survival (Roff,
2002). These traits cannot be simultaneously maximized within the same
individual because the available amount of nutrients and other resources
are in limited supply (Stearns, 1989). Increased resource allocation
into one trait will, therefore, come at the cost of reduced allocation
into other traits (Agnew et al., 2000). In each given environment, the
optimal way to resolve these trade-offs (i.e., the optimal
strategy for maximizing fitness) is the one achieving the highest
possible reproductive success (Agnew et al., 2000; Pianka, 1976;
Stearns, 1989). For instance, if adult mortality increases within a
population (e.g., due to increased predation), individuals that
mature relatively earlier and invest relatively more into current
reproduction versus future survival will be favoured by natural
selection (Fredensborg and Poulin, 2006).
For fish, both natural predation and fishing (i.e., predation by
humans) are important selective factors that drive adaptive changes in
life history traits such as developmental rates and timing of
reproduction (Heino and Godø, 2002; Jorgensen et al., 2007; Jørgensen et
al., 2009; Sharpe et al., 2012). Fishing practices and predation are
usually non-random factors, as gears are often designed to selectively
take larger and older fish in the population (Law, 2000). In this case,
smaller fish are likely to have a higher probability of survival than
the larger ones, and among them, those that can mature and reproduce
early will be selected favourably. Assuming that early maturation is
heritable to some extent, this should result in life histories changing
towards earlier reproduction at smaller sizes.
Parasitism can also affect the future reproductive success of hosts
(Fredensborg and Poulin, 2006) and thus select for changes in host life
history traits (Adamo, 1999; Agnew et al., 1999; Lafferty, 1993b;
McCurdy et al., 1999; Perrin et al., 1996; Polak and Starmer, 1998;
Richner and Tripet, 1999; Sorci et al., 1996; Thomas et al., 2000; Yan
et al., 1997). For instance, an increase in the prevalence of parasites
causing castration (i.e., destruction or alteration of the host’s
gonadal tissues by the parasite; (Noble and Noble, 1971)) (Fredensborg
and Poulin, 2006; Lafferty, 1993a; Loot et al., 2002; Minchella and
Loverde, 1981) can select for earlier maturity. For the infected host,
achieving reproduction prior to castration yields clear fitness benefits
(Gooderham and Schulte-Hostedde, 2011; Lafferty, 1993a; Minchella and
Loverde, 1981), and these benefits increase along with infection risk
(Minchella and Loverde, 1981; Polak and Starmer, 1998; Sorci et al.,
1996). Increased reproductive effort in hosts exposed to castrating
parasites has been reported in a number of species. So far, however,
most documented life history changes seem to result from adaptive
plastic responses of hosts to parasitic exposure, more than life history
evolution following a change in parasite-mediated selection (Chadwick
and Little, 2005; Hudson et al., 2019; Vale and Little, 2012).
Ligula intestinalis (L. 1758) is a common and widespread cestode,
that use cyprinid fish as intermediate hosts (Dubinina, 1980). In Lake
Nyasa this cestode has been increasingly reported since it was first
noted by Mwambungu et al. (1996), where it infects the endemic pelagic
fish Engraulicypris sardella (Figure 1) (Gabagambi et al., 2019;
Gabagambi and Skorping, 2018; Msafiri et al., 2014; Rusuwa et al.,
2014). L. intestinalis is known to induce castration in several
intermediate hosts (Cowx et al., 2008; Hoole et al., 2010; Kennedy et
al., 2001; Loot et al., 2002; Wyatt and Kennedy, 1988) and has therefore
been suggested to cause population crashes of its host (Burrough et al.,
1979; Kennedy et al., 2001). This could sometimes lead to local
extinction of the parasite in small ecosystems (Kennedy et al., 2001).
Recent results, however, indicate that local extinction of this parasite
is unlikely in Lake Nyasa due to spatial and temporal variations in
transmission rates (Gabagambi and Skorping, 2018).
Under such conditions of recent parasite invasion, we hypothesize that
the cestode L. intestinalis should select for a shift in resource
investment from somatic growth towards reproduction in its intermediate
fish host E. sardella . Using data collected from 2005 to 2013 and
then in 2015 in the northern part of Lake Nyasa, we address the
following three questions:
(i) What are the effects of L. intestinalis on the fecundity ofE. sardella ? (ii) has reproductive investment at maturity ofE. sardella increased over time? and (iii) has the average size
at maturity of E. sardella decreased since the parasite was first
identified in the lake?
We then further discuss the selective roles of parasitic invasionversus other environmental factors that may recently have changed
in Lake Nyasa.